这是学习《android开发艺术探究》的笔记，如果有什么不对的地方，欢迎指出。

1.验证P143第三点

验证 某个ViewGroup一旦决定拦截事件,那么这一个事件序列的其他事件只能由它来处理，并且它的onInterceptTouchEvent不会再被调用(事件直接由onTouchEvent处理)。

情景1. onInterceptTouchEvent始终拦截事件（补充：ViewGroup默认是不拦截事件的）

public class MyViewGroup extends FrameLayout {
 private static final String TAG = "MyViewGroup";


 @Override
 public boolean onInterceptTouchEvent(MotionEvent ev) {
     int action = ev.getAction();
    String ActionStr = Util.getActionStr(action);
     boolean consume;
     consume = true;
     Log.i(TAG, "onInterceptTouchEvent"+(consume?"拦截":"不拦截") + ActionStr);
     return consume;
 }

 @Override
 public boolean onTouchEvent(MotionEvent event) {
     int action = event.getAction();
       String ActionStr = Util.getActionStr(action);
     Log.i(TAG, "onTouchEvent: " + ActionStr);
     return true;
 }
}

结果：ACTOIN_DOWN被拦截后，后续事件直接交由onTouchEvent处理，没有经过onInterceptTouchEvent。

onInterceptTouchEvent拦截ACTION_DOWN
onTouchEvent: ACTION_DOWN
onTouchEvent: ACTION_MOVE
onTouchEvent: ACTION_MOVE
onTouchEvent: ACTION_MOVE
onTouchEvent: ACTION_MOVE
onTouchEvent: ACTION_MOVE
onTouchEvent: ACTION_MOVE
onTouchEvent: ACTION_MOVE
onTouchEvent: ACTION_MOVE
onTouchEvent: ACTION_MOVE
onTouchEvent: ACTION_UP

情景2. onInterceptTouchEvent不拦截ACTION_DOWN,拦截ACTION_MOVE

public class MyViewGroup extends FrameLayout {

  @Override
  public boolean onInterceptTouchEvent(MotionEvent ev) {
      int action = ev.getAction();
      String ActionStr = Util.getActionStr(action);
      boolean consume;
      if (action == MotionEvent.ACTION_DOWN) {
          consume = false;
      } else {
          consume = true;
      }
      Log.i(TAG, "onInterceptTouchEvent"+(consume?"拦截":"不拦截") + ActionStr);
      return consume;
  }
  .....
}

结果：ACTOIN_MOVE被拦截后，后续事件直接交由onTouchEvent处理，没有经过onInterceptTouchEvent。

onInterceptTouchEvent不拦截ACTION_DOWN
onInterceptTouchEvent拦截ACTION_MOVE
onTouchEvent: ACTION_MOVE
onTouchEvent: ACTION_MOVE
onTouchEvent: ACTION_MOVE
onTouchEvent: ACTION_MOVE
onTouchEvent: ACTION_MOVE
onTouchEvent: ACTION_MOVE
onTouchEvent: ACTION_UP

2.源码分析

    final boolean intercepted;
 if (actionMasked == MotionEvent.ACTION_DOWN
   || mFirstTouchTarget != null) {
         final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
         if (!disallowIntercept) {
                intercepted = onInterceptTouchEvent(ev);
                ev.setAction(action); // restore action in case it was changed
            } else {
                intercepted = false;
            }
 } else {
    // There are no touch targets and this action is not an initial down
    // so this view group continues to intercept touches.
    intercepted = true;
}

2.1 关于源码的几点说明

1. actionMasked == MotionEvent.ACTION_DOWN的含义
   按下动作，即事件序列的起始动作
2. mFirstTouchTarget != null的含义
   mFirsetTouchTarget表示的是处理事件的子元素
   mFirstTouchTarget != null表示有子元素处理事件
   mFirstTouchTarget == null表示没有子元素处理事件
3. actionMasked == MotionEvent.ACTION_DOWN
   || mFirstTouchTarget != null 的含义
   事件序列的起始动作或者已有子元素处理事件（注意，一旦ViewGroup拦截了事件，mFirstTouchTarget会被清空）

2.2 源码逻辑

如果是事件序列的起始动作或者已有子元素在处理事件，则调用onInterceptionTouchEvent判断是否拦截事件,否则默认拦截该事件

而如果"某个ViewGroup一旦决定拦截事件"则

1. 后续事件不可能是ACTION_DOWN 即 actionMasked == MotionEvent.ACTION_DOWN不成立

2. mFirstTouchTarget也会被置空,即mFirstTouchTarget！=null不成立（下面这段代码会将mFirstTouchTarget置空）

              TouchTarget predecessor = null;
              TouchTarget target = mFirstTouchTarget;
               while (target != null) {
                   final TouchTarget next = target.next;
                   if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) {
                       handled = true;
                   } else {
                       final boolean cancelChild = resetCancelNextUpFlag(target.child)
                               || intercepted;//拦截时，cancelChild为true
                       if (dispatchTransformedTouchEvent(ev, cancelChild,
                               target.child, target.pointerIdBits)) {
                           handled = true;
                       }
                       if (cancelChild) {
                           if (predecessor == null) {
                               mFirstTouchTarget = next;//会将mFirsrtTouchTarget置空
                           } else {
                               predecessor.next = next;
                           }
                           target.recycle();
                           target = next;
                           continue;
                       }
                   }
                   predecessor = target;
                   target = next;
               }
           }
   

所以actionMasked == MotionEvent.ACTION_DOWN|| mFirstTouchTarget != null不成立，onInterceptTouchEvent不会被调用，intercept直接赋值为true,所以书中原文才这样描述

某个ViewGroup一旦决定拦截事件,那么这一个事件序列的其他事件只能由它来处理，并且它的onInterceptTouchEvent不会再被调用