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
java.lang.ClassCastException: android.view.ViewGroup$LayoutParams cannot be cast to android.view.ViewGroup$MarginLayoutParams at 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)
public class MainActivity extends Activity { @Override protected 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); } }}