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ViewGroup.dispatchTouchEvent()分析

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ViewGroup.dispatchTouchEvent()分析

流程图：
dispatchTouchEvent

拦截检查

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// Check for interception.
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;
}

分析：

  1. 如果是ACTION_DOWN事件，则检查是否需要拦截。如果不是ACTION_DOWN事件，比如是ACTION_MOVE事件，则看mFirstTouchTarget的情况，若在之前的事件中有childView想处理touch事件，则mFirstTouchTarget不为空，即需要向下传递touch事件，这时候也需要检查是否需要拦截。
  2. 不是ACTION_DOWN事件, 并且mFirstTouchTarget为空，即不需要向下传递touch事件, 这时候可以直接设置intercepted=true，后续便不会向下传递事件。

不拦截，查找可以接收事件的子View

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if (!canceled && !intercepted) {
    if (actionMasked == MotionEvent.ACTION_DOWN
            || (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN)
            || actionMasked == MotionEvent.ACTION_HOVER_MOVE) {

    }
}

分析：

  1. 如果不是取消动作，并且不拦截事件，再如果是ACTION_DOWN事件，则查找一个可以接收这次事件的childView。若存在的话，则为mFirstTouchTarget赋值，不再为空。
  2. 只有添加到mFirstTouchTarget的childView才会收到后续的touch事件，childView需要在ACTION_DOWN事件中return true才会被添加到mFirstTouchTarget。

如何分发

//最后一组操作

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// Dispatch to touch targets.
if (mFirstTouchTarget == null) {
    // No touch targets so treat this as an ordinary view.
    handled = dispatchTransformedTouchEvent(ev, canceled, null,
            TouchTarget.ALL_POINTER_IDS);
} else {
    // Dispatch to touch targets, excluding the new touch target if we already
    // dispatched to it.  Cancel touch targets if necessary.
    TouchTarget predecessor = null;
    TouchTarget target = mFirstTouchTarget;
    while (target != null) {
        final TouchTarget next = target.next;
        if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) {
            handled = true;
        } else {
            final boolean cancelChild = resetCancelNextUpFlag(target.child)
                    || intercepted;
            if (dispatchTransformedTouchEvent(ev, cancelChild,
                    target.child, target.pointerIdBits)) {
                handled = true;
            }
            if (cancelChild) {
                if (predecessor == null) {
                    mFirstTouchTarget = next;
                } else {
                    predecessor.next = next;
                }
                target.recycle();
                target = next;
                continue;
            }
        }
        predecessor = target;
        target = next;
    }
}

分析：

  1. mFirstTouchTarget为空，即不需要往下传递，则最终会调用super.dispatchTouchEvent(), ViewGroup继承于View，即执行了View.dispatchTouchEvent()；
  2. 对mFirstTouchTarget中的childView执行dispatchTransformedTouchEvent();

dispatchTransformedTouchEvent分析：

貌似就两种可能：
case 1: handled = super.dispatchTouchEvent(event);
case 2: handled = child.dispatchTouchEvent(event);