面试常问的ACTION_CANCEL到底何时触发,滑出子View范围会发生什么?

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()

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;
}

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detacheddetached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_MOVEACTION_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()

结论:
  • 滑出view范围后,如果父view没有拦截事件,则会继续受到ACTION_MOVEACTION_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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