看完本文你將了解：

• ACTION_CANCEL的觸發(fā)時機
• 滑出子View區(qū)域會發(fā)生什么椒功？為什么不響應(yīng)onClick()事件

首先看一下官方的解釋：

/**
 * 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;

說人話就是：當前的手勢被中止了，你不會再收到任何事件了讼呢，你可以把它當做一個ACTION_UP事件，但是不要執(zhí)行正常情況下的邏輯节沦。

ACTION_CANCEL的觸發(fā)時機

有四種情況會觸發(fā)ACTION_CANCEL:

• 在子View處理事件的過程中甫贯，父View對事件攔截
• ACTION_DOWN初始化操作
• 在子View處理事件的過程中被從父View中移除時
• 子View被設(shè)置了PFLAG_CANCEL_NEXT_UP_EVENT標記時

1叫搁，父view攔截事件

首先要了解ViewGroup什么情況下會攔截事件，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;
        // 判斷條件一
        if (actionMasked == MotionEvent.ACTION_DOWN
                || mFirstTouchTarget != null) {
            final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
            // 判斷條件二
            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;
        }
        ...
    }
    ...
}

有兩個條件

• MotionEvent.ACTION_DOWN事件或者mFirstTouchTarget非空，也就是有子view在處理事件
• 子view沒有做攔截惨奕，也就是沒有調(diào)用ViewParent#requestDisallowInterceptTouchEvent(true)

如果滿足上面的兩個條件才會執(zhí)行onInterceptTouchEvent(ev)梨撞。

如果ViewGroup攔截了事件，則intercepted變量為true时肿，接著往下看：

@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) {
                // 當mFirstTouchTarget != null螃成，也就是子view處理了事件
                // 此時如果父ViewGroup攔截了事件锈颗，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 {
                    // 判斷一：此時cancelChild == true
                    final boolean cancelChild = resetCancelNextUpFlag(target.child)
                            || intercepted;

                    // 判斷二：給child發(fā)送cancel事件
                    if (dispatchTransformedTouchEvent(ev, cancelChild,
                            target.child, target.pointerIdBits)) {
                        handled = true;
                    }
                    ...
                }
                ...
            }
        }
        ...
    }
    ...
    return handled;
}

以上判斷一處cancelChild為true淋淀，然后進入判斷二中一看究竟：

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) {
        // 將event設(shè)置成ACTION_CANCEL
        event.setAction(MotionEvent.ACTION_CANCEL);
        if (child == null) {
            ...
        } else {
            // 分發(fā)給child
            handled = child.dispatchTouchEvent(event);
        }
        event.setAction(oldAction);
        return handled;
    }
    ...
}

當參數(shù)cancel為ture時會將event設(shè)置為MotionEvent.ACTION_CANCEL朵纷，然后分發(fā)給child袍辞。

2搅吁，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目標
            cancelAndClearTouchTargets(ev);
            resetTouchState();
        }
        ...
    }
    ...
}

系統(tǒng)可能會由于App切換谎懦、ANR等原因丟失了up界拦，cancel事件梗劫。

因此需要在ACTION_DOWN時丟棄掉所有前面的狀態(tài)，具體代碼如下：

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);
            // 分發(fā)事件同情況一
            dispatchTransformedTouchEvent(event, true, target.child, target.pointerIdBits);
        }
        ...
    }
}

PS：在dispatchDetachedFromWindow()中也會調(diào)用cancelAndClearTouchTargets()

3，在子View處理事件的過程中被從父View中移除時

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) {
            ...
            // 創(chuàng)建ACTION_CANCEL事件
            MotionEvent event = MotionEvent.obtain(now, now,
                    MotionEvent.ACTION_CANCEL, 0.0f, 0.0f, 0);
            event.setSource(InputDevice.SOURCE_TOUCHSCREEN);
            分發(fā)給目標view
            view.dispatchTouchEvent(event);
            event.recycle();
            return;
        }
        predecessor = target;
        target = next;
    }
}

4瓷翻，子View被設(shè)置了PFLAG_CANCEL_NEXT_UP_EVENT標記時

在情況一種的兩個判斷處：

// 判斷一：此時cancelChild == true
final boolean cancelChild = resetCancelNextUpFlag(target.child)
|| intercepted;

// 判斷二：給child發(fā)送cancel事件
if (dispatchTransformedTouchEvent(ev, cancelChild,
    target.child, target.pointerIdBits)) {
    handled = true;
}

當resetCancelNextUpFlag(target.child)為true時同樣也會導(dǎo)致cancel，查看代碼：

/**
 * 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;
}

根據(jù)注釋大概意思是妒牙，該view暫時detached湘今，detached是什么意思摩瞎？就是和attached相反的那個旗们，具體什么時候打了這個標記构灸，我覺得沒必要深究。

以上四種情況最重要的就是第一種稠氮，后面的只需了解即可隔披。

滑出子View區(qū)域會發(fā)生什么寂拆？

了解了什么情況下會觸發(fā)ACTION_CANCEL纠永，那么針對問題：滑出子View區(qū)域會觸發(fā)ACTION_CANCEL嗎渺蒿？這個問題就很明確了：不會茂装。

實踐是檢驗真理的唯一標準少态，代碼擼起來：

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。

為什么有人會認為滑出view后會收到ACTION_CANCEL呢？

我想是因為滑出view后屋摇，view的onClick()不會觸發(fā)了炮温，所以有人就以為是觸發(fā)了ACTION_CANCEL牵舵。

那么為什么滑出view后不會觸發(fā)onClick呢畸颅？再來看看View的源碼：

在view的onTouchEvent()中：

case MotionEvent.ACTION_MOVE:
    // Be lenient about moving outside of buttons
    // 判斷是否超出view的邊界
    if (!pointInView(x, y, mTouchSlop)) {
        // Outside button
        if ((mPrivateFlags & PRESSED) != 0) {
            // 這里改變狀態(tài)為 not PRESSED
            // Need to switch from pressed to not pressed
            mPrivateFlags &= ~PRESSED;
        }
    }
    break;
    
case MotionEvent.ACTION_UP:
    boolean prepressed = (mPrivateFlags & PFLAG_PREPRESSED) != 0;
    // 可以看到當move出view范圍后没炒，這里走不進去了
    if ((mPrivateFlags & PFLAG_PRESSED) != 0 || prepressed) {
        ...
        performClick();
        ...
    }
    mIgnoreNextUpEvent = false;
    break;

1，在ACTION_MOVE中會判斷事件的位置是否超出view的邊界祖很，如果超出邊界則將mPrivateFlags置為not PRESSED狀態(tài)漾脂。

2骨稿，在ACTION_UP中判斷只有當mPrivateFlags包含PRESSED狀態(tài)時才會執(zhí)行performClick()等。

因此滑出view后不會執(zhí)行onClick()形耗。

結(jié)論：

• 滑出view范圍后激涤，如果父view沒有攔截事件倦踢，則會繼續(xù)受到ACTION_MOVE和ACTION_UP等事件辱挥。
• 一旦滑出view范圍晤碘，view會被移除PRESSED標記，這個是不可逆的宠蚂，然后在ACTION_UP中不會執(zhí)行performClick()等邏輯肥矢。