Android中ACTION_CANCEL的触发机制与滑出子view的情况
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}
*/
@Override
public 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：
@Override
public 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==true
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;
}
...
// 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 == true
final 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_CANCEL
event.setAction(MotionEvent.ACTION_CANCEL);
if (child == null) {
...
} else {
// 分发给child
handled = 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()
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view
view.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 == true
final 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 {
@Override
public 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 button
if ((mPrivateFlags & PRESSED) != 0) {
// 这里改变状态为 not PRESSED
// Need to switch from pressed to not pressed
mPrivateFlags &= ~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()。
滑出view范围后，如果父view没有拦截事件，则会继续受到ACTION_MOVE和ACTION_UP等事件。
一旦滑出view范围，view会被移除PRESSED标记，这个是不可逆的，然后在ACTION_UP中不会执行performClick()等逻辑。
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