Android：事件分发机制（二）

这篇主要是第一篇回顾之后，补充一些上一篇没写到的两个点。
第一个的切入点是这个。【处理层叠的view，想要执行下一层的view的点击事件】其背后的原理。

处理层叠的view，要执行下一层的view的点击事件

我们知道，方法是将上一层的view设置setOnTouchListener的onTouch() return false;

iv_right.setOnTouchListener { _, _ ->
    false
}

那么，原理是啥？其实看源码就可以了解。
首先，viewGroup的 dispatchTouchEvent 在 onInterceptTouchEvent不拦截的情况下， 传递Event到 view的 dispatchTouchEvent，然后在
其方法体中，实现原理如一下源代码：

   if (onFilterTouchEventForSecurity(event)) {
        if ((mViewFlags & ENABLED_MASK) == ENABLED && handleScrollBarDragging(event)) {
            result = true;
        }
        //noinspection SimplifiableIfStatement
        ListenerInfo li = mListenerInfo;
        if (li != null && li.mOnTouchListener != null
                && (mViewFlags & ENABLED_MASK) == ENABLED
                && li.mOnTouchListener.onTouch(this, event)) {
            result = true;
        }

        if (!result && onTouchEvent(event)) {
            result = true;
        }
    }
    //...
     return result;

而onTouchEvent默认是false。因此，最后的result就是返回了false。这个时候，会回到ViewGroup层。再回过来看其 dispatchTouchEvent的这段代码：

//...(此处省略部分源码)
final int childrenCount = mChildrenCount;
  if (newTouchTarget == null && childrenCount != 0) {
      final float x =
              isMouseEvent ? ev.getXCursorPosition() : ev.getX(actionIndex);
      final float y =
              isMouseEvent ? ev.getYCursorPosition() : ev.getY(actionIndex);
      // Find a child that can receive the event.
      // Scan children from front to back.
      final ArrayList<View> preorderedList = buildTouchDispatchChildList();
      final boolean customOrder = preorderedList == null
              && isChildrenDrawingOrderEnabled();
   final View[] children = mChildren;
   for (int i = childrenCount - 1; i >= 0; i--) {
       final int childIndex = getAndVerifyPreorderedIndex(
               childrenCount, i, customOrder);
       final View child = getAndVerifyPreorderedView(
               preorderedList, children, childIndex);
       if (!child.canReceivePointerEvents()
               || !isTransformedTouchPointInView(x, y, child, null)) {
           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);
   }

由此，可知viewgroup会在这个方法中，遍历对应区域下的所有view。如果所有view都没消费掉这个Event的时候，dispatchTouchEvent会继续执行接下来的代码，

 // 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);

这这段代码中，可以看到，进入了if语句，语句中执行了dispatchTransformedTouchEvent()方法。可以看到，它的源码中，它会在这个场景下，回调super.dispatchTouchEvent(event);最终执行了viewGroup自身的onTouchEvent（）方法。

 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;
        }
	//...(此处省略部分源码)
}

这样，最终形成闭环。也就是上一篇文章所画的流程了。第一篇在这：Android：事件分发机制

click在整体流程中的哪个节点上

这个其实在第一篇的4.3节有提到过。但是没说得很多，在这里补充一下。
这个问题的切入点，我们从最常见的设置点击事件开始说起。

mTv.setOnClickListener(v -> {});

这里，设置给了View的mOnClickListener。然后，这个回调会在performClick中被调用。

 public boolean performClick() {
        // We still need to call this method to handle the cases where performClick() was called
        // externally, instead of through performClickInternal()
        notifyAutofillManagerOnClick();

        final boolean result;
        final ListenerInfo li = mListenerInfo;
        if (li != null && li.mOnClickListener != null) {
            playSoundEffect(SoundEffectConstants.CLICK);
            li.mOnClickListener.onClick(this);
            result = true;
        } else {
            result = false;
        }

        sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_CLICKED);

        notifyEnterOrExitForAutoFillIfNeeded(true);

        return result;
    }

然后，它会在onTouchEvent中被调用。具体是在MotionEvent.ACTION_UP中执行了performClickInternal()。
看部分源码如下：

//...(此处省略部分源码)
if (clickable || (viewFlags & TOOLTIP) == TOOLTIP) {
            switch (action) {
                case MotionEvent.ACTION_UP:
                    mPrivateFlags3 &= ~PFLAG3_FINGER_DOWN;
                    if ((viewFlags & TOOLTIP) == TOOLTIP) {
                        handleTooltipUp();
                    }
                    if (!clickable) {
                        removeTapCallback();
                        removeLongPressCallback();
                        mInContextButtonPress = false;
                        mHasPerformedLongPress = false;
                        mIgnoreNextUpEvent = false;
                        break;
                    }
                    boolean prepressed = (mPrivateFlags & PFLAG_PREPRESSED) != 0;
                    if ((mPrivateFlags & PFLAG_PRESSED) != 0 || prepressed) {
                        // take focus if we don't have it already and we should in
                        // touch mode.
                        boolean focusTaken = false;
                        if (isFocusable() && isFocusableInTouchMode() && !isFocused()) {
                            focusTaken = requestFocus();
                        }

                        if (prepressed) {
                            // The button is being released before we actually
                            // showed it as pressed.  Make it show the pressed
                            // state now (before scheduling the click) to ensure
                            // the user sees it.
                            setPressed(true, x, y);
                        }

                        if (!mHasPerformedLongPress && !mIgnoreNextUpEvent) {
                            // This is a tap, so remove the longpress check
                            removeLongPressCallback();

                            // Only perform take click actions if we were in the pressed state
                            if (!focusTaken) {
                                // Use a Runnable and post this rather than calling
                                // performClick directly. This lets other visual state
                                // of the view update before click actions start.
                                if (mPerformClick == null) {
                                    mPerformClick = new PerformClick();
                                }
                                if (!post(mPerformClick)) {
                                    performClickInternal();
                                }
                            }
                        }
                        //...(此处省略部分源码)