• 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。