Android中ACTION_CANCEL的触发机制与滑出子view的情况 Android中ACTION_CANCEL的触发机制与

ACTION_CANCEL的触发时机

1，父view拦截事件
2，ACTION_DOWN初始化操作
3，在子View处理事件的过程中被从父View中移除时
4，子View被设置了PFLAG_CANCEL_NEXT_UP_EVENT标记时

滑出子View区域会发生什么？

结论：

看完本文你将了解：

ACTION_CANCEL的触发时机
滑出子View区域会发生什么？为什么不响应onClick()事件

首先看一下官方的解释：

/** * Constant for {@link #getActionMasked}: The current gesture has been aborted. * You will not receive any more points in it.You should treat this as * an up event, but not perform any action that you normally would. */public static final int ACTION_CANCEL= 3;

说人话就是：当前的手势被中止了，你不会再收到任何事件了，你可以把它当做一个ACTION_UP事件，但是不要执行正常情况下的逻辑。

ACTION_CANCEL的触发时机有四种情况会触发ACTION_CANCEL:

在子View处理事件的过程中，父View对事件拦截
ACTION_DOWN初始化操作
在子View处理事件的过程中被从父View中移除时
子View被设置了PFLAG_CANCEL_NEXT_UP_EVENT标记时

1，父view拦截事件
首先要了解ViewGroup什么情况下会拦截事件，Look the Fuck Resource Code：

/** * {@inheritDoc} */@Overridepublic boolean dispatchTouchEvent(MotionEvent ev) { ...boolean handled = false; if (onFilterTouchEventForSecurity(ev)) {final int action = ev.getAction(); final int actionMasked = action & MotionEvent.ACTION_MASK; ...// Check for interception.final boolean intercepted; // 判断条件一if (actionMasked == MotionEvent.ACTION_DOWN|| mFirstTouchTarget != null) {final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0; // 判断条件二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; }...}...}

有两个条件

MotionEvent.ACTION_DOWN事件或者mFirstTouchTarget非空也就是有子view在处理事件
子view没有做拦截，也就是没有调用ViewParent#requestDisallowInterceptTouchEvent(true)

如果满足上面的两个条件才会执行onInterceptTouchEvent(ev)。
如果ViewGroup拦截了事件，则intercepted变量为true，接着往下看：

@Overridepublic boolean dispatchTouchEvent(MotionEvent ev) {boolean handled = false; if (onFilterTouchEventForSecurity(ev)) {...// Check for interception.final boolean intercepted; if (actionMasked == MotionEvent.ACTION_DOWN|| mFirstTouchTarget != null) {final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0; if (!disallowIntercept) {// 当mFirstTouchTarget != null，也就是子view处理了事件// 此时如果父ViewGroup拦截了事件，intercepted==trueintercepted = 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; }...// Dispatch to touch targets.if (mFirstTouchTarget == null) {...} 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) {...} else {// 判断一：此时cancelChild == truefinal boolean cancelChild = resetCancelNextUpFlag(target.child)|| intercepted; // 判断二：给child发送cancel事件if (dispatchTransformedTouchEvent(ev, cancelChild,target.child, target.pointerIdBits)) {handled = true; }...}...}}...}...return handled; }

以上判断一处cancelChild为true，然后进入判断二中一看究竟：

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) {// 将event设置成ACTION_CANCELevent.setAction(MotionEvent.ACTION_CANCEL); if (child == null) {...} else {// 分发给childhandled = child.dispatchTouchEvent(event); }event.setAction(oldAction); return handled; }...}

当参数cancel为ture时会将event设置为MotionEvent.ACTION_CANCEL，然后分发给child。

2，ACTION_DOWN初始化操作

public boolean dispatchTouchEvent(MotionEvent ev) {boolean handled = false; if (onFilterTouchEventForSecurity(ev)) {final int action = ev.getAction(); final int actionMasked = action & MotionEvent.ACTION_MASK; // Handle an initial down.if (actionMasked == MotionEvent.ACTION_DOWN) {// Throw away all previous state when starting a new touch gesture.// The framework may have dropped the up or cancel event for the previous gesture// due to an app switch, ANR, or some other state change.// 取消并清除所有的Touch目标cancelAndClearTouchTargets(ev); resetTouchState(); }...}...}

系统可能会由于App切换、ANR等原因丢失了up，cancel事件。
因此需要在ACTION_DOWN时丢弃掉所有前面的状态，具体代码如下：

private void cancelAndClearTouchTargets(MotionEvent event) {if (mFirstTouchTarget != null) {boolean syntheticEvent = false; if (event == null) {final long now = SystemClock.uptimeMillis(); event = MotionEvent.obtain(now, now,MotionEvent.ACTION_CANCEL, 0.0f, 0.0f, 0); event.setSource(InputDevice.SOURCE_TOUCHSCREEN); syntheticEvent = true; }for (TouchTarget target = mFirstTouchTarget; target != null; target = target.next) {resetCancelNextUpFlag(target.child); // 分发事件同情况一dispatchTransformedTouchEvent(event, true, target.child, target.pointerIdBits); }...}}

PS：在dispatchDetachedFromWindow()中也会调用cancelAndClearTouchTargets()

3，在子View处理事件的过程中被从父View中移除时

public void removeView(View view) {if (removeViewInternal(view)) {requestLayout(); invalidate(true); }}private boolean removeViewInternal(View view) {final int index = indexOfChild(view); if (index >= 0) {removeViewInternal(index, view); return true; }return false; }private void removeViewInternal(int index, View view) {...cancelTouchTarget(view); ...}private void cancelTouchTarget(View view) {TouchTarget predecessor = null; TouchTarget target = mFirstTouchTarget; while (target != null) {final TouchTarget next = target.next; if (target.child == view) {...// 创建ACTION_CANCEL事件MotionEvent event = MotionEvent.obtain(now, now,MotionEvent.ACTION_CANCEL, 0.0f, 0.0f, 0); event.setSource(InputDevice.SOURCE_TOUCHSCREEN); 分发给目标viewview.dispatchTouchEvent(event); event.recycle(); return; }predecessor = target; target = next; }}

4，子View被设置了PFLAG_CANCEL_NEXT_UP_EVENT标记时
在情况一种的两个判断处：

// 判断一：此时cancelChild == truefinal boolean cancelChild = resetCancelNextUpFlag(target.child)|| intercepted; // 判断二：给child发送cancel事件if (dispatchTransformedTouchEvent(ev, cancelChild,target.child, target.pointerIdBits)) {handled = true; }

当 resetCancelNextUpFlag(target.child) 为true时同样也会导致cancel，查看代码：

/** * Indicates whether the view is temporarily detached. * * @hide */static final int PFLAG_CANCEL_NEXT_UP_EVENT= 0x04000000; private static boolean resetCancelNextUpFlag(View view) {if ((view.mPrivateFlags & PFLAG_CANCEL_NEXT_UP_EVENT) != 0) {view.mPrivateFlags &= ~PFLAG_CANCEL_NEXT_UP_EVENT; return true; }return false; }

根据注释大概意思是，该view暂时detached，detached是什么意思？就是和attached相反的那个，具体什么时候打了这个标记，我觉得没必要深究。
以上四种情况最重要的就是第一种，后面的只需了解即可。

滑出子View区域会发生什么？了解了什么情况下会触发ACTION_CANCEL，那么针对问题：滑出子View区域会触发ACTION_CANCEL吗？这个问题就很明确了：不会。
实践是检验真理的唯一标准，代码撸起来：

public class MyButton extends androidx.appcompat.widget.AppCompatButton { @Overridepublic boolean onTouchEvent(MotionEvent event) {switch (event.getAction()) {case MotionEvent.ACTION_DOWN:LogUtil.d("ACTION_DOWN"); break; case MotionEvent.ACTION_MOVE:LogUtil.d("ACTION_MOVE"); break; case MotionEvent.ACTION_UP:LogUtil.d("ACTION_UP"); break; case MotionEvent.ACTION_CANCEL:LogUtil.d("ACTION_CANCEL"); break; }return super.onTouchEvent(event); }}

一波操作以后日志如下：

(MyButton.java:32) -->ACTION_DOWN
(MyButton.java:36) -->ACTION_MOVE
(MyButton.java:36) -->ACTION_MOVE
(MyButton.java:36) -->ACTION_MOVE
(MyButton.java:36) -->ACTION_MOVE
(MyButton.java:36) -->ACTION_MOVE
(MyButton.java:39) -->ACTION_UP

滑出view后依然可以收到ACTION_MOVE和ACTION_UP事件。
为什么有人会认为滑出view后会收到ACTION_CANCEL呢？
我想是因为滑出view后，view的onClick()不会触发了，所以有人就以为是触发了ACTION_CANCEL。
那么为什么滑出view后不会触发onClick呢？再来看看View的源码：
在view的onTouchEvent()中：

case MotionEvent.ACTION_MOVE:// Be lenient about moving outside of buttons // 判断是否超出view的边界if (!pointInView(x, y, mTouchSlop)) {// Outside buttonif ((mPrivateFlags & PRESSED) != 0) {// 这里改变状态为 not PRESSED// Need to switch from pressed to not pressedmPrivateFlags &= ~PRESSED; }}break; case MotionEvent.ACTION_UP:boolean prepressed = (mPrivateFlags & PFLAG_PREPRESSED) != 0; // 可以看到当move出view范围后，这里走不进去了if ((mPrivateFlags & PFLAG_PRESSED) != 0 || prepressed) {...performClick(); ...}mIgnoreNextUpEvent = false; break;

1，在ACTION_MOVE中会判断事件的位置是否超出view的边界，如果超出边界则将mPrivateFlags置为not PRESSED状态。
2，在ACTION_UP中判断只有当mPrivateFlags包含PRESSED状态时才会执行performClick()等。
因此滑出view后不会执行onClick()。
【Android中ACTION_CANCEL的触发机制与滑出子view的情况】
结论：