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 SimplifiableIfStatementListenerInfo 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 indexfor (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 checkremoveLongPressCallback();// Only perform take click actions if we were in the pressed stateif (!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)