Android中GUI系统的Event路由机制
前两天在论坛上看到有人发了一个帖子，询问一个Android GUI Event处理的问题：有一个LinearLayout，里面有很多的child view，他问如何监听这个LinearLayout的Click事件？他的做法是：
setClickable(true);
setOnClickListener(listener);
最后他发现listener中的回调函数根本不会被调用。
事实上，在Android的GUI系统的中，硬件触发的Event（KeyEvent、 TouchEvent、 TrackballEvent等）最开始是Window拿到了，Window将Event转发给了前台的Activity。但是Activity同样不能马上自己处理掉，而是将Event传递给了它里面的ContentView。
如果ContentView是一个容器View（继承自ViewGroup类型），它一般都是先判断这个Event落在哪一个Child View上。然后将该Event Dispatch给这个Child View了。
我们以Touch Event为例，看看ViewGroup类中的dispatchTouchEvent()函数：
/** * {@inheritDoc} */ @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. } } } } } boolean isUpOrCancel = (action == MotionEvent.ACTION_UP) || (action == MotionEvent.ACTION_CANCEL); if (isUpOrCancel) { // Note, we've already copied the previous state to our local // variable, so this takes effect on the next event mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT; } // The event wasn't an ACTION_DOWN, dispatch it to our target if // we have one. final View target = mMotionTarget; if (target == null) { // We don't have a target, this means we're handling the // event as a regular view. 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 have a target, see if we're allowed to and want to intercept its // events 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)) { // target didn't handle ACTION_CANCEL. not much we can do // but they should have. } // clear the target mMotionTarget = null; // Don't dispatch this event to our own view, because we already // saw it when intercepting; we just want to give the following // event to the normal onTouchEvent(). return true; } 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); 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); }
我们可以看到ViewGroup通过调用Child View的dispatchTouchEvent()函数，将Event按照View树状结构一级一级的Dispatch下去（Child View， GrandChild View ……）。
最终，最里层的Child View拿到了这个Event，而它又没有Child View了。于是它就开始处理Event（也就是响应事件）。
我们还以Touch Event为例，看看View类中的dispatchTouchEvent()函数：
/** * Pass the touch screen motion event down to the target view, or this * view if it is the target. * * @param event The motion event to be dispatched. * @return True if the event was handled by the view, false otherwise. */ public boolean dispatchTouchEvent(MotionEvent event) { if (!onFilterTouchEventForSecurity(event)) { return false; } if (mOnTouchListener != null && (mViewFlags & ENABLED_MASK) == ENABLED && mOnTouchListener.onTouch(this, event)) { return true; } return onTouchEvent(event); }
首先，它是判断这个Touch Event是否安全，如果不安全，返回。
然后，判断是否设置了Touch Event监听器，如果设置了，就调用监听器的OnTouchListener的处理函数，返回。
如果没有Touch Event监听器，就调用自身定义onTouchEvent()方法，返回。
在这种事件处理函数中，都要返回一个boolean类型的值。如果返回了false，它的父容器还能再次拿到了事件的。如果返回了true，也就相当于告诉它的父容器：这事我管了，你就别过问了。
因此，Activity拥有的处理Event权限是最低级别的。
我们同时也会注意到，除了硬件触发的Event（KeyEvent、 TouchEvent、 TrackballEvent等）外，还存在一些Event，如：Click、LongPress、DoubleClick等。这些Event通常是人为的将“硬件触发的Event”封装得来的。例如Click Event就是使用了TouchEvent 中的MotionEvent.ACTION_UP和MotionEvent.ACTION_DOWN封装而来的。默认情况下，它们使得onTouchEvent处理函数返回了true。因此，Linearlayout的孩子们一旦设置了自己的Click事件监听器，Linearlayout本身就拿不到事件了，因为它的孩子已经进行了处理。这正是论坛中问题的答案所在！
不过，这里也存在一种解决方案。我们在看ViewGroup的dispatchTouchEvent()函数时，应该注意到ViewGroup在将Event Dispatch给Child View之前，先调用了自己的onInterceptTouchEvent（）函数（Intercept就是拦截的意思）。因此，我们可以通过重写Linearlayout的onInterceptTouchEvent（）方法，来拦截我们想要处理的Event，它会在事件传给孩子之前被调用的。
