View的工作原理（一）——从ViewRoot和DecorView说起

来源：互联网发布：大学java专业入门课程编辑：程序博客网时间：2024/04/30 08:22

前言

本文参考《Android开发艺术与探索》第四章内容及网上几篇博客，里面融入笔者的个人理解。希望能对大家理解View有所帮助。

基本概念介绍

介绍View的工作原理之前我们首先要理解DecorView和ViewRoot两个概念：

1、DecorView

DecorView是Windows中的View的最顶层View。我们可以根据下面一副图来认识它：

由这幅图我们可以看到，其实DecorView是一个FrameLayout，里面是一个垂直的线性布局，在线性布局中分上下两部分FrameLayout,上面一部分是TitleBar，下面是android.R.id.content,我们平常的setContentView就是将布局加载在android.R.id.content中。

2、ViewRoot

ViewRoot是连接WindowsManager和DecorView的桥梁对应于ViewRootImpl。

View的绘制流程就是从ViewRootImpl的performTraversala（）方法开始的，包含三大流程：

1、Measure（）：［测量流程］

2、Layout（）：［布局流程］

3、Draw（）：［绘制流程］

这三大流程也就是View绘制的三大流程。我们可以通过下面两幅图来理解performTrarsala（）方法；

什么是MeasureSpec

MeasureSpec 从名字上来看看起来是“测量规格“或是”测量说明书“。大致意思就是决定View的Measure过程。我们可以这样来理解：”MeasureSpec在很大程度上决定了一个View的尺寸规格，之所以这样说是很大程度上是因为这个过程还受父容器的影响，因为父容器影响View的MeasureSpec的创建过程。“

1、MeasureSpec

MeasureSpec代表一个32位int值，高2位代表SPecMode（测量模式），低30位代表SpecSize（在某种测量模式下的规格大小）。

SpecMode有三类如下所示：

UNSPECIFIED（未指定模式）

父容器不对View有任何限制，要多大给多大。

EXACTLY（确定模式）

父容器已检测出View所需要的精确大小，这时候View的最终大小就是SpecSize指定的值，它对应于LayoutParams中的match_parent和具体指定的数值这两种模式。

AT_MOST（最多模式）

父容器指定一个可用大小即SpecSize，View的代销不能大于这个值。对应于wrap_content。

2、MeasureSPec 和LayoutParams的对应关系

为什么要说MeasureSpec和LayoutParams对应昵？那是因为View在测量的时候会将我们设置的LayoutParams在父容器的约束条件下转换成MeasureSpec，然后再根据MeasureSpec来确定View测量后的宽和高。

有些读者会问：那么顶级View（DecorView）怎么转换昵？

对于顶级View：MeasureSpec有窗口尺寸和自身的LayoutParams来共同决定。

对于普通View：由父容器的MeasureSpec和自身的LayoutParams共同来决定。

通过下面一段代码我们来理解DecorView的创建MeasureSpec过程，desiredWindowWidth和desiredWindowHeight是屏幕尺寸：

childWidthMeasureSpec = getRootMeasureSpec(desiredWindowWidth, lp.width);childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height);host.measure(childWidthMeasureSpec, childHeightMeasureSpec);

我们来再看看getRootMeasureSpec方法：

private int getRootMeasureSpec(int windowSize, int rootDimension) {        int measureSpec;        switch (rootDimension) {        case ViewGroup.LayoutParams.MATCH_PARENT:            // Window can't resize. Force root view to be windowSize.            measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);            break;        case ViewGroup.LayoutParams.WRAP_CONTENT:            // Window can resize. Set max size for root view.            measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);            break;        default:            // Window wants to be an exact size. Force root view to be that size.            measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);            break;        }        return measureSpec;    }

在android.view.ViewRootImpl 中可以看到其对应关系LayoutParams 中这三个值在内部有个对应关系，那就是

LayoutParams.MATCH_PARENT 对应 MeasureSpec.EXACTLY

.LayoutParams.WRAP_CONTENT对应 MeasureSpec.AT_MOST

默认值（也就是具体值）对应 MeasureSpec.EXACTLY

也就是内部只有两种模式 EXACTLY 精确模式和 AT_MOST 最大模式！

对于普通View它的measure是由ViewGroup传递而来，我们看一下ViewGroup的measureChildWidthMargins方法:

/**     * Ask one of the children of this view to measure itself, taking into     * account both the MeasureSpec requirements for this view and its padding     * and margins. The child must have MarginLayoutParams The heavy lifting is     * done in getChildMeasureSpec.     *     * @param child The child to measure     * @param parentWidthMeasureSpec The width requirements for this view     * @param widthUsed Extra space that has been used up by the parent     *        horizontally (possibly by other children of the parent)     * @param parentHeightMeasureSpec The height requirements for this view     * @param heightUsed Extra space that has been used up by the parent     *        vertically (possibly by other children of the 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 // 考虑上parent的padding和child的margin                        + widthUsed, lp.width);        final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,                mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin                        + heightUsed, lp.height);        // 这里如果child是个ViewGroup类型，则实际会递归下去。。。        child.measure(childWidthMeasureSpec, childHeightMeasureSpec);    }

我们可以清楚的看到子View的MeasureSpec创建与父容器的MeasureSpec和子元素本身的LayoutParams有关，此外还和VIew的margin 和 padding 有关，我们来看下ViewGroup的getChildMeasureSpec:

/**     * Does the hard part of measureChildren: figuring out the MeasureSpec to     * pass to a particular child. This method figures out the right MeasureSpec     * for one dimension (height or width) of one child view.     *     * The goal is to combine information from our MeasureSpec with the     * LayoutParams of the child to get the best possible results. For example,     * if the this view knows its size (because its MeasureSpec has a mode of     * EXACTLY), and the child has indicated in its LayoutParams that it wants     * to be the same size as the parent, the parent should ask the child to     * layout given an exact size.     *     * @param spec The requirements for this view     * @param padding The padding of this view for the current dimension and     *        margins, if applicable     * @param childDimension How big the child wants to be in the current     *        dimension     * @return a MeasureSpec integer for the child     */     public static int getChildMeasureSpec(int spec, int padding, int childDimension) {        // 这个方法是协商型的，最终结果既可能直接由spec（parent提供的），也可能由childDimension决定        // 所以我们知道了，一个View的大小不是简单的单方面决定的，而是通过一系列条件协商的结果，        // 有时会尊重parent的spec，有时会坚持自己的dimension要求        int specMode = MeasureSpec.getMode(spec);        int specSize = MeasureSpec.getSize(spec);        int size = Math.max(0, specSize - padding); // 可用的大小        int resultSize = 0;        int resultMode = 0;        switch (specMode) {        // Parent has imposed an exact size on us        case MeasureSpec.EXACTLY: // parent说child你应该是个确定的大小            if (childDimension >= 0) { // child正好设置了确定的大小                resultSize = childDimension; // 让child是那个确定的大小                resultMode = MeasureSpec.EXACTLY; // 设置mode为EXACTLY            } else if (childDimension == LayoutParams.MATCH_PARENT) {                // Child wants to be our size. So be it.                resultSize = size; // 其他情况下都是parent spec中的大小，只是mode不同                resultMode = MeasureSpec.EXACTLY;            } else if (childDimension == LayoutParams.WRAP_CONTENT) {                // Child wants to determine its own size. It can't be                // bigger than us.                resultSize = size; // 不能超过size                resultMode = MeasureSpec.AT_MOST;            }            break;        // Parent has imposed a maximum size on us        case MeasureSpec.AT_MOST: // parent说child你应该最大是某个值。。。            if (childDimension >= 0) { // child指定确定值了，则听child的                // Child wants a specific size... so be it                resultSize = childDimension;                resultMode = MeasureSpec.EXACTLY;            } else if (childDimension == LayoutParams.MATCH_PARENT) {                // Child wants to be our size, but our size is not fixed.                // Constrain child to not be bigger than us.                resultSize = size;                resultMode = MeasureSpec.AT_MOST;            } else if (childDimension == LayoutParams.WRAP_CONTENT) {                // Child wants to determine its own size. It can't be                // bigger than us.                resultSize = size;                resultMode = MeasureSpec.AT_MOST;            }            break;        // Parent asked to see how big we want to be        case MeasureSpec.UNSPECIFIED: // parent没对child的大小有啥要求            if (childDimension >= 0) { // child指定了确定的值，听child的                // Child wants a specific size... let him have it                resultSize = childDimension;                resultMode = MeasureSpec.EXACTLY;            } else if (childDimension == LayoutParams.MATCH_PARENT) {                // Child wants to be our size... find out how big it should                // be                resultSize = 0;                resultMode = MeasureSpec.UNSPECIFIED;            } else if (childDimension == LayoutParams.WRAP_CONTENT) {                // Child wants to determine its own size.... find out how                // big it should be                resultSize = 0;                resultMode = MeasureSpec.UNSPECIFIED;            }            break;        }        return MeasureSpec.makeMeasureSpec(resultSize, resultMode);    }

从上述方法中我们不难理解，子View的MeasureSpec创建与父容器的MeasureSpec和子元素本身的LayoutParams有关。

下面一张表格对MeasureSpec和LayoutParams对应做个总结：

普通View的MeasureSpec创建规则

parentSpecMode

childLayoutParams

EXACTLY

AT_MOST

UNSPECIFIED

dp/px

EXACTLY

chiladSize

EXACTLY

chiladSize

EXACTLY

childSize

match_parent

EXACTLY

parentSize

AT_MOST

parentSize

UNSPECIFIED

wrap_content

EXACTLY

parentSize

AT_MOST

parentSize

UNSPECIFIED

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