Android View measure （五） 支持margin属性，从一个异常说起

    先来看下代码

一、查看夏目

1. 自定义控件

public class CustomViewGroup extends ViewGroup {    ......    @Override    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {        super.onMeasure(widthMeasureSpec, heightMeasureSpec);        // 遍历所有子视图，进行measure操作        for (int i = 0; i < getChildCount(); i++) {            View child = getChildAt(i);            if (child != null && child.getVisibility() != View.GONE) {                measureChild(child, widthMeasureSpec, heightMeasureSpec);                // 支持子视图设置的android:layout_margin属性                MarginLayoutParams layoutParams = (MarginLayoutParams) child.getLayoutParams();                int marginLeft = layoutParams.leftMargin;            }        }                setMeasuredDimension(widthMeasureSpec, heightMeasureSpec);    }    @Override    protected void onLayout(boolean changed, int left, int top, int right, int bottom) {        for (int i = 0; i < getChildCount(); i++) {            View childView = getChildAt(i);// 只为最简单代码复现BUG，所有子视图都随便放            childView.layout(left, top, left + childView.getMeasuredWidth(), top + childView.getMeasuredHeight());        }    }}

    继承自ViewGroup，主要是演示自定义视图中如何支持margin属性，重点在child.getLayoutParams()一行，接下来看下布局文件中如何使用


2. 布局文件

        <com.example.android.apis.CustomViewGroup            android:id="@+id/custom_view_group"            android:layout_width="match_parent"            android:layout_height="wrap_content" >            <TextView                android:layout_width="wrap_content"                android:layout_height="wrap_content"android:layout_margin="10dip"                android:text="love_world_" />        </com.example.android.apis.CustomViewGroup>

重头戏：android:layout_margin="10dip" margin属性

3. 异常

java.lang.ClassCastException: android.view.ViewGroup$LayoutParams cannot be cast to android.view.ViewGroup$MarginLayoutParamsat com.example.android.apis.CustomViewGroup.onMeasure(CustomViewGroup.java:50)at android.view.View.measure(View.java:16831)at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5245)at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1410)at android.widget.LinearLayout.measureHorizontal(LinearLayout.java:1052)at android.widget.LinearLayout.onMeasure(LinearLayout.java:590)at android.view.View.measure(View.java:16831)at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5245)at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1410)at android.widget.LinearLayout.measureVertical(LinearLayout.java:695)at android.widget.LinearLayout.onMeasure(LinearLayout.java:588)at android.view.View.measure(View.java:16831)at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5245)at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)at android.view.View.measure(View.java:16831)at android.widget.LinearLayout.measureVertical(LinearLayout.java:847)at android.widget.LinearLayout.onMeasure(LinearLayout.java:588)at android.view.View.measure(View.java:16831)at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5245)at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2586)at android.view.View.measure(View.java:16831)at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2189)at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1352)at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1535)at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1249)at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:6364)at android.view.Choreographer$CallbackRecord.run(Choreographer.java:791)at android.view.Choreographer.doCallbacks(Choreographer.java:591)at android.view.Choreographer.doFrame(Choreographer.java:561)at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:777)at android.os.Handler.handleCallback(Handler.java:730)at android.os.Handler.dispatchMessage(Handler.java:92)at android.os.Looper.loop(Looper.java:176)at android.app.ActivityThread.main(ActivityThread.java:5419)at java.lang.reflect.Method.invokeNative(Native Method)at java.lang.reflect.Method.invoke(Method.java:525)at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1209)at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1025)at dalvik.system.NativeStart.main(Native Method)

出现以上异常的原因，LayoutParams从哪里来的？
视图的加载有两种方式一种是代码addView 一种是inflate 。

1. inflate 方法加载并添加LayoutParams
Android中measure过程、WRAP_CONTENT详解以及xml布局文件解析流程浅析(上)

2. 以下演示addView方式添加LayoutParams

public class MainActivity extends Activity {    @Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);CustomViewGroup customViewGroup = (CustomViewGroup) findViewById(R.id.custom_view_group);customViewGroup.addView(createTextView("Love_world_"));}    private View createTextView(String value) {        TextView textView = new TextView(this);          textView.setText("a child view");        textView.setText(value);        textView.setLayoutParams(new ViewGroup.MarginLayoutParams(LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT));        return textView;    }}

2. ViewGroup.addView源码，查找何处添加LayoutParams

public abstract class ViewGroup extends View implements ViewParent, ViewManager {    public void addView(View child) {        addView(child, -1);    }    public void addView(View child, int index) {        LayoutParams params = child.getLayoutParams();// 子视图LayoutParams为为空是处理方式        if (params == null) {            params = generateDefaultLayoutParams();            if (params == null) {                throw new IllegalArgumentException("generateDefaultLayoutParams() cannot return null");            }        }        addView(child, index, params);    }    public void addView(View child, int width, int height) {        final LayoutParams params = generateDefaultLayoutParams();        params.width = width;        params.height = height;        addView(child, -1, params);    }    public void addView(View child, LayoutParams params) {        addView(child, -1, params);    }    public void addView(View child, int index, LayoutParams params) {        if (DBG) {            System.out.println(this + " addView");        }        // addViewInner() will call child.requestLayout() when setting the new LayoutParams        // therefore, we call requestLayout() on ourselves before, so that the child's request        // will be blocked at our level        requestLayout();        invalidate();        addViewInner(child, index, params, false);    }// 子视图默认LayoutParams实例protected LayoutParams generateDefaultLayoutParams() {        return new LayoutParams(LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT);    }// 定义LayoutParams类public static class LayoutParams {public int width;public int height;public LayoutParams(int width, int height) {            this.width = width;            this.height = height;        }}}

以下是关键

public abstract class ViewGroup extends View implements ViewParent, ViewManager {    private void addViewInner(View child, int index, LayoutParams params,            boolean preventRequestLayout) {        if (child.getParent() != null) {            throw new IllegalStateException("The specified child already has a parent. " +                    "You must call removeView() on the child's parent first.");        }        if (!checkLayoutParams(params)) {            params = generateLayoutParams(params);        }        if (preventRequestLayout) {            child.mLayoutParams = params;        } else {            child.setLayoutParams(params);        }        ......    }protected boolean checkLayoutParams(ViewGroup.LayoutParams p) {        return  p != null;    }}

   通过查看以上addView添加LayoutParams代码可以发现解决方案，复写这些函数，创建当前自定义视图的LayoutParams继承自MarginLayoutParams。

public class CustomViewGroup extends ViewGroup {@Override    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {......}@Override    protected void onLayout(boolean changed, int left, int top, int right, int bottom) {......}    @Override    protected LayoutParams generateLayoutParams(ViewGroup.LayoutParams p) {        return new LayoutParams(p);    }        @Override    protected LayoutParams generateDefaultLayoutParams() {        return new LayoutParams(LayoutParams.MATCH_PARENT, LayoutParams.WRAP_CONTENT);    }            @Override    public LayoutParams generateLayoutParams(AttributeSet attrs) {        return new LayoutParams(getContext(), attrs);    }        // 继承自margin，支持子视图android:layout_margin属性    public static class LayoutParams extends MarginLayoutParams {        public LayoutParams(Context c, AttributeSet attrs) {            super(c, attrs);        }        public LayoutParams(int width, int height) {            super(width, height);        }        public LayoutParams(ViewGroup.LayoutParams source) {            super(source);        }        public LayoutParams(ViewGroup.MarginLayoutParams source) {            super(source);        }    }}