Android触屏分发机制（二）

上一篇博客 Android触屏分发机制（一） http://blog.csdn.net/u010429311/article/details/50978987

在上一篇博客中讲到了Button的点击事件处理，即View的点击事件是如何分发的，那么这篇文章更深入的理解触屏分发机制，讲述ViewGroup是如何实现的。大家可能都知道各种布局如RelativeLayout,LinearLayout都是继承自ViewGroup，简单来说它是View的集合,就像一个RelativeLayout包含了许多控件一样。其关系如图所示。

关于ViewGroup和View的具体信息，可以在百度里找到大量的资料。
回到正题，ViewGroup是如何实现触屏分发机制的呢？同样，先做一个测试：

Button btn = (Button) findViewById(R.id.button);        btn.setOnClickListener(new View.OnClickListener() {            @Override            public void onClick(View v) {                Log.i(TAG, "on click");            }        });RelativeLayout layout = (RelativeLayout) findViewById(R.id.layout);        layout.setOnTouchListener(new View.OnTouchListener() {            @Override            public boolean onTouch(View v, MotionEvent event) {                Log.i(TAG, "layout on touch");                return false;            }        });

当我们分别点击Button和Button外的空白处时，分别输出了两个结果：


我们可以看到，当点击按钮时layout的onTouch事件并没有执行！为什么作为ViewGroup的RelativeLayout没有执行onTouch事件呢？上一节讲到了当按钮点击时会执行其dispatchTouchEvent()方法，同样我们可以看看ViewGroup是如何实现其dispatchTouchEvent()方法的：

@Overridepublic boolean dispatchTouchEvent(MotionEvent ev) {    //省略一部分    final boolean intercepted;    if (actionMasked == MotionEvent.ACTION_DOWN                    || mFirstTouchTarget != null) {                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;            }            // If intercepted, start normal event dispatch. Also if there is already            // a view that is handling the gesture, do normal event dispatch.            if (intercepted || mFirstTouchTarget != null) {                ev.setTargetAccessibilityFocus(false);            }            // Check for cancelation.            final boolean canceled = resetCancelNextUpFlag(this)                    || actionMasked == MotionEvent.ACTION_CANCEL;            // Update list of touch targets for pointer down, if needed.            final boolean split = (mGroupFlags & FLAG_SPLIT_MOTION_EVENTS) != 0;            TouchTarget newTouchTarget = null;            boolean alreadyDispatchedToNewTouchTarget = false;            if (!canceled && !intercepted) {                // If the event is targeting accessiiblity focus we give it to the                // view that has accessibility focus and if it does not handle it                // we clear the flag and dispatch the event to all children as usual.                // We are looking up the accessibility focused host to avoid keeping                // state since these events are very rare.                View childWithAccessibilityFocus = ev.isTargetAccessibilityFocus()                        ? findChildWithAccessibilityFocus() : null;                if (actionMasked == MotionEvent.ACTION_DOWN                        || (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN)                        || actionMasked == MotionEvent.ACTION_HOVER_MOVE) {                    final int actionIndex = ev.getActionIndex(); // always 0 for down                    final int idBitsToAssign = split ? 1 << ev.getPointerId(actionIndex)                            : TouchTarget.ALL_POINTER_IDS;                    // Clean up earlier touch targets for this pointer id in case they                    // have become out of sync.                    removePointersFromTouchTargets(idBitsToAssign);                    final int childrenCount = mChildrenCount;                    if (newTouchTarget == null && childrenCount != 0) {                        final float x = ev.getX(actionIndex);                        final float y = ev.getY(actionIndex);                        // Find a child that can receive the event.                        // Scan children from front to back.                        final ArrayList<View> preorderedList = buildOrderedChildList();                        final boolean customOrder = preorderedList == null                                && isChildrenDrawingOrderEnabled();                        final View[] children = mChildren;                        for (int i = childrenCount - 1; i >= 0; i--) {                            final int childIndex = customOrder                                    ? getChildDrawingOrder(childrenCount, i) : i;                            final View child = (preorderedList == null)                                    ? children[childIndex] : preorderedList.get(childIndex);                            // If there is a view that has accessibility focus we want it                            // to get the event first and if not handled we will perform a                            // normal dispatch. We may do a double iteration but this is                            // safer given the timeframe.                            if (childWithAccessibilityFocus != null) {                                if (childWithAccessibilityFocus != child) {                                    continue;                                }                                childWithAccessibilityFocus = null;                                i = childrenCount - 1;                            }                            if (!canViewReceivePointerEvents(child)                                    || !isTransformedTouchPointInView(x, y, child, null)) {                                ev.setTargetAccessibilityFocus(false);                                continue;                            }                            newTouchTarget = getTouchTarget(child);                            if (newTouchTarget != null) {                                // Child is already receiving touch within its bounds.                                // Give it the new pointer in addition to the ones it is handling.                                newTouchTarget.pointerIdBits |= idBitsToAssign;                                break;                            }                            resetCancelNextUpFlag(child);                            if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {                                // Child wants to receive touch within its bounds.                                mLastTouchDownTime = ev.getDownTime();                                if (preorderedList != null) {                                    // childIndex points into presorted list, find original index                                    for (int j = 0; j < childrenCount; j++) {                                        if (children[childIndex] == mChildren[j]) {                                            mLastTouchDownIndex = j;                                            break;                                        }                                    }                                } else {                                    mLastTouchDownIndex = childIndex;                                }                                mLastTouchDownX = ev.getX();                                mLastTouchDownY = ev.getY();                                newTouchTarget = addTouchTarget(child, idBitsToAssign);                                alreadyDispatchedToNewTouchTarget = true;                                break;                            }                            // The accessibility focus didn't handle the event, so clear                            // the flag and do a normal dispatch to all children.                            ev.setTargetAccessibilityFocus(false);                        }                        if (preorderedList != null) preorderedList.clear();                    }                //省略一部分}

已经省略一部分了，代码还是有点长。我们看到第9行中这句代码
intercepted = onInterceptTouchEvent(ev)中的 onInterceptTouchEvent()方法，该方法很简单：

public boolean onInterceptTouchEvent(MotionEvent ev) {        return false;    }

Intercept为“拦截的意思”，顾名思义，我们可以联想到分发事件的拦截。经过测试，可以知道，到返回值为true时，拦截分发，点击RelativeLayout中的Button无法响应，只输出了RelativeLayout的onTouch事件。了解了这个方法后，我们继续回到之前的dispatchTouchEvent()方法：
我们看到34行： if (!canceled && !intercepted)，当onInterceptTouchEvent()方法返回false时，可通过该条件，然后跳到65行里的for循环，该循坏是遍历ViewGroup中所有的子ViewGroup或是View，找到用户所点击的View，并在第89行赋值newTouchTarget = getTouchTarget(child)。
之后我们在跳到98行的 if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign))这个条件语句，再看看 idispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)这个方法实现方式：

private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,            View child, int desiredPointerIdBits) {        final boolean handled;        // Canceling motions is a special case.  We don't need to perform any transformations        // or filtering.  The important part is the action, not the contents.        final int oldAction = event.getAction();        if (cancel || oldAction == MotionEvent.ACTION_CANCEL) {            event.setAction(MotionEvent.ACTION_CANCEL);            if (child == null) {                handled = super.dispatchTouchEvent(event);            } else {                handled = child.dispatchTouchEvent(event);            }            event.setAction(oldAction);            return handled;    }        //省略一部分  }   

幸运！我们终于在第13行中看到了child.dispatchTouchEvent(event)这句话，明显如果找到了点击控件child不为null，执行该语句。即回到了上一节所讲的View的dispatchTouchEvent()方法的触屏分发机制。
至此，ViewGroup的触屏分发机制也清晰了。最后，画个图总结一下：