Android - View 事件分发机制

1、事件调用顺序

以Button为例，以下分别为 onClick、onTouch　

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button.setOnClickListener(new OnClickListener() {      @Override      public void onClick(View v) {          Log.d("TAG", "onClick execute");      }  });

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button.setOnTouchListener(new OnTouchListener() {      @Override      public boolean onTouch(View v, MotionEvent event) {          Log.d("TAG", "onTouch execute, action " + event.getAction());          return false;      }  });

　　调用顺序：onTouch->onClick, 并且OnTouch调用两次(ACTION_DOWN、ACTION_UP)

　　onTouch有返回值，true表示事件被onTouch消费掉了，不会继续向下传递；

2、控件的dispatchTouchEvent方法

　　Button控件的dispatchTouchEvent继承于View （Button->TextView->View）

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public boolean dispatchTouchEvent(MotionEvent event) {      if (mOnTouchListener != null && (mViewFlags & ENABLED_MASK) == ENABLED &&              mOnTouchListener.onTouch(this, event)) {          return true;      }      return onTouchEvent(event);  }

　　如果onTouchListener监听器不为空， 当前点击的控件是enable的，mOnTouchListener.onTouch(this, event)返回TRUE，就返回TRUE，否则就去执行onTouchEvent(event)方法并返回；

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public boolean onTouchEvent(MotionEvent event) {      final int viewFlags = mViewFlags;      if ((viewFlags & ENABLED_MASK) == DISABLED) {          // A disabled view that is clickable still consumes the touch          // events, it just doesn't respond to them.          return (((viewFlags & CLICKABLE) == CLICKABLE ||                  (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE));      }      if (mTouchDelegate != null) {          if (mTouchDelegate.onTouchEvent(event)) {              return true;          }      }      if (((viewFlags & CLICKABLE) == CLICKABLE ||              (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)) {          switch (event.getAction()) {              case MotionEvent.ACTION_UP:                  boolean prepressed = (mPrivateFlags & PREPRESSED) != 0;                  if ((mPrivateFlags & PRESSED) != 0 || prepressed) {                      // take focus if we don't have it already and we should in                      // touch mode.                      boolean focusTaken = false;                      if (isFocusable() && isFocusableInTouchMode() && !isFocused()) {                          focusTaken = requestFocus();                      }                      if (!mHasPerformedLongPress) {                          // This is a tap, so remove the longpress check                          removeLongPressCallback();                          // Only perform take click actions if we were in the pressed state                          if (!focusTaken) {                              // Use a Runnable and post this rather than calling                              // performClick directly. This lets other visual state                              // of the view update before click actions start.                              if (mPerformClick == null) {                                  mPerformClick = new PerformClick();                              }                              if (!post(mPerformClick)) {                                  performClick();                              }                          }                      }                      if (mUnsetPressedState == null) {                          mUnsetPressedState = new UnsetPressedState();                      }                      if (prepressed) {                          mPrivateFlags |= PRESSED;                          refreshDrawableState();                          postDelayed(mUnsetPressedState,                                  ViewConfiguration.getPressedStateDuration());                      } else if (!post(mUnsetPressedState)) {                          // If the post failed, unpress right now                          mUnsetPressedState.run();                      }                      removeTapCallback();                  }                  break;              case MotionEvent.ACTION_DOWN:                  if (mPendingCheckForTap == null) {                      mPendingCheckForTap = new CheckForTap();                  }                  mPrivateFlags |= PREPRESSED;                  mHasPerformedLongPress = false;                  postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout());                  break;              case MotionEvent.ACTION_CANCEL:                  mPrivateFlags &= ~PRESSED;                  refreshDrawableState();                  removeTapCallback();                  break;              case MotionEvent.ACTION_MOVE:                  final int x = (int) event.getX();                  final int y = (int) event.getY();                  // Be lenient about moving outside of buttons                  int slop = mTouchSlop;                  if ((x < 0 - slop) || (x >= getWidth() + slop) ||                          (y < 0 - slop) || (y >= getHeight() + slop)) {                      // Outside button                      removeTapCallback();                      if ((mPrivateFlags & PRESSED) != 0) {                          // Remove any future long press/tap checks                          removeLongPressCallback();                          // Need to switch from pressed to not pressed                          mPrivateFlags &= ~PRESSED;                          refreshDrawableState();                      }                  }                  break;          }          return true;      }      return false;  }

　　首先在第14行我们可以看出，如果该控件是可以点击的就会进入到第16行的switch判断中去，而如果当前的事件是抬起手指，则会进入到MotionEvent.ACTION_UP这个case当中。

　　在经过种种判断之后，会执行到第38行的performClick()方法： mOnClickListener.onClick(this) 就是在此时被调用；

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public boolean performClick() {      sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_CLICKED);      if (mOnClickListener != null) {          playSoundEffect(SoundEffectConstants.CLICK);          mOnClickListener.onClick(this);          return true;      }      return false;  }

　　3、Touch事件的层级传递

　　我们都知道如果给一个控件注册了touch事件，每次点击它的时候都会触发一系列的ACTION_DOWN，ACTION_MOVE，ACTION_UP等事件。

　　这里需要注意，如果处理ACTION_DOWN时，dispatchTouchEvent返回FALSE，后面一系列其它的action就不会再得到处理。总之，当dispatchTouchEvent在进行事件分发的时候，只有前一个action返回true，才会触发后一个action。

 　  参考前面的源码，首先在OnTouchListener之onTouch事件里返回了false，代码执行进入到控件onTouchEvent方法中，然后我们来看一下onTouchEvent方法的细节。

　　由于我们点击了按钮，代码执行进入到第14行的IF分支，不管action类型，最终都会返回一个true。如果将Button换成ImageView，因为ImageView默认不可点击，onTouchEvent中代码执行进入else分支，dispatchTouchEvent返回false；

　　4、总结

　　  OnTouchListener之OnTouch事件，返回False，代码执行进入控件OnTouchEvent方法；

　　  控件OnTouchEvent方法中，返回True，才会继续触发后面的action(ACTION_DOWN,ACTION_MOVE,ACTION_UP)

　　  比如点击事件,先响应ACTION_DOWN,返回true,然后手抬起,又从dispatchTouchEvent()分发,再响应ACTION_UP

　　

　参考链接：简书 http://www.jianshu.com/p/b1ee0985da16

　　　　　　郭神 http://blog.csdn.net/guolin_blog/article/details/9097463