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

Posted 2021-09-23 涂程

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;
}

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()。

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_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。