从ViewRootImpl类分析View绘制的流程

从上两篇博客 《从setContentView方法分析Android加载布局流程》 和 《从LayoutInflater分析XML布局解析成View的树形结构的过程》 中我们了解到Activity视图UI是怎么添加到Activity的根布局DecorView上面的。

我们知道Activity中的PhoneView对象帮我们创建了一个PhoneView内部类DecorView（父类为FrameLayout）窗口顶层视图，

然后通过LayoutInflater将xml内容布局解析成View树形结构添加到DecorView顶层视图中id为content的FrameLayout父容器上面。到此，我们已经知道Activity的content内容布局最终

会添加到DecorView窗口顶层视图上面，相信很多人也会有这样的疑惑：窗口顶层视图DecorView是怎么绘制到我们的手机屏幕上的呢？

这篇博客来尝试着分析DecorView的绘制流程。

顶层视图DecorView添加到窗口的过程

DecorView是怎么添加到窗口的呢？这时候我们不得不从Activity是怎么启动的说起，当Activity初始化 Window和将布局添加到

PhoneWindow的内部类DecorView类之后，ActivityThread类会调用handleResumeActivity方法将顶层视图DecorView添加到PhoneWindow窗口，来看看handlerResumeActivity方法的实现：

0-1

Step1

<code class="hljs lasso has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;">final <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">void</span> handleResumeActivity(IBinder token,            boolean clearHide, boolean isForward, boolean reallyResume) {            <span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span>            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (r<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>window <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">==</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">null</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">&&</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">!</span>a<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>mFinished <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">&&</span> willBeVisible) {                <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//获得当前Activity的PhoneWindow对象</span>                r<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>window <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> r<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>activity<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>getWindow();                <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//获得当前phoneWindow内部类DecorView对象</span>                View decor <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> r<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>window<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>getDecorView();                <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//设置窗口顶层视图DecorView可见度</span>                decor<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>setVisibility(View<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>INVISIBLE);                <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//得当当前Activity的WindowManagerImpl对象</span>                ViewManager wm <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> a<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>getWindowManager();                WindowManager<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>LayoutParams l <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> r<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>window<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>getAttributes();                a<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>mDecor <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> decor;                l<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">type</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> WindowManager<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>LayoutParams<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>TYPE_BASE_APPLICATION;                l<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>softInputMode <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">|=</span> forwardBit;                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (a<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>mVisibleFromClient) {                    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//标记根布局DecorView已经添加到窗口</span>                    a<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>mWindowAdded <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">true</span>;                    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//将根布局DecorView添加到当前Activity的窗口上面</span>                    wm<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>addView(decor, l);            <span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li></ul>

分析：详细步骤以上代码都有详细注释，这里就不一一解释。handlerResumeActivity（）方法主要就是通过第 23 行代码将

Activity的顶层视图DecorView添加到窗口视图上。我们来看看WindowManagerImpl类的addView（）方法。

<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;">@Override    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">addView</span>(View view, ViewGroup.LayoutParams <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">params</span>) {        mGlobal.addView(view, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">params</span>, mDisplay, mParentWindow);    }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li></ul>

源码很简单，直接调用了 mGlobal对象的addView（）方法。继续跟踪，mGlobal对象是WindowManagerGlobal类。进入WindowManagerGlobal类看addView（）方法。

0-2

Step2

<code class="hljs lasso has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">void</span> addView(View view, ViewGroup<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>LayoutParams <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">params</span>,            Display display, Window parentWindow) {        <span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span>        ViewRootImpl root;        View panelParentView <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">null</span>;        <span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span>        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//获得ViewRootImpl对象root</span>         root <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">new</span> ViewRootImpl(view<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>getContext(), display);        <span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">..</span>        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// do this last because it fires off messages to start doing things</span>        try {            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//将传进来的参数DecorView设置到root中</span>            root<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>setView(view, wparams, panelParentView);        } catch (RuntimeException e) {          <span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">..</span>        }    }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li></ul>

该方法中创建了一个ViewRootImpl对象root，然后调用ViewRootImpl类中的setView成员方法（）。继续跟踪代码进入ViewRootImpl类分析：

0-3

Step3

<code class="hljs lasso has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">void</span> setView(View view, WindowManager<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>LayoutParams attrs, View panelParentView) {        synchronized (this) {            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (mView <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">==</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">null</span>) {            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//将顶层视图DecorView赋值给全局的mView</span>                mView <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> view;            <span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//标记已添加DecorView</span>             mAdded <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">true</span>;            <span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//请求布局</span>            requestLayout();            <span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>             } }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li></ul>

该方法实现有点长，我省略了其他代码，直接看以上几行代码：

1. 将外部参数DecorView赋值给mView成员变量
2. 标记DecorView已添加到ViewRootImpl
3. 调用requestLayout方法请求布局

0-4

跟踪代码进入到 requestLayout()方法： 
Step4

<code class="hljs lasso has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;">@Override    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">void</span> requestLayout() {        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">!</span>mHandlingLayoutInLayoutRequest) {            checkThread();            mLayoutRequested <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">true</span>;            scheduleTraversals();        }    }    <span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span><span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">void</span> scheduleTraversals() {        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">!</span>mTraversalScheduled) {            mTraversalScheduled <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">true</span>;            mTraversalBarrier <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> mHandler<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>getLooper()<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>postSyncBarrier();            mChoreographer<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>postCallback(                    Choreographer<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>CALLBACK_TRAVERSAL, mTraversalRunnable, <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">null</span>);            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">!</span>mUnbufferedInputDispatch) {                scheduleConsumeBatchedInput();            }            notifyRendererOfFramePending();        }    }<span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">..</span>final class TraversalRunnable implements Runnable {        @Override        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">void</span> run() {            doTraversal();        }    }final TraversalRunnable mTraversalRunnable <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">new</span> TraversalRunnable();<span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span> <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">void</span> doTraversal() {        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (mTraversalScheduled) {            mTraversalScheduled <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">false</span>;            mHandler<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>getLooper()<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>removeSyncBarrier(mTraversalBarrier);            try {                performTraversals();            } finally {                Trace<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>traceEnd(Trace<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>TRACE_TAG_VIEW);            }        }    }<span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li><li style="box-sizing: border-box; padding: 0px 5px;">38</li><li style="box-sizing: border-box; padding: 0px 5px;">39</li><li style="box-sizing: border-box; padding: 0px 5px;">40</li><li style="box-sizing: border-box; padding: 0px 5px;">41</li><li style="box-sizing: border-box; padding: 0px 5px;">42</li><li style="box-sizing: border-box; padding: 0px 5px;">43</li><li style="box-sizing: border-box; padding: 0px 5px;">44</li><li style="box-sizing: border-box; padding: 0px 5px;">45</li><li style="box-sizing: border-box; padding: 0px 5px;">46</li><li style="box-sizing: border-box; padding: 0px 5px;">47</li><li style="box-sizing: border-box; padding: 0px 5px;">48</li><li style="box-sizing: border-box; padding: 0px 5px;">49</li><li style="box-sizing: border-box; padding: 0px 5px;">50</li></ul>

跟踪代码，最后DecorView的绘制会进入到ViewRootImpl类中的performTraversals()成员方法，这个过程可以参考上面的代码流程图。现在我们主要来分析下 ViewRootImpl类中的performTraversals（）方法。

0-5

Step5

<code class="hljs lasso has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">void</span> performTraversals() {        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// cache mView since it is used so much below...</span>        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//我们在Step3知道，mView就是DecorView根布局</span>        final View host <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> mView;        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//在Step3 成员变量mAdded赋值为true，因此条件不成立</span>        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (host <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">==</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">null</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">||</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">!</span>mAdded)            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span>;        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//是否正在遍历</span>        mIsInTraversal <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">true</span>;        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//是否马上绘制View</span>        mWillDrawSoon <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">true</span>;        <span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//顶层视图DecorView所需要窗口的宽度和高度</span>        int desiredWindowWidth;        int desiredWindowHeight;        <span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span>        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//在构造方法中mFirst已经设置为true，表示是否是第一次绘制DecorView</span>        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (mFirst) {            mFullRedrawNeeded <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">true</span>;            mLayoutRequested <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">true</span>;            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//如果窗口的类型是有状态栏的，那么顶层视图DecorView所需要窗口的宽度和高度就是除了状态栏</span>            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (lp<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">type</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">==</span> WindowManager<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>LayoutParams<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>TYPE_STATUS_BAR_PANEL                    <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">||</span> lp<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">type</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">==</span> WindowManager<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>LayoutParams<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>TYPE_INPUT_METHOD) {                <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// NOTE -- system code, won't try to do compat mode.</span>                Point size <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">new</span> Point();                mDisplay<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>getRealSize(size);                desiredWindowWidth <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> size<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>x;                desiredWindowHeight <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> size<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>y;            } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> {<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//否则顶层视图DecorView所需要窗口的宽度和高度就是整个屏幕的宽高</span>                DisplayMetrics packageMetrics <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span>                    mView<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>getContext()<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>getResources()<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>getDisplayMetrics();                desiredWindowWidth <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> packageMetrics<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>widthPixels;                desiredWindowHeight <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> packageMetrics<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>heightPixels;            }    }<span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//获得view宽高的测量规格，mWidth和mHeight表示窗口的宽高，lp.widthhe和lp.height表示DecorView根布局宽和高</span> int childWidthMeasureSpec <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> getRootMeasureSpec(mWidth, lp<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>width); int childHeightMeasureSpec <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> getRootMeasureSpec(mHeight, lp<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>height);  <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Ask host how big it wants to be</span>  <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//执行测量操作</span>  performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);<span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//执行布局操作</span> performLayout(lp, desiredWindowWidth, desiredWindowHeight);<span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">..</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//执行绘制操作</span>performDraw();}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li><li style="box-sizing: border-box; padding: 0px 5px;">38</li><li style="box-sizing: border-box; padding: 0px 5px;">39</li><li style="box-sizing: border-box; padding: 0px 5px;">40</li><li style="box-sizing: border-box; padding: 0px 5px;">41</li><li style="box-sizing: border-box; padding: 0px 5px;">42</li><li style="box-sizing: border-box; padding: 0px 5px;">43</li><li style="box-sizing: border-box; padding: 0px 5px;">44</li><li style="box-sizing: border-box; padding: 0px 5px;">45</li><li style="box-sizing: border-box; padding: 0px 5px;">46</li><li style="box-sizing: border-box; padding: 0px 5px;">47</li><li style="box-sizing: border-box; padding: 0px 5px;">48</li><li style="box-sizing: border-box; padding: 0px 5px;">49</li><li style="box-sizing: border-box; padding: 0px 5px;">50</li><li style="box-sizing: border-box; padding: 0px 5px;">51</li><li style="box-sizing: border-box; padding: 0px 5px;">52</li><li style="box-sizing: border-box; padding: 0px 5px;">53</li><li style="box-sizing: border-box; padding: 0px 5px;">54</li><li style="box-sizing: border-box; padding: 0px 5px;">55</li></ul>

该方法主要流程就体现了View绘制渲染的三个主要步骤，分别是测量，布局，绘制三个阶段。

这里先给出Android系统View的绘制流程：依次执行View类里面的如下三个方法：

1. measure(int ,int) :测量View的大小
2. layout(int ,int ,int ,int) ：设置子View的位置
3. draw(Canvas) ：绘制View内容到Canvas画布上

测量measure

1-1

从performTraversals方法我们可以看到，在执行performMeasure测量之前要通过getRootMeasureSpec方法获得顶层视图DecorView的测量规格，跟踪代码进入getRootMeasureSpec（）方法：

<code class="hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;">  <span class="hljs-javadoc" style="color: rgb(136, 0, 0); box-sizing: border-box;">/**     * Figures out the measure spec for the root view in a window based on it's     * layout params.     *     *<span class="hljs-javadoctag" style="color: rgb(102, 0, 102); box-sizing: border-box;"> @param</span> windowSize     *            The available width or height of the window     *     *<span class="hljs-javadoctag" style="color: rgb(102, 0, 102); box-sizing: border-box;"> @param</span> rootDimension     *            The layout params for one dimension (width or height) of the     *            window.     *     *<span class="hljs-javadoctag" style="color: rgb(102, 0, 102); box-sizing: border-box;"> @return</span> The measure spec to use to measure the root view.     */</span>    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> <span class="hljs-title" style="box-sizing: border-box;">getRootMeasureSpec</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> windowSize, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> rootDimension) {        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> measureSpec;        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">switch</span> (rootDimension) {        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//匹配父容器时，测量模式为MeasureSpec.EXACTLY，测量大小直接为屏幕的大小，也就是充满真个屏幕</span>        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">case</span> ViewGroup.LayoutParams.MATCH_PARENT:            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Window can't resize. Force root view to be windowSize.</span>            measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">break</span>;        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//包裹内容时，测量模式为MeasureSpec.AT_MOST，测量大小直接为屏幕大小，也就是充满真个屏幕</span>        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">case</span> ViewGroup.LayoutParams.WRAP_CONTENT:            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Window can resize. Set max size for root view.</span>            measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">break</span>;        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//其他情况时，测量模式为MeasureSpec.EXACTLY，测量大小为DecorView顶层视图布局设置的大小。</span>        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">default</span>:            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Window wants to be an exact size. Force root view to be that size.</span>            measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">break</span>;        }        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> measureSpec;    }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li></ul>

分析：该方法主要作用是在整个窗口的基础上计算出root view（顶层视图DecorView）的测量规格，该方法的两个参数分别表示：

1. windowSize：当前手机窗口的有效宽和高，一般都是除了通知栏的屏幕宽和高
2. rootDimension 根布局DecorView请求的宽和高，由前面的博客我们知道是MATCH_PARENT

由 《从setContentView方法分析Android加载布局流程》可知，我们的DecorView根布局宽和高都是MATCH_PARENT，

因此DecorView根布局的测量模式就是MeasureSpec.EXACTLY，测量大小一般都是整个屏幕大小，所以一般我们的Activity

窗口都是全屏的。因此上面代码走第一个分支，通过调用MeasureSpec.makeMeasureSpec方法将

DecorView的测量模式和测量大小封装成DecorView的测量规格。

1-2

由于performMeasure（）方法调用了 View中measure（）方法俩进行测量，并且DecorView（继承自FrameLayout）的父类是

ViewGroup，祖父类是View。因此我们从View的成员函数measure开始分析整个测量过程。

这个过程分为 3 步，我们来一一分析。

Step1

<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;">    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> mOldWidthMeasureSpec = Integer.MIN_VALUE;    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> mOldHeightMeasureSpec = Integer.MIN_VALUE; <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> final <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">measure</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> widthMeasureSpec, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> heightMeasureSpec) {        ..................        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//如果上一次的测量规格和这次不一样，则条件满足，重新测量视图View的大小</span>        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ((mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ||                widthMeasureSpec != mOldWidthMeasureSpec ||                heightMeasureSpec != mOldHeightMeasureSpec) {            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// first clears the measured dimension flag</span>            mPrivateFlags &= ~PFLAG_MEASURED_DIMENSION_SET;            resolveRtlPropertiesIfNeeded();            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> cacheIndex = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ? -<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span> :                    mMeasureCache.indexOfKey(key);            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (cacheIndex < <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span> || sIgnoreMeasureCache) {                <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// measure ourselves, this should set the measured dimension flag back</span>                onMeasure(widthMeasureSpec, heightMeasureSpec);                mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;            } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> {                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">long</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">value</span> = mMeasureCache.valueAt(cacheIndex);                <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Casting a long to int drops the high 32 bits, no mask needed</span>                setMeasuredDimensionRaw((<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span>) (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">value</span> >> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">32</span>), (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span>) <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">value</span>);                mPrivateFlags3 |= PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;            }            mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;        }        mOldWidthMeasureSpec = widthMeasureSpec;        mOldHeightMeasureSpec = heightMeasureSpec;    }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li><li style="box-sizing: border-box; padding: 0px 5px;">38</li></ul>

分析： 
1.代码第10行：判断当前视图View是否需要重新测量，当上一次视图View测量的规格和本次视图View测量规格不一样时，就说明视图View的大小有改变，因此需要重新测量。

2.代码第23行：调用了onMeasure方法进行测量，说明View主要的测量逻辑是在该方法中实现。

3.代码第35-36行：保存本次视图View的测量规格到mOldWidthMeasureSpec和mOldHeightMeasureSpec以便下次测量条件的判断是否需要重新测量。

1-3

跟踪代码，进入View类的 onMeasure方法

<code class="hljs coffeescript has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"> /**     * <p>     * Measure the view <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">and</span> its content to determine the measured width <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">and</span> the     * measured height. This method <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">is</span> invoked <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">by</span> {<span class="hljs-property" style="box-sizing: border-box;">@link</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">#measure(int, int)} and</span>     * should be overriden <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">by</span> subclasses to provide accurate <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">and</span> efficient     * measurement <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">of</span> their contents.     * </p>     *     * <p>     * <strong><span class="hljs-attribute" style="box-sizing: border-box; color: rgb(0, 136, 0);">CONTRACT</span>:</strong> When overriding <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span> method, you     * <em>must</em> call {<span class="hljs-property" style="box-sizing: border-box;">@link</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">#setMeasuredDimension(int, int)} to store the</span>     * measured width <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">and</span> height <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">of</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span> view. Failure to <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">do</span> so will trigger an     * <code>IllegalStateException</code>, thrown <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">by</span>     * {<span class="hljs-property" style="box-sizing: border-box;">@link</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">#measure(int, int)}. Calling the superclass'</span>     * {<span class="hljs-property" style="box-sizing: border-box;">@link</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">#onMeasure(int, int)} is a valid use.</span>     * </p>     *     * <p>     * The base <span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">implementation</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">of</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">measure</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">defaults</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">to</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">the</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">background</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">size</span>,</span>     * <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">unless</span> a larger size <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">is</span> allowed <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">by</span> the MeasureSpec. Subclasses should     * override {<span class="hljs-property" style="box-sizing: border-box;">@link</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">#onMeasure(int, int)} to provide better measurements of</span>     * their content.     * </p>     *     * <p>     * If <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span> method <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">is</span> overridden, it <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">is</span> the subclass<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'s responsibility to make     * sure the measured height and width are at least the view'</span>s minimum height     * <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">and</span> width ({<span class="hljs-property" style="box-sizing: border-box;">@link</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">#getSuggestedMinimumHeight()} and</span>     * {<span class="hljs-property" style="box-sizing: border-box;">@link</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">#getSuggestedMinimumWidth()}).</span>     * </p>     *     * <span class="hljs-property" style="box-sizing: border-box;">@param</span> widthMeasureSpec horizontal space requirements as imposed <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">by</span> the parent.     *                         The requirements are encoded <span class="hljs-reserved" style="box-sizing: border-box;">with</span>     *                         {<span class="hljs-property" style="box-sizing: border-box;">@link</span> android.view.View.MeasureSpec}.     * <span class="hljs-property" style="box-sizing: border-box;">@param</span> heightMeasureSpec vertical space requirements as imposed <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">by</span> the parent.     *                         The requirements are encoded <span class="hljs-reserved" style="box-sizing: border-box;">with</span>     *                         {<span class="hljs-property" style="box-sizing: border-box;">@link</span> android.view.View.MeasureSpec}.     *     * <span class="hljs-property" style="box-sizing: border-box;">@see</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">#getMeasuredWidth()</span>     * <span class="hljs-property" style="box-sizing: border-box;">@see</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">#getMeasuredHeight()</span>     * <span class="hljs-property" style="box-sizing: border-box;">@see</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">#setMeasuredDimension(int, int)</span>     * <span class="hljs-property" style="box-sizing: border-box;">@see</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">#getSuggestedMinimumHeight()</span>     * <span class="hljs-property" style="box-sizing: border-box;">@see</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">#getSuggestedMinimumWidth()</span>     * <span class="hljs-property" style="box-sizing: border-box;">@see</span> android.view.View.MeasureSpec<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">#getMode(int)</span>     * <span class="hljs-property" style="box-sizing: border-box;">@see</span> android.view.View.MeasureSpec<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">#getSize(int)</span>     */    protected <span class="hljs-reserved" style="box-sizing: border-box;">void</span> onMeasure(int widthMeasureSpec, int heightMeasureSpec) {        setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),                getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));    }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li><li style="box-sizing: border-box; padding: 0px 5px;">38</li><li style="box-sizing: border-box; padding: 0px 5px;">39</li><li style="box-sizing: border-box; padding: 0px 5px;">40</li><li style="box-sizing: border-box; padding: 0px 5px;">41</li><li style="box-sizing: border-box; padding: 0px 5px;">42</li><li style="box-sizing: border-box; padding: 0px 5px;">43</li><li style="box-sizing: border-box; padding: 0px 5px;">44</li><li style="box-sizing: border-box; padding: 0px 5px;">45</li><li style="box-sizing: border-box; padding: 0px 5px;">46</li><li style="box-sizing: border-box; padding: 0px 5px;">47</li><li style="box-sizing: border-box; padding: 0px 5px;">48</li><li style="box-sizing: border-box; padding: 0px 5px;">49</li><li style="box-sizing: border-box; padding: 0px 5px;">50</li><li style="box-sizing: border-box; padding: 0px 5px;">51</li></ul>

分析： 
该方法的实现也很简单，直接调用setMeasuredDimension方法完成视图View的测量。我们知道，Android中所有的视图组件都是继承自View实现的。因此该方法提供了一个默认测量视图View大小的实现。

1-4

言外之意，如果你不想你自己的View使用默认实现来测量View的宽高的话，你可以在子类中重写onMeasure方法来自定义测量方法。我们先来看看默认测量宽高的实现。跟踪代码进入getDefaultSize方法：

<code class="hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"> <span class="hljs-javadoc" style="color: rgb(136, 0, 0); box-sizing: border-box;">/**     * Utility to return a default size. Uses the supplied size if the     * MeasureSpec imposed no constraints. Will get larger if allowed     * by the MeasureSpec.     *     *<span class="hljs-javadoctag" style="color: rgb(102, 0, 102); box-sizing: border-box;"> @param</span> size Default size for this view     *<span class="hljs-javadoctag" style="color: rgb(102, 0, 102); box-sizing: border-box;"> @param</span> measureSpec Constraints imposed by the parent     *<span class="hljs-javadoctag" style="color: rgb(102, 0, 102); box-sizing: border-box;"> @return</span> The size this view should be.     */</span>    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> <span class="hljs-title" style="box-sizing: border-box;">getDefaultSize</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> size, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> measureSpec) {        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> result = size;        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//获得测量模式</span>        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> specMode = MeasureSpec.getMode(measureSpec);        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//获得父亲容器留给子视图View的大小</span>        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> specSize = MeasureSpec.getSize(measureSpec);        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">switch</span> (specMode) {        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">case</span> MeasureSpec.UNSPECIFIED:            result = size;            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">break</span>;        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">case</span> MeasureSpec.AT_MOST:        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">case</span> MeasureSpec.EXACTLY:            result = specSize;            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">break</span>;        }        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> result;    }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li></ul>

分析：该方法的作用是根据View布局设置的宽高和父View传递的测量规格重新计算View的测量宽高。由此可以知道，我们布局的

子View最终的大小是由布局大小和父容器的测量规格共同决定的。如果自定义View你没有重写onMeasure使用系统默认方法的

话，测量模式MeasureSpec.AT_MOST和MeasureSpec.EXACTLY下的测量大小是一样的。我们来总结一下测量模式的种类：

1. MeasureSpec.EXACTLY：确定模式，父容器希望子视图View的大小是固定，也就是specSize大小。
2. MeasureSpec.AT_MOST：最大模式，父容器希望子视图View的大小不超过父容器希望的大小，也就是不超过specSize大小。
3. MeasureSpec.UNSPECIFIED: 不确定模式，子视图View请求多大就是多大，父容器不限制其大小范围，也就是size大小。

从上面代码可以看出，当测量模式是MeasureSpec.UNSPECIFIED时，View的测量值为size，当测量模式为

MeasureSpec.AT_MOST或者case MeasureSpec.EXACTLY时，View的测量值为specSize。我们知道，specSize是由父容器决

定，那么size是怎么计算出来的呢？getDefaultSize方法的第一个参数是调用getSuggestedMinimumWidth方法获得。进入getSuggestedMinimumWidth方法看看实现：

<code class="hljs applescript has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;">/**     * Returns <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">the</span> suggested minimum width <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">that</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">the</span> view should use. This     * returns <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">the</span> maximum <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">of</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">the</span> view's minimum width)     * <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">and</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">the</span> background's minimum width     *  ({@link android.graphics.drawable.Drawable<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">#getMinimumWidth()}).</span>     * <p>     * When being used <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">in</span> {@link <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">#onMeasure(int, int)}, the caller should still</span>     * ensure <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">the</span> returned width <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">is</span> within <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">the</span> requirements <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">of</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">the</span> parent.     *     * @<span class="hljs-constant" style="box-sizing: border-box;">return</span> The suggested minimum width <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">of</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">the</span> view.     */    protected int getSuggestedMinimumWidth() {<span class="hljs-command" style="box-sizing: border-box;">        return</span> (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth());    }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li></ul>

原来size大小是获取View属性当中的最小值，也就是 android:minWidth和 android:minHeight的值，前提是View没有设置背景属性。否则就在最小值和背景的最小值中间取最大值。

sizeSpec大小是有父容器决定的，我们由 1-1节知道父容器DecorView的测量模式是MeasureSpec.EXACTLY，测量大小sizeSpec是整个屏幕的大小。

setp2 
而DecorView是继承自FrameLayout的，那么我们来看看FrameLayout类中的onMeasure方法的实现

<code class="hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"> <span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">protected</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">onMeasure</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> widthMeasureSpec, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> heightMeasureSpec) {        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> count = getChildCount();        ..............        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> maxHeight = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> maxWidth = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> childState = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> i = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>; i < count; i++) {            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> View child = getChildAt(i);            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (mMeasureAllChildren || child.getVisibility() != GONE) {                <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//测量FrameLayout下每个子视图View的宽和高</span>                measureChildWithMargins(child, widthMeasureSpec, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>, heightMeasureSpec, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>);                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> LayoutParams lp = (LayoutParams) child.getLayoutParams();                maxWidth = Math.max(maxWidth,                        child.getMeasuredWidth() + lp.leftMargin + lp.rightMargin);                maxHeight = Math.max(maxHeight,                        child.getMeasuredHeight() + lp.topMargin + lp.bottomMargin);                childState = combineMeasuredStates(childState, child.getMeasuredState());                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (measureMatchParentChildren) {                    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (lp.width == LayoutParams.MATCH_PARENT ||                            lp.height == LayoutParams.MATCH_PARENT) {                        mMatchParentChildren.add(child);                    }                }            }        }        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Account for padding too</span>        maxWidth += getPaddingLeftWithForeground() + getPaddingRightWithForeground();        maxHeight += getPaddingTopWithForeground() + getPaddingBottomWithForeground();        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Check against our minimum height and width</span>        maxHeight = Math.max(maxHeight, getSuggestedMinimumHeight());        maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Check against our foreground's minimum height and width</span>        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> Drawable drawable = getForeground();        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (drawable != <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>) {            maxHeight = Math.max(maxHeight, drawable.getMinimumHeight());            maxWidth = Math.max(maxWidth, drawable.getMinimumWidth());        }        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//设置当前FrameLayout测量结果，此方法的调用表示当前View测量的结束。</span>        setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),                resolveSizeAndState(maxHeight, heightMeasureSpec,                        childState << MEASURED_HEIGHT_STATE_SHIFT));}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li><li style="box-sizing: border-box; padding: 0px 5px;">38</li><li style="box-sizing: border-box; padding: 0px 5px;">39</li><li style="box-sizing: border-box; padding: 0px 5px;">40</li><li style="box-sizing: border-box; padding: 0px 5px;">41</li><li style="box-sizing: border-box; padding: 0px 5px;">42</li><li style="box-sizing: border-box; padding: 0px 5px;">43</li><li style="box-sizing: border-box; padding: 0px 5px;">44</li><li style="box-sizing: border-box; padding: 0px 5px;">45</li><li style="box-sizing: border-box; padding: 0px 5px;">46</li><li style="box-sizing: border-box; padding: 0px 5px;">47</li></ul>

分析：由以上代码发现，ViewGroup测量结果都是带边距的，代码第9-27行就是遍历测量FrameLayout下子视图View的大小了。

代码第44行，最后调用setMeasuredDimension方法设置当前View的测量结果，此方法的调用表示当前View测量结束。

那么我们来分析下代码第12行measureChildWithMargins方法测量FrameLayout下的子视图View的大小，跟踪源码：

Step3： 
由于FrameLayout父类是ViewGroup，measureChildWithMargins方法在ViewGroup下

<code class="hljs applescript has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;">/**     * Ask one <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">of</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">the</span> children <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">of</span> this view <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">to</span> measure itself, taking <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">into</span>     * account both <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">the</span> MeasureSpec requirements <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> this view <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">and</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">its</span> padding     * <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">and</span> margins. The child must have MarginLayoutParams The heavy lifting <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">is</span>     * done <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">in</span> getChildMeasureSpec.     *     * @param child The child <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">to</span> measure     * @param parentWidthMeasureSpec The width requirements <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> this view     * @param widthUsed Extra <span class="hljs-constant" style="box-sizing: border-box;">space</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">that</span> has been used up <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">by</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">the</span> parent     *        horizontally (possibly <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">by</span> other children <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">of</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">the</span> parent)     * @param parentHeightMeasureSpec The height requirements <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> this view     * @param heightUsed Extra <span class="hljs-constant" style="box-sizing: border-box;">space</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">that</span> has been used up <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">by</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">the</span> parent     *        vertically (possibly <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">by</span> other children <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">of</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">the</span> parent)     */    protected void measureChildWithMargins(View child,            int parentWidthMeasureSpec, int widthUsed,            int parentHeightMeasureSpec, int heightUsed) {        final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();        final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,                mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin                        + widthUsed, lp.width);        final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,                mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin                        + heightUsed, lp.height);        child.measure(childWidthMeasureSpec, childHeightMeasureSpec);    }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li></ul>

分析：该方法中调用getChildMeasureSpec方法来获得ViewGroup下的子视图View的测量规格。然后将测量规格最为参数传递给

View的measure方法，最终完成所有子视图View的测量。来看看这里是怎么获得子视图View的测量规格的，进入getChildMeasureSpec方法：

<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> <span class="hljs-title" style="box-sizing: border-box;">getChildMeasureSpec</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> spec, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> padding, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> childDimension) {        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> specMode = MeasureSpec.getMode(spec);        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> specSize = MeasureSpec.getSize(spec);        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> size = Math.max(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>, specSize - padding);        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> resultSize = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> resultMode = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">switch</span> (specMode) {        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Parent has imposed an exact size on us</span>        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">case</span> MeasureSpec.EXACTLY:            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (childDimension >= <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>) {                resultSize = childDimension;                resultMode = MeasureSpec.EXACTLY;            } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (childDimension == LayoutParams.MATCH_PARENT) {                <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Child wants to be our size. So be it.</span>                resultSize = size;                resultMode = MeasureSpec.EXACTLY;            } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (childDimension == LayoutParams.WRAP_CONTENT) {                <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Child wants to determine its own size. It can't be</span>                <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// bigger than us.</span>                resultSize = size;                resultMode = MeasureSpec.AT_MOST;            }            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">break</span>;       ...........        }        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> MeasureSpec.makeMeasureSpec(resultSize, resultMode);    }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li></ul>

分析：由1-1节我们知道根布局DecorView的测量规格中的测量模式是MeasureSpec.EXACTLY，测量大小是整个窗口大小。因此上面代码分支走MeasureSpec.EXACTLY。子视图View的测量规格由其宽和高参数决定。

1. 当DecorView根布局的子视图View宽高为一个确定值childDimension时，该View的测量模式为MeasureSpec.EXACTLY，测量大小就是childDimension。
2. 当子视图View宽高为MATCH_PARENT时，该View的测量模式为MeasureSpec.EXACTLY，测量大小是父容器DecorView规定的大小，为整个屏幕大小MATCH_PARENT。
3. 当子视图View宽高为WRAP_CONTENT时，该View的测量模式为MeasureSpec.AT_MOST，测量大小是父容器DecorView规定的大小，为整个屏幕大小MATCH_PARENT。

这里我们来验证一下以上的结论，目的是进一步理解 View的几种测量模式和View的测量规格。

1.定义一个布局activity_main.xml如下：

<code class="hljs avrasm has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">com</span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.xjp</span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.layoutdemo</span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.MyView</span>    xmlns:android=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"http://schemas.android.com/apk/res/android"</span>    android:layout_width=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"wrap_content"</span>    android:layout_height=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"wrap_content"</span>    android:text=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Button"</span>    android:gravity=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"start"</span>/></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li></ul>

这个布局很简单，直接将自定义的MyView作为Activity的内容布局。 
2.自定义MyView代码如下：

<code class="hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">MyView</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">extends</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">View</span> {</span>    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> String TAG = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"MyCustomView"</span>;    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> String titleText = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Hello world"</span>;    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> titleColor = Color.BLACK;    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> titleBackgroundColor = Color.RED;    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> titleSize = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">16</span>;    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> Paint mPaint;    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> Rect mBound;    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-title" style="box-sizing: border-box;">MyView</span>(Context context) {        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>(context, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>);    }    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-title" style="box-sizing: border-box;">MyView</span>(Context context, AttributeSet attrs) {        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>(context, attrs, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>);    }    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-title" style="box-sizing: border-box;">MyView</span>(Context context, AttributeSet attrs, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> defStyleAttr) {        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">super</span>(context, attrs, defStyleAttr);        init();    }    <span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">protected</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">onMeasure</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> widthMeasureSpec, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> heightMeasureSpec) {        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> specMode = MeasureSpec.getMode(widthMeasureSpec);        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> specSize = MeasureSpec.getSize(widthMeasureSpec);        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">switch</span> (specMode) {            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">case</span> MeasureSpec.UNSPECIFIED:                Log.e(TAG, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"UNSPECIFIED....."</span>);                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">break</span>;            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">case</span> MeasureSpec.AT_MOST:                Log.e(TAG, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"AT_MOST....."</span>);                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">break</span>;            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">case</span> MeasureSpec.EXACTLY:                Log.e(TAG, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"EXACTLY....."</span>);                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">break</span>;        }        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">super</span>.onMeasure(widthMeasureSpec, heightMeasureSpec);    }    <span class="hljs-javadoc" style="color: rgb(136, 0, 0); box-sizing: border-box;">/**     * 初始化     */</span>    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">init</span>() {        mPaint = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Paint(Paint.ANTI_ALIAS_FLAG);        mPaint.setTextSize(titleSize);        <span class="hljs-javadoc" style="color: rgb(136, 0, 0); box-sizing: border-box;">/**         * 得到自定义View的titleText内容的宽和高         */</span>        mBound = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Rect();        mPaint.getTextBounds(titleText, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>, titleText.length(), mBound);    }    <span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">protected</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">onDraw</span>(Canvas canvas) {        mPaint.setColor(titleBackgroundColor);        canvas.drawCircle(getWidth() / <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>f, getWidth() / <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>f, getWidth() / <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>f, mPaint);        mPaint.setColor(titleColor);        canvas.drawText(titleText, getWidth() / <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span> - mBound.width() / <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>, getHeight() / <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span> + mBound.height() / <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>, mPaint);    }}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li><li style="box-sizing: border-box; padding: 0px 5px;">38</li><li style="box-sizing: border-box; padding: 0px 5px;">39</li><li style="box-sizing: border-box; padding: 0px 5px;">40</li><li style="box-sizing: border-box; padding: 0px 5px;">41</li><li style="box-sizing: border-box; padding: 0px 5px;">42</li><li style="box-sizing: border-box; padding: 0px 5px;">43</li><li style="box-sizing: border-box; padding: 0px 5px;">44</li><li style="box-sizing: border-box; padding: 0px 5px;">45</li><li style="box-sizing: border-box; padding: 0px 5px;">46</li><li style="box-sizing: border-box; padding: 0px 5px;">47</li><li style="box-sizing: border-box; padding: 0px 5px;">48</li><li style="box-sizing: border-box; padding: 0px 5px;">49</li><li style="box-sizing: border-box; padding: 0px 5px;">50</li><li style="box-sizing: border-box; padding: 0px 5px;">51</li><li style="box-sizing: border-box; padding: 0px 5px;">52</li><li style="box-sizing: border-box; padding: 0px 5px;">53</li><li style="box-sizing: border-box; padding: 0px 5px;">54</li><li style="box-sizing: border-box; padding: 0px 5px;">55</li><li style="box-sizing: border-box; padding: 0px 5px;">56</li><li style="box-sizing: border-box; padding: 0px 5px;">57</li><li style="box-sizing: border-box; padding: 0px 5px;">58</li><li style="box-sizing: border-box; padding: 0px 5px;">59</li><li style="box-sizing: border-box; padding: 0px 5px;">60</li><li style="box-sizing: border-box; padding: 0px 5px;">61</li><li style="box-sizing: border-box; padding: 0px 5px;">62</li><li style="box-sizing: border-box; padding: 0px 5px;">63</li><li style="box-sizing: border-box; padding: 0px 5px;">64</li><li style="box-sizing: border-box; padding: 0px 5px;">65</li><li style="box-sizing: border-box; padding: 0px 5px;">66</li></ul>

 
自定义的MyView也很简单，仅仅重写了onDraw方法，onMeasure方法调用父类方法。代码运行之后你会发现，

1.布局中设置的MyView大小是wrap_content包裹内容的，但是View视图却充满整个屏幕。看打印发现当前的测量模式是MeasureSpec.AT_MOST。

2.当MyView大小是match_parent填满父容器时，View视图也是充满整个屏幕，看打印发现测量模式是MeasureSpec.EXACTLY。

3.当MyView大小是固定值，比如是1200dp和1200dp时，View视图是超出整个屏幕的。 

原因是此处的Activity内容布局的父容器也是一个id为content的FrameLayout布局。这里就不解释以上三种情况的原因了，参考Stpe3解释的很详细了。

至此，整个View树型结构的布局测量流程可以归纳如下：

measure总结

1. View的measure方法是final类型的，子类不可以重写，子类可以通过重写onMeasure方法来测量自己的大小，当然也可以不重写onMeasure方法使用系统默认测量大小。
2. View测量结束的标志是调用了View类中的setMeasuredDimension成员方法，言外之意是，如果你需要在自定义的View中重写onMeasure方法，在你测量结束之前你必须调用setMeasuredDimension方法测量才有效。
3. 在Activity生命周期onCreate和onResume方法中调用View.getWidth()和View.getMeasuredHeight()返回值为0的，是因为当前View的测量还没有开始，这里关系到Activity启动过程，文章开头说了当ActivityThread类中的performResumeActivity方法执行之后才将DecorView添加到PhoneWindow窗口上，开始测量。在Activity生命周期onCreate在中performResumeActivity还为执行，因此调用View.getMeasuredHeight(）返回值为0。
4. 子视图View的大小是由父容器View和子视图View布局共同决定的。

布局Layout

由 0-5节可知，View视图绘制流程中的布局layout是由ViewRootImpl中的performLayout成员方法开始的，看源码：

2-1

<code class="hljs lasso has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">void</span> performLayout(WindowManager<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>LayoutParams lp, int desiredWindowWidth,            int desiredWindowHeight) {        <span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span>        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//标记当前开始布局</span>        mInLayout <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">true</span>;        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//mView就是DecorView</span>        final View host <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> mView;        <span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span>        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//DecorView请求布局</span>        host<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>layout(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>, host<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>getMeasuredWidth(), host<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>getMeasuredHeight());        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//标记布局结束</span>        mInLayout <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">false</span>;        <span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span>}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li></ul>

分析： 
代码第10行发现，DecorView的四个位置左=0，顶=0，右=屏幕宽，底=屏幕宽，说明DecorView布局的位置是从屏幕最左最顶端开始布局，到屏幕最低最右结束。因此DecorView根布局是充满整个屏幕的。

该方法主要调用了View类的layout方法，跟踪代码进入View类的layout方法瞧瞧吧

2-2

<code class="hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-javadoc" style="color: rgb(136, 0, 0); box-sizing: border-box;">/**     * Assign a size and position to a view and all of its     * descendants     *     * <p>This is the second phase of the layout mechanism.     * (The first is measuring). In this phase, each parent calls     * layout on all of its children to position them.     * This is typically done using the child measurements     * that were stored in the measure pass().</p>     *     * <p>Derived classes should not override this method.     * Derived classes with children should override     * onLayout. In that method, they should     * call layout on each of their children.</p>     *     *<span class="hljs-javadoctag" style="color: rgb(102, 0, 102); box-sizing: border-box;"> @param</span> l Left position, relative to parent     *<span class="hljs-javadoctag" style="color: rgb(102, 0, 102); box-sizing: border-box;"> @param</span> t Top position, relative to parent     *<span class="hljs-javadoctag" style="color: rgb(102, 0, 102); box-sizing: border-box;"> @param</span> r Right position, relative to parent     *<span class="hljs-javadoctag" style="color: rgb(102, 0, 102); box-sizing: border-box;"> @param</span> b Bottom position, relative to parent     */</span>    <span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@SuppressWarnings</span>({<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"unchecked"</span>})    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">layout</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> l, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> t, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> r, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> b) {        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//判断是否需要重新测量</span>        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ((mPrivateFlags3 & PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT) != <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>) {            onMeasure(mOldWidthMeasureSpec, mOldHeightMeasureSpec);            mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;        }        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//保存上一次View的四个位置</span>        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> oldL = mLeft;        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> oldT = mTop;        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> oldB = mBottom;        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> oldR = mRight;        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//设置当前视图View的左，顶，右，底的位置，并且判断布局是否有改变</span>        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">boolean</span> changed = isLayoutModeOptical(mParent) ?                setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//如果布局有改变，条件成立，则视图View重新布局</span>            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//调用onLayout，将具体布局逻辑留给子类实现</span>            onLayout(changed, l, t, r, b);            mPrivateFlags &= ~PFLAG_LAYOUT_REQUIRED;            ListenerInfo li = mListenerInfo;            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (li != <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span> && li.mOnLayoutChangeListeners != <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>) {                ArrayList<OnLayoutChangeListener> listenersCopy =                        (ArrayList<OnLayoutChangeListener>)li.mOnLayoutChangeListeners.clone();                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> numListeners = listenersCopy.size();                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> i = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>; i < numListeners; ++i) {                    listenersCopy.get(i).onLayoutChange(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>, l, t, r, b, oldL, oldT, oldR, oldB);                }            }        }        mPrivateFlags &= ~PFLAG_FORCE_LAYOUT;        mPrivateFlags3 |= PFLAG3_IS_LAID_OUT;    }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li><li style="box-sizing: border-box; padding: 0px 5px;">38</li><li style="box-sizing: border-box; padding: 0px 5px;">39</li><li style="box-sizing: border-box; padding: 0px 5px;">40</li><li style="box-sizing: border-box; padding: 0px 5px;">41</li><li style="box-sizing: border-box; padding: 0px 5px;">42</li><li style="box-sizing: border-box; padding: 0px 5px;">43</li><li style="box-sizing: border-box; padding: 0px 5px;">44</li><li style="box-sizing: border-box; padding: 0px 5px;">45</li><li style="box-sizing: border-box; padding: 0px 5px;">46</li><li style="box-sizing: border-box; padding: 0px 5px;">47</li><li style="box-sizing: border-box; padding: 0px 5px;">48</li><li style="box-sizing: border-box; padding: 0px 5px;">49</li><li style="box-sizing: border-box; padding: 0px 5px;">50</li><li style="box-sizing: border-box; padding: 0px 5px;">51</li><li style="box-sizing: border-box; padding: 0px 5px;">52</li><li style="box-sizing: border-box; padding: 0px 5px;">53</li><li style="box-sizing: border-box; padding: 0px 5px;">54</li><li style="box-sizing: border-box; padding: 0px 5px;">55</li><li style="box-sizing: border-box; padding: 0px 5px;">56</li></ul>

分析： 
1.代码第23-32行保存本次布局的四个位置，用于布局变化的监听事件，如果用户设置了布局变化的监听事件，则代码第43-50就会执行设置监听事件。

2.代码第34-35行设置当前View的布局位置，也就是当调用了setFrame(l, t, r, b)方法之后，当前View布局基本完成，既然这样为什么还要第39行 onLayout方法呢？稍后解答，这里来分析一下setFrame是怎么设置当前View的布局位置的。

进入setFrame方法

2-3

<code class="hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-javadoc" style="color: rgb(136, 0, 0); box-sizing: border-box;">/**     * Assign a size and position to this view.     *     * This is called from layout.     *     *<span class="hljs-javadoctag" style="color: rgb(102, 0, 102); box-sizing: border-box;"> @param</span> left Left position, relative to parent     *<span class="hljs-javadoctag" style="color: rgb(102, 0, 102); box-sizing: border-box;"> @param</span> top Top position, relative to parent     *<span class="hljs-javadoctag" style="color: rgb(102, 0, 102); box-sizing: border-box;"> @param</span> right Right position, relative to parent     *<span class="hljs-javadoctag" style="color: rgb(102, 0, 102); box-sizing: border-box;"> @param</span> bottom Bottom position, relative to parent     *<span class="hljs-javadoctag" style="color: rgb(102, 0, 102); box-sizing: border-box;"> @return</span> true if the new size and position are different than the     *         previous ones     * {@hide}     */</span>    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">protected</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">boolean</span> <span class="hljs-title" style="box-sizing: border-box;">setFrame</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> left, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> top, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> right, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> bottom) {        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">boolean</span> changed = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">false</span>;        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//当上，下，左，右四个位置有一个和上次的值不一样都会重新布局</span>        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (mLeft != left || mRight != right || mTop != top || mBottom != bottom) {            changed = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">true</span>;            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Remember our drawn bit</span>            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> drawn = mPrivateFlags & PFLAG_DRAWN;            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//得到本次和上次的宽和高</span>            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> oldWidth = mRight - mLeft;            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> oldHeight = mBottom - mTop;            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> newWidth = right - left;            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> newHeight = bottom - top;            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//判断本次View的宽高和上次View的宽高是否相等</span>            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">boolean</span> sizeChanged = (newWidth != oldWidth) || (newHeight != oldHeight);            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Invalidate our old position</span>            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//清楚上次布局的位置</span>            invalidate(sizeChanged);            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//保存当前View的最新位置</span>            mLeft = left;            mTop = top;            mRight = right;            mBottom = bottom;            mRenderNode.setLeftTopRightBottom(mLeft, mTop, mRight, mBottom);            mPrivateFlags |= PFLAG_HAS_BOUNDS;            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//如果当前View的尺寸有所变化</span>            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (sizeChanged) {                sizeChange(newWidth, newHeight, oldWidth, oldHeight);            }            ...............        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> changed;    }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li><li style="box-sizing: border-box; padding: 0px 5px;">38</li><li style="box-sizing: border-box; padding: 0px 5px;">39</li><li style="box-sizing: border-box; padding: 0px 5px;">40</li><li style="box-sizing: border-box; padding: 0px 5px;">41</li><li style="box-sizing: border-box; padding: 0px 5px;">42</li><li style="box-sizing: border-box; padding: 0px 5px;">43</li><li style="box-sizing: border-box; padding: 0px 5px;">44</li><li style="box-sizing: border-box; padding: 0px 5px;">45</li><li style="box-sizing: border-box; padding: 0px 5px;">46</li><li style="box-sizing: border-box; padding: 0px 5px;">47</li><li style="box-sizing: border-box; padding: 0px 5px;">48</li><li style="box-sizing: border-box; padding: 0px 5px;">49</li><li style="box-sizing: border-box; padding: 0px 5px;">50</li></ul>

分析： 
1.代码第17行，如果当前View视图的最新位置和上一次不一样时，则View会重新布局。

2.代码第32-38行，保存当前View的最新位置，到此当前View的布局基本结束。从这里我们可以看到，四个全局变量 mLeft，mTop，mRight，mBottom在此刻赋值，联想我们平时使用的View.getWidth（）方法获得View的宽高，你可以发现，其实View.getWidth()方法的实现如下：

<code class="hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> <span class="hljs-title" style="box-sizing: border-box;">getWidth</span>() {        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> mRight - mLeft;    } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> <span class="hljs-title" style="box-sizing: border-box;">getHeight</span>() {        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> mBottom - mTop;    }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li></ul>

也就是说，以上两个方法是获得View布局时候的宽高，因此，我们只有在View 布局完之后调用getWidth才能真正获取到大于0的值。

2-4

细心的你会发现，既然2-3小节调用了setFrame方法，也就是当前View的布局结束了，那么View中的onLayout方法又是干嘛的呢？进入onLayout方法：

<code class="hljs vbscript has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;">/**     * Called from layout when this view should     * assign a size <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">and</span> position <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">to</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">each</span> of its children.     *     * <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">Derived</span> classes <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">with</span> children should override     * this method <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">and</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">call</span> layout <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">on</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">each</span> of     * their children.     * @param changed This <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">is</span> a <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> size <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">or</span> position <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> this view     * @param <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">left</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">Left</span> position, relative <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">to</span> parent     * @param top Top position, relative <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">to</span> parent     * @param <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">right</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">Right</span> position, relative <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">to</span> parent     * @param bottom Bottom position, relative <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">to</span> parent     */    protected void onLayout(boolean changed, <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">int</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">left</span>, <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">int</span> top, <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">int</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">right</span>, <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">int</span> bottom) {    }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li></ul>

分析：原来这是一个空方法，既然是空方法，那么该方法的实现应该在子类中。前面分析过，DecorView是继承自FrameLayout的，那么进入FarmeLayout类中看看 onLayout方法的实现吧：

<code class="hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;">  * {<span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@inheritDoc</span>}     */    <span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">protected</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">onLayout</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">boolean</span> changed, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> left, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> top, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> right, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> bottom) {        layoutChildren(left, top, right, bottom, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">false</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">/* no force left gravity */</span>);    }    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> layoutChildren(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> left, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> top, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> right, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> bottom,                                  <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">boolean</span> forceLeftGravity) {        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> count = getChildCount();        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> parentLeft = getPaddingLeftWithForeground();        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> parentRight = right - left - getPaddingRightWithForeground();        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> parentTop = getPaddingTopWithForeground();        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> parentBottom = bottom - top - getPaddingBottomWithForeground();        mForegroundBoundsChanged = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">true</span>;        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//遍历当前FrameLayout下的子View</span>        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> i = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>; i < count; i++) {            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> View child = getChildAt(i);            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//当子视图View可见度设置为GONE时，不进行当前子视图View的布局，这就是为什么当你布局中使用Visibility=GONE时，该view是不占据空间的。</span>            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (child.getVisibility() != GONE) {                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> LayoutParams lp = (LayoutParams) child.getLayoutParams();                <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//获得子视图View的宽高</span>                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> width = child.getMeasuredWidth();                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> height = child.getMeasuredHeight();                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> childLeft;                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> childTop;                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> gravity = lp.gravity;                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (gravity == -<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>) {                    gravity = DEFAULT_CHILD_GRAVITY;                }                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> layoutDirection = getLayoutDirection();                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> verticalGravity = gravity & Gravity.VERTICAL_GRAVITY_MASK;                <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//一下代码获得子视图View的四个位置，用于子视图View布局。</span>                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">switch</span> (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {                    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">case</span> Gravity.CENTER_HORIZONTAL:                        childLeft = parentLeft + (parentRight - parentLeft - width) / <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span> +                        lp.leftMargin - lp.rightMargin;                        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">break</span>;                    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">case</span> Gravity.RIGHT:                        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (!forceLeftGravity) {                            childLeft = parentRight - width - lp.rightMargin;                            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">break</span>;                        }                    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">case</span> Gravity.LEFT:                    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">default</span>:                        childLeft = parentLeft + lp.leftMargin;                }                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">switch</span> (verticalGravity) {                    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">case</span> Gravity.TOP:                        childTop = parentTop + lp.topMargin;                        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">break</span>;                    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">case</span> Gravity.CENTER_VERTICAL:                        childTop = parentTop + (parentBottom - parentTop - height) / <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span> +                        lp.topMargin - lp.bottomMargin;                        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">break</span>;                    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">case</span> Gravity.BOTTOM:                        childTop = parentBottom - height - lp.bottomMargin;                        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">break</span>;                    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">default</span>:                        childTop = parentTop + lp.topMargin;                }                <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//子视图布局</span>                child.layout(childLeft, childTop, childLeft + width, childTop + height);            }        }    }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li><li style="box-sizing: border-box; padding: 0px 5px;">38</li><li style="box-sizing: border-box; padding: 0px 5px;">39</li><li style="box-sizing: border-box; padding: 0px 5px;">40</li><li style="box-sizing: border-box; padding: 0px 5px;">41</li><li style="box-sizing: border-box; padding: 0px 5px;">42</li><li style="box-sizing: border-box; padding: 0px 5px;">43</li><li style="box-sizing: border-box; padding: 0px 5px;">44</li><li style="box-sizing: border-box; padding: 0px 5px;">45</li><li style="box-sizing: border-box; padding: 0px 5px;">46</li><li style="box-sizing: border-box; padding: 0px 5px;">47</li><li style="box-sizing: border-box; padding: 0px 5px;">48</li><li style="box-sizing: border-box; padding: 0px 5px;">49</li><li style="box-sizing: border-box; padding: 0px 5px;">50</li><li style="box-sizing: border-box; padding: 0px 5px;">51</li><li style="box-sizing: border-box; padding: 0px 5px;">52</li><li style="box-sizing: border-box; padding: 0px 5px;">53</li><li style="box-sizing: border-box; padding: 0px 5px;">54</li><li style="box-sizing: border-box; padding: 0px 5px;">55</li><li style="box-sizing: border-box; padding: 0px 5px;">56</li><li style="box-sizing: border-box; padding: 0px 5px;">57</li><li style="box-sizing: border-box; padding: 0px 5px;">58</li><li style="box-sizing: border-box; padding: 0px 5px;">59</li><li style="box-sizing: border-box; padding: 0px 5px;">60</li><li style="box-sizing: border-box; padding: 0px 5px;">61</li><li style="box-sizing: border-box; padding: 0px 5px;">62</li><li style="box-sizing: border-box; padding: 0px 5px;">63</li><li style="box-sizing: border-box; padding: 0px 5px;">64</li><li style="box-sizing: border-box; padding: 0px 5px;">65</li><li style="box-sizing: border-box; padding: 0px 5px;">66</li><li style="box-sizing: border-box; padding: 0px 5px;">67</li><li style="box-sizing: border-box; padding: 0px 5px;">68</li><li style="box-sizing: border-box; padding: 0px 5px;">69</li><li style="box-sizing: border-box; padding: 0px 5px;">70</li><li style="box-sizing: border-box; padding: 0px 5px;">71</li><li style="box-sizing: border-box; padding: 0px 5px;">72</li><li style="box-sizing: border-box; padding: 0px 5px;">73</li><li style="box-sizing: border-box; padding: 0px 5px;">74</li><li style="box-sizing: border-box; padding: 0px 5px;">75</li></ul>

分析：在FrameLayout中的onLayout方法中仅仅是调用了layoutChildren方法，从该方法名称我们不难看出，原来该方法的作用是

给子视图View进行布局的。也就是说FrameLayout布局其实在View类中的layout方法中已经实现，布局的逻辑实现是在父视图中

实现的，不像View视图的measure测量，通过子类实现onMeasure方法来实现测量逻辑。

1.代码第20-71行，遍历获得FrameLayout的子视图View的四个位置，然后调用child.layout对子视图View进行布局操作。

2.代码第23行，对每个子视图View的可见度进行了判断，如果当前子视图View可见度类型为GONE,则当前子视图View不进行布局，这也就是为什么可见度GONE类型时是不占据屏幕空间的，而其他两种VISIBLE和INVISIBLE是占据屏幕空间的。

2-5

由于FrameLayout类是继承自ViewGroup类的，那么我们进入ViewGroup类去窥探一下onLayout方法具体做了什么？

<code class="hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-javadoc" style="color: rgb(136, 0, 0); box-sizing: border-box;">/**     * {@inheritDoc}     */</span>    <span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">protected</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">abstract</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">onLayout</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">boolean</span> changed,            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> l, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> t, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> r, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> b);</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li></ul>

你会惊讶的发现，在ViewGroup类中的onLayout方法居然是一个抽象方法，现在你明白了吧？我们的FrameLayout继承自ViewGroup，自然FrameLayout就必须实现ViewGroup中的抽象方法onLayout，而FrameLyayout中的onLayout方法的作用是用来设置它的子视图View的布局位置。

到此，我们的布局流程可以用如下图表示：

layout布局总结

1.视图View的布局逻辑是由父View，也就是ViewGroup容器布局来实现的。因此，我们如果自定义View一般都无需重写onMeasure方法，但是如果自定义一个ViewGroup容器的话，就必须实现onMeasure方法，因为该方法在ViewGroup是抽象的，所有ViewGroup的所有子类必须实现onMeasure方法。

2.当我们的视图View在布局中使用 android:visibility=”gone” 属性时，是不占据屏幕空间的，因为在布局时ViewGroup会遍历每个子视图View，判断当前子视图View是否设置了 Visibility==GONE,如果设置了，当前子视图View就会添加到父容器上，因此也就不占据屏幕空间。具体可以参考2-4节。

3.必须在View布局完之后调用getHeight（）和getWidth（）方法获取到的View的宽高才大于0。具体可以参考2-3节。

View的绘制Draw

由 0-5节可知，View视图绘制流程中的最后一步绘制draw是由ViewRootImpl中的performDraw成员方法开始的，跟踪代码，最后会在ViewRootImpl类中的drawSoftware方法绘制View：

3-1

<code class="hljs lasso has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;">  <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff,            boolean scalingRequired, Rect dirty) {        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Draw with software renderer.</span>        final Canvas canvas;        try {            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//从surface对象中获得canvas变量</span>            canvas <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> mSurface<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>lockCanvas(dirty);            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// If this bitmap's format includes an alpha channel, we</span>            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// need to clear it before drawing so that the child will</span>            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// properly re-composite its drawing on a transparent</span>            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// background. This automatically respects the clip/dirty region</span>            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// or</span>            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// If we are applying an offset, we need to clear the area</span>            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// where the offset doesn't appear to avoid having garbage</span>            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// left in the blank areas.</span>            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">!</span>canvas<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>isOpaque() <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">||</span> yoff <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">!=</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">||</span> xoff <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">!=</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>) {                canvas<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>drawColor(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>, PorterDuff<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>Mode<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>CLEAR);            }           <span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>            try {                <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//调整画布的位置</span>                canvas<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>translate(<span class="hljs-attribute" style="box-sizing: border-box;">-xoff</span>, <span class="hljs-attribute" style="box-sizing: border-box;">-yoff</span>);                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (mTranslator <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">!=</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">null</span>) {                    mTranslator<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>translateCanvas(canvas);                }                canvas<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>setScreenDensity(scalingRequired <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">?</span> mNoncompatDensity : <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>);                attachInfo<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>mSetIgnoreDirtyState <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">false</span>;                <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//调用View类中的成员方法draw开始绘制View视图</span>                mView<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>draw(canvas);            }         <span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span><span class="hljs-attribute" style="box-sizing: border-box;">...</span>        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">true</span>;    }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li><li style="box-sizing: border-box; padding: 0px 5px;">38</li><li style="box-sizing: border-box; padding: 0px 5px;">39</li></ul>

分析：代码第8行，从mSurface对象中获得canvas画布，然后将变量canvas变量作为参数传递给第33行代码中的draw方法。由此

可知，我们的视图View最终是绘制到Surface中去的，关于Surface相关的知识，可以参考这篇大神的博客：

Android应用程序窗口（Activity）的绘图表面（Surface）的创建过程分析 

跟踪代码，进入View的draw方法分析源码：

3-2

<code class="hljs mel has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;">public void draw(Canvas <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">canvas</span>) {        final <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> privateFlags = mPrivateFlags;        final boolean dirtyOpaque = (privateFlags & PFLAG_DIRTY_MASK) == PFLAG_DIRTY_OPAQUE &&                (mAttachInfo == null || !mAttachInfo.mIgnoreDirtyState);        mPrivateFlags = (privateFlags & ~PFLAG_DIRTY_MASK) | PFLAG_DRAWN;        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">/*         * Draw traversal performs several drawing steps which must be executed         * in the appropriate order:         *         *      1. Draw the background         *      2. If necessary, save the canvas' layers to prepare for fading         *      3. Draw view's content         *      4. Draw children         *      5. If necessary, draw the fading edges and restore layers         *      6. Draw decorations (scrollbars for instance)         */</span>        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Step 1, draw the background, if needed</span>        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> saveCount;        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (!dirtyOpaque) {            drawBackground(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">canvas</span>);        }        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// skip step 2 & 5 if possible (common case)</span>        final <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> viewFlags = mViewFlags;        boolean horizontalEdges = (viewFlags & FADING_EDGE_HORIZONTAL) != <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;        boolean verticalEdges = (viewFlags & FADING_EDGE_VERTICAL) != <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (!verticalEdges && !horizontalEdges) {            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Step 3, draw the content</span>            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (!dirtyOpaque) onDraw(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">canvas</span>);            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Step 4, draw the children</span>            dispatchDraw(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">canvas</span>);            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Step 6, draw decorations (scrollbars)</span>            onDrawScrollBars(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">canvas</span>);            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (mOverlay != null && !mOverlay.isEmpty()) {                mOverlay.getOverlayView().dispatchDraw(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">canvas</span>);            }            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// we're done...</span>            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span>;        }        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">/*         * Here we do the full fledged routine...         * (this is an uncommon case where speed matters less,         * this is why we repeat some of the tests that have been         * done above)         */</span>        boolean drawTop = false;        boolean drawBottom = false;        boolean drawLeft = false;        boolean drawRight = false;        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">float</span> topFadeStrength = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0.0</span>f;        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">float</span> bottomFadeStrength = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0.0</span>f;        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">float</span> leftFadeStrength = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0.0</span>f;        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">float</span> rightFadeStrength = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0.0</span>f;        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Step 2, save the canvas' layers</span>        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> paddingLeft = mPaddingLeft;        final boolean offsetRequired = isPaddingOffsetRequired();        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (offsetRequired) {            paddingLeft += getLeftPaddingOffset();        }        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> left = mScrollX + paddingLeft;        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> right = left + mRight - mLeft - mPaddingRight - paddingLeft;        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> top = mScrollY + getFadeTop(offsetRequired);        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> bottom = top + getFadeHeight(offsetRequired);        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (offsetRequired) {            right += getRightPaddingOffset();            bottom += getBottomPaddingOffset();        }        final ScrollabilityCache scrollabilityCache = mScrollCache;        final <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">float</span> fadeHeight = scrollabilityCache.fadingEdgeLength;        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> length = (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span>) fadeHeight;        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// clip the fade length if top and bottom fades overlap</span>        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// overlapping fades produce odd-looking artifacts</span>        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (verticalEdges && (top + length > bottom - length)) {            length = (bottom - top) / <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>;        }        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// also clip horizontal fades if necessary</span>        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (horizontalEdges && (left + length > right - length)) {            length = (right - left) / <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>;        }        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (verticalEdges) {            topFadeStrength = Math.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">max</span>(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0.0</span>f, Math.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">min</span>(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1.0</span>f, getTopFadingEdgeStrength()));            drawTop = topFadeStrength * fadeHeight > <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1.0</span>f;            bottomFadeStrength = Math.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">max</span>(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0.0</span>f, Math.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">min</span>(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1.0</span>f, getBottomFadingEdgeStrength()));            drawBottom = bottomFadeStrength * fadeHeight > <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1.0</span>f;        }        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (horizontalEdges) {            leftFadeStrength = Math.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">max</span>(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0.0</span>f, Math.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">min</span>(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1.0</span>f, getLeftFadingEdgeStrength()));            drawLeft = leftFadeStrength * fadeHeight > <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1.0</span>f;            rightFadeStrength = Math.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">max</span>(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0.0</span>f, Math.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">min</span>(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1.0</span>f, getRightFadingEdgeStrength()));            drawRight = rightFadeStrength * fadeHeight > <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1.0</span>f;        }        saveCount = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">canvas</span>.getSaveCount();        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> solidColor = getSolidColor();        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (solidColor == <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>) {            final <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> flags = Canvas.HAS_ALPHA_LAYER_SAVE_FLAG;            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (drawTop) {                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">canvas</span>.saveLayer(left, top, right, top + length, null, flags);            }            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (drawBottom) {                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">canvas</span>.saveLayer(left, bottom - length, right, bottom, null, flags);            }            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (drawLeft) {                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">canvas</span>.saveLayer(left, top, left + length, bottom, null, flags);            }            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (drawRight) {                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">canvas</span>.saveLayer(right - length, top, right, bottom, null, flags);            }        } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> {            scrollabilityCache.setFadeColor(solidColor);        }        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Step 3, draw the content</span>        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (!dirtyOpaque) onDraw(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">canvas</span>);        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Step 4, draw the children</span>        dispatchDraw(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">canvas</span>);        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Step 5, draw the fade effect and restore layers</span>        final Paint p = scrollabilityCache.paint;        final Matrix <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">matrix</span> = scrollabilityCache.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">matrix</span>;        final Shader fade = scrollabilityCache.shader;        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (drawTop) {            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">matrix</span>.setScale(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>, fadeHeight * topFadeStrength);            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">matrix</span>.postTranslate(left, top);            fade.setLocalMatrix(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">matrix</span>);            p.setShader(fade);            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">canvas</span>.drawRect(left, top, right, top + length, p);        }        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (drawBottom) {            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">matrix</span>.setScale(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>, fadeHeight * bottomFadeStrength);            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">matrix</span>.postRotate(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">180</span>);            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">matrix</span>.postTranslate(left, bottom);            fade.setLocalMatrix(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">matrix</span>);            p.setShader(fade);            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">canvas</span>.drawRect(left, bottom - length, right, bottom, p);        }        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (drawLeft) {            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">matrix</span>.setScale(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>, fadeHeight * leftFadeStrength);            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">matrix</span>.postRotate(-<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">90</span>);            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">matrix</span>.postTranslate(left, top);            fade.setLocalMatrix(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">matrix</span>);            p.setShader(fade);            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">canvas</span>.drawRect(left, top, left + length, bottom, p);        }        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (drawRight) {            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">matrix</span>.setScale(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>, fadeHeight * rightFadeStrength);            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">matrix</span>.postRotate(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">90</span>);            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">matrix</span>.postTranslate(right, top);            fade.setLocalMatrix(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">matrix</span>);            p.setShader(fade);            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">canvas</span>.drawRect(right - length, top, right, bottom, p);        }        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">canvas</span>.restoreToCount(saveCount);        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Step 6, draw decorations (scrollbars)</span>        onDrawScrollBars(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">canvas</span>);        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (mOverlay != null && !mOverlay.isEmpty()) {            mOverlay.getOverlayView().dispatchDraw(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">canvas</span>);        }    }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li><li style="box-sizing: border-box; padding: 0px 5px;">38</li><li style="box-sizing: border-box; padding: 0px 5px;">39</li><li style="box-sizing: border-box; padding: 0px 5px;">40</li><li style="box-sizing: border-box; padding: 0px 5px;">41</li><li style="box-sizing: border-box; padding: 0px 5px;">42</li><li style="box-sizing: border-box; padding: 0px 5px;">43</li><li style="box-sizing: border-box; padding: 0px 5px;">44</li><li style="box-sizing: border-box; padding: 0px 5px;">45</li><li style="box-sizing: border-box; padding: 0px 5px;">46</li><li style="box-sizing: border-box; padding: 0px 5px;">47</li><li style="box-sizing: border-box; padding: 0px 5px;">48</li><li style="box-sizing: border-box; padding: 0px 5px;">49</li><li style="box-sizing: border-box; padding: 0px 5px;">50</li><li style="box-sizing: border-box; padding: 0px 5px;">51</li><li style="box-sizing: border-box; padding: 0px 5px;">52</li><li style="box-sizing: border-box; padding: 0px 5px;">53</li><li style="box-sizing: border-box; padding: 0px 5px;">54</li><li style="box-sizing: border-box; padding: 0px 5px;">55</li><li style="box-sizing: border-box; padding: 0px 5px;">56</li><li style="box-sizing: border-box; padding: 0px 5px;">57</li><li style="box-sizing: border-box; padding: 0px 5px;">58</li><li style="box-sizing: border-box; padding: 0px 5px;">59</li><li style="box-sizing: border-box; padding: 0px 5px;">60</li><li style="box-sizing: border-box; padding: 0px 5px;">61</li><li style="box-sizing: border-box; padding: 0px 5px;">62</li><li style="box-sizing: border-box; padding: 0px 5px;">63</li><li style="box-sizing: border-box; padding: 0px 5px;">64</li><li style="box-sizing: border-box; padding: 0px 5px;">65</li><li style="box-sizing: border-box; padding: 0px 5px;">66</li><li style="box-sizing: border-box; padding: 0px 5px;">67</li><li style="box-sizing: border-box; padding: 0px 5px;">68</li><li style="box-sizing: border-box; padding: 0px 5px;">69</li><li style="box-sizing: border-box; padding: 0px 5px;">70</li><li style="box-sizing: border-box; padding: 0px 5px;">71</li><li style="box-sizing: border-box; padding: 0px 5px;">72</li><li style="box-sizing: border-box; padding: 0px 5px;">73</li><li style="box-sizing: border-box; padding: 0px 5px;">74</li><li style="box-sizing: border-box; padding: 0px 5px;">75</li><li style="box-sizing: border-box; padding: 0px 5px;">76</li><li style="box-sizing: border-box; padding: 0px 5px;">77</li><li style="box-sizing: border-box; padding: 0px 5px;">78</li><li style="box-sizing: border-box; padding: 0px 5px;">79</li><li style="box-sizing: border-box; padding: 0px 5px;">80</li><li style="box-sizing: border-box; padding: 0px 5px;">81</li><li style="box-sizing: border-box; padding: 0px 5px;">82</li><li style="box-sizing: border-box; padding: 0px 5px;">83</li><li style="box-sizing: border-box; padding: 0px 5px;">84</li><li style="box-sizing: border-box; padding: 0px 5px;">85</li><li style="box-sizing: border-box; padding: 0px 5px;">86</li><li style="box-sizing: border-box; padding: 0px 5px;">87</li><li style="box-sizing: border-box; padding: 0px 5px;">88</li><li style="box-sizing: border-box; padding: 0px 5px;">89</li><li style="box-sizing: border-box; padding: 0px 5px;">90</li><li style="box-sizing: border-box; padding: 0px 5px;">91</li><li style="box-sizing: border-box; padding: 0px 5px;">92</li><li style="box-sizing: border-box; padding: 0px 5px;">93</li><li style="box-sizing: border-box; padding: 0px 5px;">94</li><li style="box-sizing: border-box; padding: 0px 5px;">95</li><li style="box-sizing: border-box; padding: 0px 5px;">96</li><li style="box-sizing: border-box; padding: 0px 5px;">97</li><li style="box-sizing: border-box; padding: 0px 5px;">98</li><li style="box-sizing: border-box; padding: 0px 5px;">99</li><li style="box-sizing: border-box; padding: 0px 5px;">100</li><li style="box-sizing: border-box; padding: 0px 5px;">101</li><li style="box-sizing: border-box; padding: 0px 5px;">102</li><li style="box-sizing: border-box; padding: 0px 5px;">103</li><li style="box-sizing: border-box; padding: 0px 5px;">104</li><li style="box-sizing: border-box; padding: 0px 5px;">105</li><li style="box-sizing: border-box; padding: 0px 5px;">106</li><li style="box-sizing: border-box; padding: 0px 5px;">107</li><li style="box-sizing: border-box; padding: 0px 5px;">108</li><li style="box-sizing: border-box; padding: 0px 5px;">109</li><li style="box-sizing: border-box; padding: 0px 5px;">110</li><li style="box-sizing: border-box; padding: 0px 5px;">111</li><li style="box-sizing: border-box; padding: 0px 5px;">112</li><li style="box-sizing: border-box; padding: 0px 5px;">113</li><li style="box-sizing: border-box; padding: 0px 5px;">114</li><li style="box-sizing: border-box; padding: 0px 5px;">115</li><li style="box-sizing: border-box; padding: 0px 5px;">116</li><li style="box-sizing: border-box; padding: 0px 5px;">117</li><li style="box-sizing: border-box; padding: 0px 5px;">118</li><li style="box-sizing: border-box; padding: 0px 5px;">119</li><li style="box-sizing: border-box; padding: 0px 5px;">120</li><li style="box-sizing: border-box; padding: 0px 5px;">121</li><li style="box-sizing: border-box; padding: 0px 5px;">122</li><li style="box-sizing: border-box; padding: 0px 5px;">123</li><li style="box-sizing: border-box; padding: 0px 5px;">124</li><li style="box-sizing: border-box; padding: 0px 5px;">125</li><li style="box-sizing: border-box; padding: 0px 5px;">126</li><li style="box-sizing: border-box; padding: 0px 5px;">127</li><li style="box-sizing: border-box; padding: 0px 5px;">128</li><li style="box-sizing: border-box; padding: 0px 5px;">129</li><li style="box-sizing: border-box; padding: 0px 5px;">130</li><li style="box-sizing: border-box; padding: 0px 5px;">131</li><li style="box-sizing: border-box; padding: 0px 5px;">132</li><li style="box-sizing: border-box; padding: 0px 5px;">133</li><li style="box-sizing: border-box; padding: 0px 5px;">134</li><li style="box-sizing: border-box; padding: 0px 5px;">135</li><li style="box-sizing: border-box; padding: 0px 5px;">136</li><li style="box-sizing: border-box; padding: 0px 5px;">137</li><li style="box-sizing: border-box; padding: 0px 5px;">138</li><li style="box-sizing: border-box; padding: 0px 5px;">139</li><li style="box-sizing: border-box; padding: 0px 5px;">140</li><li style="box-sizing: border-box; padding: 0px 5px;">141</li><li style="box-sizing: border-box; padding: 0px 5px;">142</li><li style="box-sizing: border-box; padding: 0px 5px;">143</li><li style="box-sizing: border-box; padding: 0px 5px;">144</li><li style="box-sizing: border-box; padding: 0px 5px;">145</li><li style="box-sizing: border-box; padding: 0px 5px;">146</li><li style="box-sizing: border-box; padding: 0px 5px;">147</li><li style="box-sizing: border-box; padding: 0px 5px;">148</li><li style="box-sizing: border-box; padding: 0px 5px;">149</li><li style="box-sizing: border-box; padding: 0px 5px;">150</li><li style="box-sizing: border-box; padding: 0px 5px;">151</li><li style="box-sizing: border-box; padding: 0px 5px;">152</li><li style="box-sizing: border-box; padding: 0px 5px;">153</li><li style="box-sizing: border-box; padding: 0px 5px;">154</li><li style="box-sizing: border-box; padding: 0px 5px;">155</li><li style="box-sizing: border-box; padding: 0px 5px;">156</li><li style="box-sizing: border-box; padding: 0px 5px;">157</li><li style="box-sizing: border-box; padding: 0px 5px;">158</li><li style="box-sizing: border-box; padding: 0px 5px;">159</li><li style="box-sizing: border-box; padding: 0px 5px;">160</li><li style="box-sizing: border-box; padding: 0px 5px;">161</li><li style="box-sizing: border-box; padding: 0px 5px;">162</li><li style="box-sizing: border-box; padding: 0px 5px;">163</li><li style="box-sizing: border-box; padding: 0px 5px;">164</li><li style="box-sizing: border-box; padding: 0px 5px;">165</li><li style="box-sizing: border-box; padding: 0px 5px;">166</li><li style="box-sizing: border-box; padding: 0px 5px;">167</li><li style="box-sizing: border-box; padding: 0px 5px;">168</li><li style="box-sizing: border-box; padding: 0px 5px;">169</li><li style="box-sizing: border-box; padding: 0px 5px;">170</li><li style="box-sizing: border-box; padding: 0px 5px;">171</li><li style="box-sizing: border-box; padding: 0px 5px;">172</li><li style="box-sizing: border-box; padding: 0px 5px;">173</li><li style="box-sizing: border-box; padding: 0px 5px;">174</li><li style="box-sizing: border-box; padding: 0px 5px;">175</li><li style="box-sizing: border-box; padding: 0px 5px;">176</li><li style="box-sizing: border-box; padding: 0px 5px;">177</li><li style="box-sizing: border-box; padding: 0px 5px;">178</li><li style="box-sizing: border-box; padding: 0px 5px;">179</li><li style="box-sizing: border-box; padding: 0px 5px;">180</li><li style="box-sizing: border-box; padding: 0px 5px;">181</li><li style="box-sizing: border-box; padding: 0px 5px;">182</li><li style="box-sizing: border-box; padding: 0px 5px;">183</li><li style="box-sizing: border-box; padding: 0px 5px;">184</li><li style="box-sizing: border-box; padding: 0px 5px;">185</li><li style="box-sizing: border-box; padding: 0px 5px;">186</li><li style="box-sizing: border-box; padding: 0px 5px;">187</li><li style="box-sizing: border-box; padding: 0px 5px;">188</li><li style="box-sizing: border-box; padding: 0px 5px;">189</li><li style="box-sizing: border-box; padding: 0px 5px;">190</li><li style="box-sizing: border-box; padding: 0px 5px;">191</li></ul>

分析： 
从代码第7-17行可知，视图View的绘制可以分为如下6个步骤：

1. 绘制当前视图的背景。

2. 保存当前画布的堆栈状态，并且在在当前画布上创建额外的图层，以便接下来可以用来绘制当前视图在滑动时的边框渐变效果。

3. 绘制当前视图的内容。

4. 绘制当前视图的子视图的内容。

5. 绘制当前视图在滑动时的边框渐变效果。

6. 绘制当前视图的滚动条。

Step1：绘制视图View的背景 
代码第2-5行，获取当前View是否需要绘制背景的标志dirtyOpaque ，代码第22-24行，根据这个标志来决定是否绘制视图View的背景。如果需要绘制背景，那么进入View类中的drawBackground方法：

<code class="hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-javadoc" style="color: rgb(136, 0, 0); box-sizing: border-box;">/**     * Draws the background onto the specified canvas.     *     *<span class="hljs-javadoctag" style="color: rgb(102, 0, 102); box-sizing: border-box;"> @param</span> canvas Canvas on which to draw the background     */</span>    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">drawBackground</span>(Canvas canvas) {        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> Drawable background = mBackground;        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (background == <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>) {            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span>;        }        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (mBackgroundSizeChanged) {            background.setBounds(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>,  mRight - mLeft, mBottom - mTop);            mBackgroundSizeChanged = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">false</span>;            mPrivateFlags3 |= PFLAG3_OUTLINE_INVALID;        }       ............        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> scrollX = mScrollX;        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> scrollY = mScrollY;        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ((scrollX | scrollY) == <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>) {            background.draw(canvas);        } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> {            canvas.translate(scrollX, scrollY);            background.draw(canvas);            canvas.translate(-scrollX, -scrollY);        }    }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li></ul>

该方法用来描述1，绘制当前View的背景的，在绘制背景前，调用background.setBounds(0, 0, mRight - mLeft, mBottom - mTop);方法设置背景的大小，之后调用Drawable的对象backgroud的draw方法完成背景的绘制。

Step2：保存画布canvas的边框参数

代码第27-29获取当前视图View水平或者垂直方向是否需要绘制边框渐变效果，如果不需要绘制边框的渐变效果，就无需执行上面的2，5了。那么就直接执行上面的3，4，6步骤。这里描述的就是我们的ListView滑动到最底端时，底部会有一个淡蓝色的半圆形的边框渐变背景效果。

假如我们需要绘制视图View的边框渐变效果，那么我们继续分析步骤2，3，4，5，6。

代码第55-134行：这段代码用来检查是否需要保存参数canvas所描述的一块画布的堆栈状态，并且创建额外的图层来绘制当前视图在滑动时的边框渐变效果。视图的边框是绘制在内容区域的边界位置上的，而视图的内容区域是需要排除成员变量mPaddingLeft、mPaddingRight、mPaddingTop和mPaddingBottom所描述的视图内边距的。此外，视图的边框有四个，分别位于视图的左、右、上以及下内边界上。因此，这段代码首先需要计算出当前视图的左、右、上以及下内边距的大小，以便得到边框所要绘制的区域。

Step3：绘制视图View的内容

代码第138行我们可以看到，根据条件绘制当前视图View的内容，此处调用了View的成员方法onDraw来绘制视图View的内容，我们来看看onDraw成员方法的实现：

<code class="hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-javadoc" style="color: rgb(136, 0, 0); box-sizing: border-box;">/**     * Implement this to do your drawing.     *     *<span class="hljs-javadoctag" style="color: rgb(102, 0, 102); box-sizing: border-box;"> @param</span> canvas the canvas on which the background will be drawn     */</span>    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">protected</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">onDraw</span>(Canvas canvas) {    }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li></ul>

预料之中，该方法体里面是一个空实现，也就是视图View将绘制的逻辑留给继承它的子类去实现，这也就是为什么我们在自定义View的时候必须去实现其父类的onDraw方法来完成自己对内容的绘制。

Step4：绘制当前视图View的子视图

代码第141行调用View的成员方法dispatchDraw(canvas);来绘制它的子视图，我们进入dispatchDraw(canvas);方法窥探其实现逻辑：

<code class="hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-javadoc" style="color: rgb(136, 0, 0); box-sizing: border-box;">/**     * Called by draw to draw the child views. This may be overridden     * by derived classes to gain control just before its children are drawn     * (but after its own view has been drawn).     *<span class="hljs-javadoctag" style="color: rgb(102, 0, 102); box-sizing: border-box;"> @param</span> canvas the canvas on which to draw the view     */</span>    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">protected</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">dispatchDraw</span>(Canvas canvas) {    }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li></ul>

同样你会发现，这也是一个空实现，既然是这样，那么其实现的逻辑也会在它的子类中实现了，由于只有ViewGroup容器才有其子视图，因此，该方法的实现应该在ViewGroup类中，我们进入ViewGroup类中看其源码如下：

<code class="hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"> <span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">protected</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">dispatchDraw</span>(Canvas canvas) {        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">boolean</span> usingRenderNodeProperties = canvas.isRecordingFor(mRenderNode);        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> childrenCount = mChildrenCount;        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> View[] children = mChildren;        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> flags = mGroupFlags;        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//判断当前ViewGroup容器是否设置的布局动画</span>        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ((flags & FLAG_RUN_ANIMATION) != <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span> && canAnimate()) {            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">boolean</span> cache = (mGroupFlags & FLAG_ANIMATION_CACHE) == FLAG_ANIMATION_CACHE;            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">boolean</span> buildCache = !isHardwareAccelerated();            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//遍历给每个子视图View设置动画效果</span>            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> i = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>; i < childrenCount; i++) {                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> View child = children[i];                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE) {                    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> LayoutParams params = child.getLayoutParams();                    attachLayoutAnimationParameters(child, params, i, childrenCount);                    bindLayoutAnimation(child);                    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (cache) {                        child.setDrawingCacheEnabled(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">true</span>);                        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (buildCache) {                            child.buildDrawingCache(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">true</span>);                        }                    }                }            }            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//获得布局动画的控制器</span>            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> LayoutAnimationController controller = mLayoutAnimationController;            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (controller.willOverlap()) {                mGroupFlags |= FLAG_OPTIMIZE_INVALIDATE;            }            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//开始布局动画</span>            controller.start();            mGroupFlags &= ~FLAG_RUN_ANIMATION;            mGroupFlags &= ~FLAG_ANIMATION_DONE;            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (cache) {                mGroupFlags |= FLAG_CHILDREN_DRAWN_WITH_CACHE;            }            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//设置布局动画的监听事件</span>            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (mAnimationListener != <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>) {                mAnimationListener.onAnimationStart(controller.getAnimation());            }        }        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> clipSaveCount = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//是否需要剪裁边距</span>        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">boolean</span> clipToPadding = (flags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK;        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (clipToPadding) {            clipSaveCount = canvas.save();            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//对画布进行边距剪裁</span>            canvas.clipRect(mScrollX + mPaddingLeft, mScrollY + mPaddingTop,                    mScrollX + mRight - mLeft - mPaddingRight,                    mScrollY + mBottom - mTop - mPaddingBottom);        }        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// We will draw our child's animation, let's reset the flag</span>        mPrivateFlags &= ~PFLAG_DRAW_ANIMATION;        mGroupFlags &= ~FLAG_INVALIDATE_REQUIRED;        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">boolean</span> more = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">false</span>;        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">long</span> drawingTime = getDrawingTime();        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (usingRenderNodeProperties) canvas.insertReorderBarrier();        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Only use the preordered list if not HW accelerated, since the HW pipeline will do the</span>        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// draw reordering internally</span>        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> ArrayList<View> preorderedList = usingRenderNodeProperties                ? <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span> : buildOrderedChildList();        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">boolean</span> customOrder = preorderedList == <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>                && isChildrenDrawingOrderEnabled();        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//遍历绘制当前视图的子视图View</span>        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> i = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>; i < childrenCount; i++) {            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> childIndex = customOrder ? getChildDrawingOrder(childrenCount, i) : i;            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> View child = (preorderedList == <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>)                    ? children[childIndex] : preorderedList.get(childIndex);            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>) {                more |= drawChild(canvas, child, drawingTime);            }        }        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (preorderedList != <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>) preorderedList.clear();        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Draw any disappearing views that have animations</span>        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (mDisappearingChildren != <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>) {            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> ArrayList<View> disappearingChildren = mDisappearingChildren;            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> disappearingCount = disappearingChildren.size() - <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>;            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Go backwards -- we may delete as animations finish</span>            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> i = disappearingCount; i >= <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>; i--) {                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> View child = disappearingChildren.get(i);                more |= drawChild(canvas, child, drawingTime);            }        }        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (usingRenderNodeProperties) canvas.insertInorderBarrier();        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (debugDraw()) {            onDebugDraw(canvas);        }        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (clipToPadding) {            canvas.restoreToCount(clipSaveCount);        }        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// mGroupFlags might have been updated by drawChild()</span>        flags = mGroupFlags;        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ((flags & FLAG_INVALIDATE_REQUIRED) == FLAG_INVALIDATE_REQUIRED) {            invalidate(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">true</span>);        }        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//更新布局动画的监听事件</span>        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ((flags & FLAG_ANIMATION_DONE) == <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span> && (flags & FLAG_NOTIFY_ANIMATION_LISTENER) == <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span> &&                mLayoutAnimationController.isDone() && !more) {            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// We want to erase the drawing cache and notify the listener after the</span>            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// next frame is drawn because one extra invalidate() is caused by</span>            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// drawChild() after the animation is over</span>            mGroupFlags |= FLAG_NOTIFY_ANIMATION_LISTENER;            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> Runnable end = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Runnable() {               <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">run</span>() {                   notifyAnimationListener();               }            };            post(end);        }    }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li><li style="box-sizing: border-box; padding: 0px 5px;">38</li><li style="box-sizing: border-box; padding: 0px 5px;">39</li><li style="box-sizing: border-box; padding: 0px 5px;">40</li><li style="box-sizing: border-box; padding: 0px 5px;">41</li><li style="box-sizing: border-box; padding: 0px 5px;">42</li><li style="box-sizing: border-box; padding: 0px 5px;">43</li><li style="box-sizing: border-box; padding: 0px 5px;">44</li><li style="box-sizing: border-box; padding: 0px 5px;">45</li><li style="box-sizing: border-box; padding: 0px 5px;">46</li><li style="box-sizing: border-box; padding: 0px 5px;">47</li><li style="box-sizing: border-box; padding: 0px 5px;">48</li><li style="box-sizing: border-box; padding: 0px 5px;">49</li><li style="box-sizing: border-box; padding: 0px 5px;">50</li><li style="box-sizing: border-box; padding: 0px 5px;">51</li><li style="box-sizing: border-box; padding: 0px 5px;">52</li><li style="box-sizing: border-box; padding: 0px 5px;">53</li><li style="box-sizing: border-box; padding: 0px 5px;">54</li><li style="box-sizing: border-box; padding: 0px 5px;">55</li><li style="box-sizing: border-box; padding: 0px 5px;">56</li><li style="box-sizing: border-box; padding: 0px 5px;">57</li><li style="box-sizing: border-box; padding: 0px 5px;">58</li><li style="box-sizing: border-box; padding: 0px 5px;">59</li><li style="box-sizing: border-box; padding: 0px 5px;">60</li><li style="box-sizing: border-box; padding: 0px 5px;">61</li><li style="box-sizing: border-box; padding: 0px 5px;">62</li><li style="box-sizing: border-box; padding: 0px 5px;">63</li><li style="box-sizing: border-box; padding: 0px 5px;">64</li><li style="box-sizing: border-box; padding: 0px 5px;">65</li><li style="box-sizing: border-box; padding: 0px 5px;">66</li><li style="box-sizing: border-box; padding: 0px 5px;">67</li><li style="box-sizing: border-box; padding: 0px 5px;">68</li><li style="box-sizing: border-box; padding: 0px 5px;">69</li><li style="box-sizing: border-box; padding: 0px 5px;">70</li><li style="box-sizing: border-box; padding: 0px 5px;">71</li><li style="box-sizing: border-box; padding: 0px 5px;">72</li><li style="box-sizing: border-box; padding: 0px 5px;">73</li><li style="box-sizing: border-box; padding: 0px 5px;">74</li><li style="box-sizing: border-box; padding: 0px 5px;">75</li><li style="box-sizing: border-box; padding: 0px 5px;">76</li><li style="box-sizing: border-box; padding: 0px 5px;">77</li><li style="box-sizing: border-box; padding: 0px 5px;">78</li><li style="box-sizing: border-box; padding: 0px 5px;">79</li><li style="box-sizing: border-box; padding: 0px 5px;">80</li><li style="box-sizing: border-box; padding: 0px 5px;">81</li><li style="box-sizing: border-box; padding: 0px 5px;">82</li><li style="box-sizing: border-box; padding: 0px 5px;">83</li><li style="box-sizing: border-box; padding: 0px 5px;">84</li><li style="box-sizing: border-box; padding: 0px 5px;">85</li><li style="box-sizing: border-box; padding: 0px 5px;">86</li><li style="box-sizing: border-box; padding: 0px 5px;">87</li><li style="box-sizing: border-box; padding: 0px 5px;">88</li><li style="box-sizing: border-box; padding: 0px 5px;">89</li><li style="box-sizing: border-box; padding: 0px 5px;">90</li><li style="box-sizing: border-box; padding: 0px 5px;">91</li><li style="box-sizing: border-box; padding: 0px 5px;">92</li><li style="box-sizing: border-box; padding: 0px 5px;">93</li><li style="box-sizing: border-box; padding: 0px 5px;">94</li><li style="box-sizing: border-box; padding: 0px 5px;">95</li><li style="box-sizing: border-box; padding: 0px 5px;">96</li><li style="box-sizing: border-box; padding: 0px 5px;">97</li><li style="box-sizing: border-box; padding: 0px 5px;">98</li><li style="box-sizing: border-box; padding: 0px 5px;">99</li><li style="box-sizing: border-box; padding: 0px 5px;">100</li><li style="box-sizing: border-box; padding: 0px 5px;">101</li><li style="box-sizing: border-box; padding: 0px 5px;">102</li><li style="box-sizing: border-box; padding: 0px 5px;">103</li><li style="box-sizing: border-box; padding: 0px 5px;">104</li><li style="box-sizing: border-box; padding: 0px 5px;">105</li><li style="box-sizing: border-box; padding: 0px 5px;">106</li><li style="box-sizing: border-box; padding: 0px 5px;">107</li><li style="box-sizing: border-box; padding: 0px 5px;">108</li><li style="box-sizing: border-box; padding: 0px 5px;">109</li><li style="box-sizing: border-box; padding: 0px 5px;">110</li><li style="box-sizing: border-box; padding: 0px 5px;">111</li><li style="box-sizing: border-box; padding: 0px 5px;">112</li><li style="box-sizing: border-box; padding: 0px 5px;">113</li><li style="box-sizing: border-box; padding: 0px 5px;">114</li><li style="box-sizing: border-box; padding: 0px 5px;">115</li><li style="box-sizing: border-box; padding: 0px 5px;">116</li><li style="box-sizing: border-box; padding: 0px 5px;">117</li><li style="box-sizing: border-box; padding: 0px 5px;">118</li><li style="box-sizing: border-box; padding: 0px 5px;">119</li><li style="box-sizing: border-box; padding: 0px 5px;">120</li><li style="box-sizing: border-box; padding: 0px 5px;">121</li><li style="box-sizing: border-box; padding: 0px 5px;">122</li><li style="box-sizing: border-box; padding: 0px 5px;">123</li><li style="box-sizing: border-box; padding: 0px 5px;">124</li><li style="box-sizing: border-box; padding: 0px 5px;">125</li></ul>

分析： 
1.代码第8-45行，判断当前ViewGroup布局是否设置了布局动画，如果设置了，则条件满足，给每个子视图View设置布局动画，那么什么情况下，该条件满足呢？当你在布局中使用ViewGroup容器布局，且设置了android:animateLayoutChanges=”true”属性，那么该条件满足，每个子视图View出现的时候都会有一个默认的动画效果。关于容器布局动画，具体详情可以参考我的另外一篇博客：Android属性动画Property Animation系列三之LayoutTransition（布局容器动画） ，这里面详细介绍了布局容器动画的各种使用技巧。

2.代码第49-56行，对当前视图的画布canvas进行边距裁剪，把不需要绘制内容的边距裁剪掉。

3.代码第73-80行，遍历绘制当前容器布局ViewGroup的子视图，其中调用了成员方法drawChild来完成子视图的绘制。

4.代码第83-92行，当子视图设置了消失动画时，遍历绘制布局容器中需要消失的子视图。关于子视图消失动画，具体详情可以参考我的另外一篇博客：Android属性动画Property Animation系列三之LayoutTransition（布局容器动画）

我们现在来分析下ViewGroup中的drawChild方法，看看它是怎么绘制子视图的：

<code class="hljs applescript has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;">/**     * Draw one child <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">of</span> this View Group. This method <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">is</span> responsible <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> getting     * <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">the</span> canvas <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">in</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">the</span> right state. This includes clipping, translating so     * <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">that</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">the</span> child's scrolled origin <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">is</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">at</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">and</span> applying any animation     * transformations.     *     * @param canvas The canvas <span class="hljs-function_start" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">on</span></span> which <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">to</span> draw <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">the</span> child     * @param child Who <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">to</span> draw     * @param drawingTime The <span class="hljs-property" style="box-sizing: border-box;">time</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">at</span> which draw <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">is</span> occurring     * @<span class="hljs-constant" style="box-sizing: border-box;">return</span> True <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> an invalidate() was issued     */    protected <span class="hljs-type" style="box-sizing: border-box;">boolean</span> drawChild(Canvas canvas, View child, long drawingTime) {<span class="hljs-command" style="box-sizing: border-box;">        return</span> child.draw(canvas, this, drawingTime);    }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li></ul>

是不是预料之中的事情，此处又调用View类中的draw方法来绘制视图，因此形成了一个嵌套调用，知道所有的子视图View绘制结束。到此关于视图View绘制已经基本完成。

Step5：绘制滑动时边框的渐变效果

代码第143-183：绘制当前容器视图ViewGroup的边框渐变效果。

Step6：绘制滚动条

代码第186行：绘制当前视图View的滑动条。此处调用了内部成员方法onDrawScrollBars来绘制滚动条，我们进入源码来窥探一下其如何实现：

<code class="hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">protected</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">onDrawScrollBars</span>(Canvas canvas) {        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// scrollbars are drawn only when the animation is running</span>        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> ScrollabilityCache cache = mScrollCache;        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//滚动条是否有缓存</span>        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (cache != <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>) {            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> state = cache.state;            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//滚动条不显示时，直接返回，也就是不绘制滚动条</span>            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (state == ScrollabilityCache.OFF) {                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span>;            }            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">boolean</span> invalidate = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">false</span>;            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//滚动条是否可见</span>            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (state == ScrollabilityCache.FADING) {                <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// We're fading -- get our fade interpolation</span>                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (cache.interpolatorValues == <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>) {                    cache.interpolatorValues = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">float</span>[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>];                }                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">float</span>[] values = cache.interpolatorValues;                <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Stops the animation if we're done</span>                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (cache.scrollBarInterpolator.timeToValues(values) ==                        Interpolator.Result.FREEZE_END) {                    cache.state = ScrollabilityCache.OFF;                } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> {                    cache.scrollBar.setAlpha(Math.round(values[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>]));                }                <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// This will make the scroll bars inval themselves after</span>                <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// drawing. We only want this when we're fading so that</span>                <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// we prevent excessive redraws</span>                invalidate = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">true</span>;            } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> {                <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// We're just on -- but we may have been fading before so</span>                <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// reset alpha</span>                <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//设置滚动条完全可见</span>                cache.scrollBar.setAlpha(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">255</span>);            }            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> viewFlags = mViewFlags;            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">boolean</span> drawHorizontalScrollBar =                (viewFlags & SCROLLBARS_HORIZONTAL) == SCROLLBARS_HORIZONTAL;            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">boolean</span> drawVerticalScrollBar =                (viewFlags & SCROLLBARS_VERTICAL) == SCROLLBARS_VERTICAL                && !isVerticalScrollBarHidden();            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (drawVerticalScrollBar || drawHorizontalScrollBar) {                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> width = mRight - mLeft;                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> height = mBottom - mTop;                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> ScrollBarDrawable scrollBar = cache.scrollBar;                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> scrollX = mScrollX;                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> scrollY = mScrollY;                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> inside = (viewFlags & SCROLLBARS_OUTSIDE_MASK) == <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span> ? ~<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span> : <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> left;                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> top;                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> right;                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> bottom;                <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//绘制水平滚动条</span>                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (drawHorizontalScrollBar) {                    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> size = scrollBar.getSize(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">false</span>);                    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (size <= <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>) {                        size = cache.scrollBarSize;                    }                    scrollBar.setParameters(computeHorizontalScrollRange(),                                            computeHorizontalScrollOffset(),                                            computeHorizontalScrollExtent(), <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">false</span>);                    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> verticalScrollBarGap = drawVerticalScrollBar ?                            getVerticalScrollbarWidth() : <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;                    top = scrollY + height - size - (mUserPaddingBottom & inside);                    left = scrollX + (mPaddingLeft & inside);                    right = scrollX + width - (mUserPaddingRight & inside) - verticalScrollBarGap;                    bottom = top + size;                    onDrawHorizontalScrollBar(canvas, scrollBar, left, top, right, bottom);                    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (invalidate) {                        invalidate(left, top, right, bottom);                    }                }                <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//绘制垂直滚动条</span>                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (drawVerticalScrollBar) {                    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> size = scrollBar.getSize(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">true</span>);                    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (size <= <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>) {                        size = cache.scrollBarSize;                    }                    scrollBar.setParameters(computeVerticalScrollRange(),                                            computeVerticalScrollOffset(),                                            computeVerticalScrollExtent(), <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">true</span>);                    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> verticalScrollbarPosition = mVerticalScrollbarPosition;                    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (verticalScrollbarPosition == SCROLLBAR_POSITION_DEFAULT) {                        verticalScrollbarPosition = isLayoutRtl() ?                                SCROLLBAR_POSITION_LEFT : SCROLLBAR_POSITION_RIGHT;                    }                    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">switch</span> (verticalScrollbarPosition) {                        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">default</span>:                        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">case</span> SCROLLBAR_POSITION_RIGHT:                            left = scrollX + width - size - (mUserPaddingRight & inside);                            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">break</span>;                        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">case</span> SCROLLBAR_POSITION_LEFT:                            left = scrollX + (mUserPaddingLeft & inside);                            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">break</span>;                    }                    top = scrollY + (mPaddingTop & inside);                    right = left + size;                    bottom = scrollY + height - (mUserPaddingBottom & inside);                    onDrawVerticalScrollBar(canvas, scrollBar, left, top, right, bottom);                    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (invalidate) {                        invalidate(left, top, right, bottom);                    }                }            }        }    }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; 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padding: 0px 5px;">88</li><li style="box-sizing: border-box; padding: 0px 5px;">89</li><li style="box-sizing: border-box; padding: 0px 5px;">90</li><li style="box-sizing: border-box; padding: 0px 5px;">91</li><li style="box-sizing: border-box; padding: 0px 5px;">92</li><li style="box-sizing: border-box; padding: 0px 5px;">93</li><li style="box-sizing: border-box; padding: 0px 5px;">94</li><li style="box-sizing: border-box; padding: 0px 5px;">95</li><li style="box-sizing: border-box; padding: 0px 5px;">96</li><li style="box-sizing: border-box; padding: 0px 5px;">97</li><li style="box-sizing: border-box; padding: 0px 5px;">98</li><li style="box-sizing: border-box; padding: 0px 5px;">99</li><li style="box-sizing: border-box; padding: 0px 5px;">100</li><li style="box-sizing: border-box; padding: 0px 5px;">101</li><li style="box-sizing: border-box; padding: 0px 5px;">102</li><li style="box-sizing: border-box; padding: 0px 5px;">103</li><li style="box-sizing: border-box; padding: 0px 5px;">104</li><li style="box-sizing: border-box; padding: 0px 5px;">105</li><li style="box-sizing: border-box; padding: 0px 5px;">106</li><li style="box-sizing: border-box; padding: 0px 5px;">107</li><li style="box-sizing: border-box; padding: 0px 5px;">108</li><li style="box-sizing: border-box; padding: 0px 5px;">109</li><li style="box-sizing: border-box; padding: 0px 5px;">110</li><li style="box-sizing: border-box; padding: 0px 5px;">111</li><li style="box-sizing: border-box; padding: 0px 5px;">112</li><li style="box-sizing: border-box; padding: 0px 5px;">113</li><li style="box-sizing: border-box; padding: 0px 5px;">114</li><li style="box-sizing: border-box; padding: 0px 5px;">115</li><li style="box-sizing: border-box; padding: 0px 5px;">116</li><li style="box-sizing: border-box; padding: 0px 5px;">117</li><li style="box-sizing: border-box; padding: 0px 5px;">118</li><li style="box-sizing: border-box; padding: 0px 5px;">119</li><li style="box-sizing: border-box; padding: 0px 5px;">120</li><li style="box-sizing: border-box; padding: 0px 5px;">121</li></ul>

分析： 
1.代码第9行，判断是否需要绘制当前视图View的滚动条。如果你给当前视图View设置了android:scrollbars=”none”属性，时就不会绘制滚动条，也就是不显示滚动条。

2.代码第15行，判断当前视图View的滚动条是否可消失。如果你给当前视图View设置了android:fadeScrollbars=”true”属性时，你不滑动，滚动条隐藏，你滑动时，滚动条显示，有代码可以看出，此处是通过改变滚动条的透明度来实现滚动条隐藏和显示的。

3.代码第35-39行，当前视图View的滚动条设置成完全可见，也就是你设置了该属性android:fadeScrollbars=”false”。不管你是否滑动View，滚动条一直可见。

4.代码第43-116行，都是绘制水平或者垂直滚动条的逻辑。

至此，视图View的整个绘制流程就结束了。最后上一张绘制流程图如下：

绘制Draw总结：

1.View绘制的画布参数canvas是由surface对象获得，言外之意，View视图绘制最终会绘制到Surface对象去。关于Surface内容参考3-1节。

2.由3-2小节我们了解到，父类View绘制主要是绘制背景，边框渐变效果，进度条，View具体的内容绘制调用了onDraw方法，通过该方法把View内容的绘制逻辑留给子类去实现。因此，我们在自定义View的时候都一般都需要重写父类的onDraw方法来实现View内容绘制。

3.不管任何情况，每一个View视图都会绘制 scrollBars滚动条，且绘制滚动条的逻辑是在父类View中实现，子类无需自己实现滚动条的绘制。其实TextView也是有滚动条的，可以通过代码让其显示滚动条和内容滚动效果。你只需在TextView布局设置android:scrollbars=”vertical”属性，且在代码中进行如下设置

<code class="hljs avrasm has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;">textView<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.setMovementMethod</span>(ScrollingMovementMethod<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.getInstance</span>())<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">; </span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul>

这样既可让你的TextView内容可以滑动，且有滚动条。

4.ViewGroup绘制的过程会对每个子视图View设置布局容器动画效果，如果你在ViewGroup容器布局里面设置了如下属性的话：

<code class="hljs avrasm has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-label" style="box-sizing: border-box;">android:</span>animateLayoutChanges=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"true"</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul>

总结：

通过以上分析：视图View的绘制流程基本了解清楚，主要分三个步骤：measure测量，layout布局，draw绘制。当然这三个步骤并不是都要执行，在执行每一步之前都会判断当前视图是否需要重新measure测量，是否需要重新layout布局，是否需要重新draw绘制。其中很多细节性的东西，望有意了解者可以自己跟着源码思路自己分析一遍，以便自己完全理解。