Android事件分发之ViewGroup（二）

ViewGroup触摸事件解析

最新推荐文章于 2025-12-09 16:49:27 发布

原创最新推荐文章于 2025-12-09 16:49:27 发布 · 343 阅读

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#事件分发 #android #android事件分发机制

本文详细解析了ViewGroup中dispatchTouchEvent方法的工作原理，包括ACTION_DOWN、ACTION_MOVE和ACTION_UP等不同阶段的行为，以及如何在ViewGroup中进行事件拦截。

ViewGroup的dispatchTouchEvent事件总线路：

	public boolean dispatchTouchEvent(MotionEvent ev) {
		 调用onInterceptTouchEvent检查是否拦截事件
		 if(没有拦截){
			 在ViewGroup中遍历查找目前是点击了哪个子视图
			 if(找到了){
				 调用该子视图的dispatchTouchEvent，递归下去
			 }else{
				 没找到，则将事件传给onTouchListener，没有Listener则传给onTouchEvent()
				 如果再listener或者onTouchEvent()中down事件返回了true，代表事件被消费，后续的move和up都被Listener或者onTouchEvent()处理，
				 如果down事件返回false，则后续的move，up事件将不会到这一层的Viewgroup，而直接在上一层视图被消费。
			 } 
		 }else{
			 事件被拦截了，原本被点击的子视图将接收到一个ACTION_CANCEL事件，而down事件传给onTouchListener，没有Listener则传给onTouchEvent()，依然遵从上面的down和move,up事件的关系
		 }
	}

ViewGroup的onInterceptTouchEvent方法默认返回的是false,表示当前ViewGroup不对事件进行拦截；返回true表示对事件进行拦截，外面一层的子View将不会得到该事件的响应。

1.ViewGroup的dispatchTouchEvent中的ACTION_DOWN

 @Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
        if (!onFilterTouchEventForSecurity(ev)) {
            return false;
        }
        final int action = ev.getAction();
        final float xf = ev.getX();
        final float yf = ev.getY();
        final float scrolledXFloat = xf + mScrollX;
        final float scrolledYFloat = yf + mScrollY;
        final Rect frame = mTempRect;
        boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
        if (action == MotionEvent.ACTION_DOWN) {
            if (mMotionTarget != null) {
                // this is weird, we got a pen down, but we thought it was
                // already down!
                // XXX: We should probably send an ACTION_UP to the current
                // target.
                mMotionTarget = null;
            }
            // If we're disallowing intercept or if we're allowing and we didn't
            // intercept
            if (disallowIntercept || !onInterceptTouchEvent(ev)) {
                // reset this event's action (just to protect ourselves)
                ev.setAction(MotionEvent.ACTION_DOWN);
                // We know we want to dispatch the event down, find a child
                // who can handle it, start with the front-most child.
                final int scrolledXInt = (int) scrolledXFloat;
                final int scrolledYInt = (int) scrolledYFloat;
                final View[] children = mChildren;
                final int count = mChildrenCount;
                for (int i = count - 1; i >= 0; i--) {
                    final View child = children[i];
                    if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE
                            || child.getAnimation() != null) {
                        child.getHitRect(frame);
                        if (frame.contains(scrolledXInt, scrolledYInt)) {
                            // offset the event to the view's coordinate system
                            final float xc = scrolledXFloat - child.mLeft;
                            final float yc = scrolledYFloat - child.mTop;
                            ev.setLocation(xc, yc);
                            child.mPrivateFlags &= ~CANCEL_NEXT_UP_EVENT;
                            if (child.dispatchTouchEvent(ev))  {
                                // Event handled, we have a target now.
                                mMotionTarget = child;
                                return true;
                            }
                            // The event didn't get handled, try the next view.
                            // Don't reset the event's location, it's not
                            // necessary here.
                        }
                    }
                }
            }
        }                                                                                                                                                  ....//other code omitted

16行：进入ACTION_DOWN的处理

17-23行：将mMotionTarget置为null

26行：进行判断：if(disallowIntercept || !onInterceptTouchEvent(ev))

两种可能会进入IF代码段

1、当前不允许拦截，即disallowIntercept =true，

2、当前允许拦截但是未进行拦截，即disallowIntercept =false,但是onInterceptTouchEvent(ev)返回false ;

注：disallowIntercept 可以通过viewGroup.requestDisallowInterceptTouchEvent(boolean)进行设置；而onInterceptTouchEvent(ev)可以进行复写。

36-57行：开始遍历所有的子View

41行：进行判断当前的x,y坐标是否落在子View身上，如果在47行执行child.dispatchTouchEvent(ev)，就进入了View的dispatchTouchEvent代码中了，当child.dispatchTouchEvent(ev)返回true，则为mMotionTarget=child;然后return true;

ViewGroup的ACTION_DOWN分析结束，总结一下：

ViewGroup实现捕获到DOWN事件，如果代码中不做TOUCH事件拦截，则开始查找当前x,y是否在某个子View的区域内，如果在，则把事件分发下去。

2.ACTION_MOVE

 @Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
        final int action = ev.getAction();
        final float xf = ev.getX();
        final float yf = ev.getY();
        final float scrolledXFloat = xf + mScrollX;
        final float scrolledYFloat = yf + mScrollY;
        final Rect frame = mTempRect;
        boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
       //...ACTION_DOWN
       //...ACTIN_UP or ACTION_CANCEL
        // The event wasn't an ACTION_DOWN, dispatch it to our target if
        // we have one.
	final View target = mMotionTarget;
      
        // if have a target, see if we're allowed to and want to intercept its
        // events
        if (!disallowIntercept && onInterceptTouchEvent(ev)) {
            //....
        }
        // finally offset the event to the target's coordinate system and
        // dispatch the event.
        final float xc = scrolledXFloat - (float) target.mLeft;
        final float yc = scrolledYFloat - (float) target.mTop;
        ev.setLocation(xc, yc);
        return target.dispatchTouchEvent(ev);
    }

18行：把ACTION_DOWN时赋值的mMotionTarget，付给target ;

23行：if (!disallowIntercept && onInterceptTouchEvent(ev)) 当前允许拦截且拦截了，才进入IF体，当然了默认是不会拦截的~这里执行了onInterceptTouchEvent(ev)

28-30行：把坐标系统转化为子View的坐标系统

32行：直接return target.dispatchTouchEvent(ev);

可以看到，正常流程下，ACTION_MOVE在检测完是否拦截以后，直接调用了子View.dispatchTouchEvent，事件分发下去；

3.ACTION_UP

 public boolean dispatchTouchEvent(MotionEvent ev) {
        if (!onFilterTouchEventForSecurity(ev)) {
            return false;
        }
        final int action = ev.getAction();
        final float xf = ev.getX();
        final float yf = ev.getY();
        final float scrolledXFloat = xf + mScrollX;
        final float scrolledYFloat = yf + mScrollY;
        final Rect frame = mTempRect;
        boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
        if (action == MotionEvent.ACTION_DOWN) {...}
	
	boolean isUpOrCancel = (action == MotionEvent.ACTION_UP) ||
                (action == MotionEvent.ACTION_CANCEL);
	if (isUpOrCancel) {
            mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;
        }
	final View target = mMotionTarget;
	if(target ==null ){...}
	if (!disallowIntercept && onInterceptTouchEvent(ev)) {...}
        if (isUpOrCancel) {
            mMotionTarget = null;
        }
        // finally offset the event to the target's coordinate system and
        // dispatch the event.
        final float xc = scrolledXFloat - (float) target.mLeft;
        final float yc = scrolledYFloat - (float) target.mTop;
        ev.setLocation(xc, yc);
        return target.dispatchTouchEvent(ev);
    }

17行：判断当前是否是ACTION_UP

21，28行：分别重置拦截标志位以及将DOWN赋值的mMotionTarget置为null，都UP了，当然置为null，下一次DOWN还会再赋值的~

最后，修改坐标系统，然后调用target.dispatchTouchEvent(ev);

正常情况下，即我们上例整个代码的流程我们已经走完了：

1、ACTION_DOWN中，ViewGroup捕获到事件，然后判断是否拦截，如果没有拦截，则找到包含当前x,y坐标的子View，赋值给mMotionTarget，然后调用 mMotionTarget.dispatchTouchEvent

2、ACTION_MOVE中，ViewGroup捕获到事件，然后判断是否拦截，如果没有拦截，则直接调用mMotionTarget.dispatchTouchEvent(ev)

3、ACTION_UP中，ViewGroup捕获到事件，然后判断是否拦截，如果没有拦截，则直接调用mMotionTarget.dispatchTouchEvent(ev)

当然了在分发之前都会修改下坐标系统，把当前的x，y分别减去child.left 和 child.top ，然后传给child;

问题1：如何在ViewGroup中进行事件拦截？

默认是不拦截的，即返回false；如果你需要拦截，只要在onInterceptTouchEvent()中return true就行了，该事件就不会往子View传递了，并且如果你在DOWN retrun true ，则DOWN,MOVE,UP子View都不会捕获事件；如果你在MOVE return true , 则子View在MOVE和UP都不会捕获事件。

原因很简单，当onInterceptTouchEvent(ev) return true的时候，会把mMotionTarget 置为null ;

问题2：如果ViewGroup的onInterceptTouchEvent(ev) 当ACTION_MOVE时return true ，即拦截了子View的MOVE以及UP事件；此时子View希望依然能够响应MOVE和UP时该咋办呢？

requestDisallowInterceptTouchEvent(boolean) 用于设置是否允许拦截，我们在子View的dispatchTouchEvent中直接通过getParent().requestDisallowInterceptTouchEvent(true)设置子view所在ViewGroup的disallowIntercept：

@Override
	public boolean dispatchTouchEvent(MotionEvent event)
	{
		getParent().requestDisallowInterceptTouchEvent(true);  
		int action = event.getAction();
		switch (action)
		{
		case MotionEvent.ACTION_DOWN:
			Log.e(TAG, "dispatchTouchEvent ACTION_DOWN");
			break;
		case MotionEvent.ACTION_MOVE:
			Log.e(TAG, "dispatchTouchEvent ACTION_MOVE");
			break;
		case MotionEvent.ACTION_UP:
			Log.e(TAG, "dispatchTouchEvent ACTION_UP");
			break;
		default:
			break;
		}
		return super.dispatchTouchEvent(event);
	}