图解Android的事件分发

要从源码理解整个事件传递过程，必须先从事件传递的返回值认清整个事件的消费过程，对整个事件有大致的了解，为何事件被消费，为何由子类消费

需要注意的几个点

主要这三个方法

dispatchTouchEvent() —— true消费，false不消费（即为分发）；false表示事件停止往下面的视图层级进行传递，同时开始往上面的视图层级的onTouchEvent传递，也称为回溯。
oninterceptTouchEvent() —— true消费，false不消费（即为不拦截）
onTouchEvent() —— true消费，false不消费（）

而事件分发一般会经过视图的三个层级：Activity、ViewGroup、View。下表会对视图不同的三个层级拥有的事件分发的相关方法进行整理：

事件分发相关方法	方法功能	Activity	ViewGroup	View
public boolean dispatchTouchEvent	事件分发	+	+	+
public boolean onInterceptTouchEvent	事件拦截		+
public boolean onTouchEvent	事件消费	+	+	+

1. view因为没有孩子，没有onInterceptTouchEvent

2. onTouchEvent中对点击事件的具体处理流程大概如下，只要View的CLICKABLE和LONG_CLICKABLE有一个为true，那么它就会消耗事件，返回true。总的来说，View的可不可用不影响是否消耗事件，只要clickable或者longClickable有一个为true，那么它就会消耗事件。

3. ViewGroup默认不拦截任何事件，因为从源码中可以看到ViewGroup的onInterceptTouchEvent方法默认返回false.

4. 由View中dispatchTouchEvent可知，如果一个控件是可点击的，那么点击该控件时，dispatchTouchEvent的返回值必定是true

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

• ViewGroup中dispatchTouchEvent

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;  
    if (action == MotionEvent.ACTION_DOWN) {  
        if (mMotionTarget != null) {  
            mMotionTarget = null;  
        }  
        if (disallowIntercept || !onInterceptTouchEvent(ev)) {  
            ev.setAction(MotionEvent.ACTION_DOWN);  
            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)) {  
                        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))  {  
                            mMotionTarget = child;  
                            return true;  
                        }  
                    }  
                }  
            }  
        }  
    }  
    boolean isUpOrCancel = (action == MotionEvent.ACTION_UP) ||  
            (action == MotionEvent.ACTION_CANCEL);  
    if (isUpOrCancel) {  
        mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;  
    }  
    final View target = mMotionTarget;  
    if (target == null) {  
        ev.setLocation(xf, yf);  
        if ((mPrivateFlags & CANCEL_NEXT_UP_EVENT) != 0) {  
            ev.setAction(MotionEvent.ACTION_CANCEL);  
            mPrivateFlags &= ~CANCEL_NEXT_UP_EVENT;  
        }  
        return super.dispatchTouchEvent(ev);  
    }  
    if (!disallowIntercept && onInterceptTouchEvent(ev)) {  
        final float xc = scrolledXFloat - (float) target.mLeft;  
        final float yc = scrolledYFloat - (float) target.mTop;  
        mPrivateFlags &= ~CANCEL_NEXT_UP_EVENT;  
        ev.setAction(MotionEvent.ACTION_CANCEL);  
        ev.setLocation(xc, yc);  
        if (!target.dispatchTouchEvent(ev)) {  
        }  
        mMotionTarget = null;  
        return true;  
    }  
    if (isUpOrCancel) {  
        mMotionTarget = null;  
    }  
    final float xc = scrolledXFloat - (float) target.mLeft;  
    final float yc = scrolledYFloat - (float) target.mTop;  
    ev.setLocation(xc, yc);  
    if ((target.mPrivateFlags & CANCEL_NEXT_UP_EVENT) != 0) {  
        ev.setAction(MotionEvent.ACTION_CANCEL);  
        target.mPrivateFlags &= ~CANCEL_NEXT_UP_EVENT;  
        mMotionTarget = null;  
    }  
    return target.dispatchTouchEvent(ev);  
}

1. 子view的onTouchEvent如果默认false不消费的话，会交给上级的onTouchEvent

2. viewGroup如果onInterceptTouchEvent不拦截，继续交给子view

3. viewGroup如果dispatchTouchEvent返回true，消费掉会还给上级的onTouchEvent

如果上面的点你都清楚的话，说明对事件分发是有清楚认识的，那么接下来上图

android事件分发图解.png 事件链