事件冲突原理和解决方法

1、MotionEvent事件类型

ACTION_DOWN：手指初次触摸屏幕时触发
ACTION_MOVE：手指在屏幕上滑动时触发，会多次触发
ACTION_UP：手指离开屏幕时触发
ACTION_CANCEL：事件被上层拦截时触发

2、事件分发流程

  Activity#dispatchTouchEvent()
> PhoneWindow#superDispatchTouchEvent()
> DecorView#superDispatchTouchEvent()
> ViewGroup#dispatchTouchEvent()
> View#dispatchTouchEvent()
> View#onTouchEvent()

3、onTouch和onClick冲突

当我们setOnTouchListener且在onTouch()返回true表示事件被消费， setOnClickListener的onClick不会执行

//View.java
public boolean dispatchTouchEvent(MotionEvent event) {
  //...
  boolean result = false;
  ListenerInfo li = mListenerInfo;
  if (li != null && li.mOnTouchListener != null
      && (mViewFlags & ENABLED_MASK) == ENABLED
      && li.mOnTouchListener.onTouch(this, event)) {
    result = true;
  }

  if (!result && onTouchEvent(event)) {
    result = true;
  }
  //...
  return result;
}

public boolean onTouchEvent(MotionEvent event) {
  switch (action) {
    case MotionEvent.ACTION_UP:
      performClickInternal();
      break;
    //...
  }
  //...
}

private boolean performClickInternal() {
  return performClick();
}

public boolean performClick() {
  final boolean result;
  final ListenerInfo li = mListenerInfo;
  if (li != null && li.mOnClickListener != null) {
    playSoundEffect(SoundEffectConstants.CLICK);
    //回调onClick
    li.mOnClickListener.onClick(this);
    result = true;
  } else {
    result = false;
  }
  return result;
}

我们设置了setOnClickListener/setOnTouchListener，mListenerInfo就不会为空，如果li.mOnTouchListener.onTouch(this, event)为true，则result为true，!result短路就不会执行onTouchEvent(event)方法，而这个方法在ACTION_UP中回调onClick。

4、DOWN事件分析

事件都是以DOWN开始，UP事件结束，中间多个MOVE事件，所以我们先从DOWN事件开始分析

//ViewGroup.java
@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
  //...
  if (actionMasked == MotionEvent.ACTION_DOWN) {
    // due to an app switch, ANR, or some other state change.
    cancelAndClearTouchTargets(ev);
    //①重置mGroupFlags
    resetTouchState();
  }

  // Check for interception.
  final boolean intercepted;
  if (actionMasked == MotionEvent.ACTION_DOWN
      || mFirstTouchTarget != null) {
    //②判断事件是否拦截
    //计算：mGroupFlags&~FLAG_DISALLOW_INTERCEPT&FLAG_DISALLOW_INTERCEPT=0
    //所以disallowIntercept为false，会执行onInterceptTouchEvent来判断是否拦截
    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;
  }
  //...
}

DOWN事件进来，会先重置mGroupFlags变量，计算得到disallowIntercept恒为false，进入onInterceptTouchEvent来判断是否拦截。

看下requestDisallowInterceptTouchEvent，表示请求父类不要拦截事件

//ViewGroup.java
@Override
public void requestDisallowInterceptTouchEvent(boolean disallowIntercept) {

  if (disallowIntercept == ((mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0)) {
    // We're already in this state, assume our ancestors are too
    return;
  }

  if (disallowIntercept) {
    mGroupFlags |= FLAG_DISALLOW_INTERCEPT;
  } else {
    mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;
  }

  // Pass it up to our parent
  if (mParent != null) {
    mParent.requestDisallowInterceptTouchEvent(disallowIntercept);
  }
}

当View设置requestDisallowInterceptTouchEvent为true的时候，mGroupFlags=mGroupFlags|FLAG_DISALLOW_INTERCEPT，使得 disallowIntercept=mGroupFlags|FLAG_DISALLOW_INTERCEPT & FLAG_DISALLOW_INTERCEPT != 0，计算得到disallowIntercept恒为true，不会执行onInterceptTouchEvent，父View也不会拦截子View。

但是当前事件是DOWN事件，requestDisallowInterceptTouchEvent是无效的。会调用resetTouchState重置mGroupFlags使得disallowIntercept为false，进入onInterceptTouchEvent来判断是否拦截

**1）未被拦截：**intercepted为false，事件分发给子View处理

//事件分发流程
if (!canceled && !intercepted) {
  // Scan children from front to back.
  //对子View进行排序
  final ArrayList<View> preorderedList = buildTouchDispatchChildList();
  for (int i = childrenCount - 1; i >= 0; i--) {
    //遍历拿到child
    final View child = getAndVerifyPreorderedView(preorderedList, children, childIndex);
    //判断child的位置是否是点击事件
     if (!child.canReceivePointerEvents()
         || !isTransformedTouchPointInView(x, y, child, null)) {
       ev.setTargetAccessibilityFocus(false);
       //不符合取下一个child
       continue;
     }
     //分发给子View处理事件
     if(dispatchTransformedTouchEvent(ev, false, child, idBitsToAssiif(){
       //如果子View处理了事件，会执行addTouchTarget，mFirstTouchTarget就会被赋值
       newTouchTarget = addTouchTarget(child, idBitsToAssign);
       //alreadyDispatchedToNewTouchTarget赋值为true
       alreadyDispatchedToNewTouchTarget = true;
       break; 
     }
  } 
}

private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,
            View child, int desiredPointerIdBits) {
   final boolean handled;
   if (child == null) {
     handled = super.dispatchTouchEvent(transformedEvent);
   } else {
     //child去处理事件
     handled = child.dispatchTouchEvent(transformedEvent);
   }
   return handled;
 }

**2）被拦截：**intercepted为true，会跳过if (!canceled && !intercepted)

if (mFirstTouchTarget == null) {
  // No touch targets so treat this as an ordinary view.
  //当事件为down 我们拦截事件时候
  handled = dispatchTransformedTouchEvent(ev, canceled, null,
      TouchTarget.ALL_POINTER_IDS);
} else {
  // Dispatch to touch targets, excluding the new touch target if we already
  // dispatched to it.  Cancel touch targets if necessary.
  //onInterceptTouchEvent返回false
  TouchTarget predecessor = null;
  TouchTarget target = mFirstTouchTarget;
  while (target != null) {
    //down事件next为空，具体处理在addTouchTarget方法中
    final TouchTarget next = target.next;
    if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) {
      ////down事件这两个条件都满足，到此就结束了
      handled = true;
    } else {
      //move事件进来alreadyDispatchedToNewTouchTarget会被设置为false
      final boolean cancelChild = resetCancelNextUpFlag(target.child)
        || intercepted;
      //dispatchTransformedTouchEvent会调到childView里去，所以parentView滑动无效
      if (dispatchTransformedTouchEvent(ev, cancelChild,
        target.child, target.pointerIdBits)) {
        handled = true;
      }
      if (cancelChild) {
        if (predecessor == null) {
          mFirstTouchTarget = next;
        } else {
          predecessor.next = next;
        }
        target.recycle();
        target = next;
        continue;
      }
    }
    predecessor = target;
    target = next;
  }
}

首次事件处理，mFirstTouchTarget==null，调用dispatchTransformedTouchEvent，传入的child为null，调用super.dispatchTouchEvent(transformedEvent)进入View的dispatchTouchEvent方法。

完成了DOWN事件分发和处理后，再次分发MOVE事件，还是从ViewGroup的dispatchTouchEvent方法开始，而MOVE事件是可以反复调用此方法的，MOVE事件不进行分发，直接找DOWN事件确定的对象。

5、冲突解决方式

1）外部拦截法 （在ViewGroup中对事件进行拦截）
重写parentView中的onInterceptTouchEvent

@Override
public boolean onInterceptTouchEvent(MotionEvent event) {
  switch (event.getAction()) {
    case MotionEvent.ACTION_DOWN: {
      break;
    }
    case MotionEvent.ACTION_MOVE: {
      if (parentView需要此事件) {
        //进行拦截
        return true;
      }
      break;
    }
    case MotionEvent.ACTION_UP: {
      break;
    }
    default:
      break;
  }
  return super.onInterceptTouchEvent(event);
}

2）内部拦截法（ViewGroup不拦截，子View需要事件就消耗掉，否则交给父View处理）
重写parentView的onInterceptTouchEvent

@Override
public boolean onInterceptTouchEvent(MotionEvent event) {
  //不拦截down事件
  if (event.getAction() == MotionEvent.ACTION_DOWN) {
    super.onInterceptTouchEvent(event);
    return false;
  }
  return true;
}

重写childView的dispatchTouchEvent

@Override
public boolean dispatchTouchEvent(MotionEvent event) {
  switch (event.getAction()){
    case MotionEvent.ACTION_DOWN:
      getParent().requestDisallowInterceptTouchEvent(true);
      break;
    case MotionEvent.ACTION_MOVE:
      if(parentView需要此事件){
        getParent().requestDisallowInterceptTouchEvent(false);
      }
      break;
    case MotionEvent.ACTION_UP:
      break;
  }
  return super.dispatchTouchEvent(event);
}