先来看下代码
一、查看夏目
1. 自定义控件public class CustomViewGroup extends ViewGroup {……@Overrideprotected 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);}@Overrideprotected 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.CustomViewGroupandroid:id="@+id/custom_view_group"android:layout_width="match_parent"android:layout_height="wrap_content" ><TextViewandroid: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LayoutParamsAndroid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LayoutParamspublic 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;}}初初尝试着拥抱的人，一派新鲜幸福都来不及沉浸，
