学步园

        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：

/**<br /> * {@inheritDoc}<br /> */<br /> @Override<br /> public boolean dispatchTouchEvent(MotionEvent ev) {<br /> if (!onFilterTouchEventForSecurity(ev)) {<br /> return false;<br /> }<br /> final int action = ev.getAction();<br /> final float xf = ev.getX();<br /> final float yf = ev.getY();<br /> final float scrolledXFloat = xf + mScrollX;<br /> final float scrolledYFloat = yf + mScrollY;<br /> final Rect frame = mTempRect;<br /> boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;<br /> if (action == MotionEvent.ACTION_DOWN) {<br /> if (mMotionTarget != null) {<br /> // this is weird, we got a pen down, but we thought it was<br /> // already down!<br /> // XXX: We should probably send an ACTION_UP to the current<br /> // target.<br /> mMotionTarget = null;<br /> }<br /> // If we're disallowing intercept or if we're allowing and we didn't<br /> // intercept<br /> if (disallowIntercept || !onInterceptTouchEvent(ev)) {<br /> // reset this event's action (just to protect ourselves)<br /> ev.setAction(MotionEvent.ACTION_DOWN);<br /> // We know we want to dispatch the event down, find a child<br /> // who can handle it, start with the front-most child.<br /> final int scrolledXInt = (int) scrolledXFloat;<br /> final int scrolledYInt = (int) scrolledYFloat;<br /> final View[] children = mChildren;<br /> final int count = mChildrenCount;<br /> for (int i = count - 1; i >= 0; i--) {<br /> final View child = children[i];<br /> if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE<br /> || child.getAnimation() != null) {<br /> child.getHitRect(frame);<br /> if (frame.contains(scrolledXInt, scrolledYInt)) {<br /> // offset the event to the view's coordinate system<br /> final float xc = scrolledXFloat - child.mLeft;<br /> final float yc = scrolledYFloat - child.mTop;<br /> ev.setLocation(xc, yc);<br /> child.mPrivateFlags &= ~CANCEL_NEXT_UP_EVENT;<br /> if (child.dispatchTouchEvent(ev)) {<br /> // Event handled, we have a target now.<br /> mMotionTarget = child;<br /> return true;<br /> }<br /> // The event didn't get handled, try the next view.<br /> // Don't reset the event's location, it's not<br /> // necessary here.<br /> }<br /> }<br /> }<br /> }<br /> }<br /> boolean isUpOrCancel = (action == MotionEvent.ACTION_UP) ||<br /> (action == MotionEvent.ACTION_CANCEL);<br /> if (isUpOrCancel) {<br /> // Note, we've already copied the previous state to our local<br /> // variable, so this takes effect on the next event<br /> mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;<br /> }<br /> // The event wasn't an ACTION_DOWN, dispatch it to our target if<br /> // we have one.<br /> final View target = mMotionTarget;<br /> if (target == null) {<br /> // We don't have a target, this means we're handling the<br /> // event as a regular view.<br /> ev.setLocation(xf, yf);<br /> if ((mPrivateFlags & CANCEL_NEXT_UP_EVENT) != 0) {<br /> ev.setAction(MotionEvent.ACTION_CANCEL);<br /> mPrivateFlags &= ~CANCEL_NEXT_UP_EVENT;<br /> }<br /> return super.dispatchTouchEvent(ev);<br /> }<br /> // if have a target, see if we're allowed to and want to intercept its<br /> // events<br /> if (!disallowIntercept && onInterceptTouchEvent(ev)) {<br /> final float xc = scrolledXFloat - (float) target.mLeft;<br /> final float yc = scrolledYFloat - (float) target.mTop;<br /> mPrivateFlags &= ~CANCEL_NEXT_UP_EVENT;<br /> ev.setAction(MotionEvent.ACTION_CANCEL);<br /> ev.setLocation(xc, yc);<br /> if (!target.dispatchTouchEvent(ev)) {<br /> // target didn't handle ACTION_CANCEL. not much we can do<br /> // but they should have.<br /> }<br /> // clear the target<br /> mMotionTarget = null;<br /> // Don't dispatch this event to our own view, because we already<br /> // saw it when intercepting; we just want to give the following<br /> // event to the normal onTouchEvent().<br /> return true;<br /> }<br /> if (isUpOrCancel) {<br /> mMotionTarget = null;<br /> }<br /> // finally offset the event to the target's coordinate system and<br /> // dispatch the event.<br /> final float xc = scrolledXFloat - (float) target.mLeft;<br /> final float yc = scrolledYFloat - (float) target.mTop;<br /> ev.setLocation(xc, yc);<br /> if ((target.mPrivateFlags & CANCEL_NEXT_UP_EVENT) != 0) {<br /> ev.setAction(MotionEvent.ACTION_CANCEL);<br /> target.mPrivateFlags &= ~CANCEL_NEXT_UP_EVENT;<br /> mMotionTarget = null;<br /> }<br /> return target.dispatchTouchEvent(ev);<br /> }<br />  

        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：

/**<br /> * Pass the touch screen motion event down to the target view, or this<br /> * view if it is the target.<br /> *<br /> * @param event The motion event to be dispatched.<br /> * @return True if the event was handled by the view, false otherwise.<br /> */<br /> public boolean dispatchTouchEvent(MotionEvent event) {<br /> if (!onFilterTouchEventForSecurity(event)) {<br /> return false;<br /> }<br /> if (mOnTouchListener != null && (mViewFlags & ENABLED_MASK) == ENABLED &&<br /> mOnTouchListener.onTouch(this, event)) {<br /> return true;<br /> }<br /> return onTouchEvent(event);<br /> } 

        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。