首先看一下官方的注释：
/**
* 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;
当前的手势被中支，你将不会收到任何事件了，可以把它当做一个ACTION_UP事件，但不要执行正常情况下的逻辑。
ACTION_CANCEL的触发时机分别是： 1 在子View处理事件的过程中，父View对事件拦截，
// Dispatch to touch targets.
if (mFirstTouchTarget == null) {//1
// No touch targets so treat this as an ordinary view.
handled = dispatchTransformedTouchEvent(ev, canceled, null,
TouchTarget.ALL_POINTER_IDS);
} 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) {
handled = true;
} else {
final boolean cancelChild = resetCancelNextUpFlag(target.child)
|| intercepted; //2
if (dispatchTransformedTouchEvent(ev, cancelChild, //3
target.child, target.pointerIdBits)) {
handled = true;
}
if (cancelChild) {
if (predecessor == null) {
mFirstTouchTarget = next;
} else {
predecessor.next = next;
}
target.recycle();
target = next;
continue;
}
}
predecessor = target;
target = next;
}
}
第一处代码注释地方，当有子View处理事件时，那么mFirstTouchTarget这个就不为null的。所以符合在子View处理事件的过程中这个前置条件了。 第二处代码注释地方，当父View拦截时，那么intercepted这个值就是为ture的，那么cancelChild就为true了，接着执行了第三处代码注释dispatchTransformedTouchEvent方法，根据上述的变量值可以看出
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) {
event.setAction(MotionEvent.ACTION_CANCEL);//5
if (child == null) {
handled = super.dispatchTouchEvent(event);
} else {
handled = child.dispatchTouchEvent(event);//4
}
event.setAction(oldAction);
return handled;
}
第四处代码注释：就执行了child的dispatchTouchEvent的方法来自第五处代码注释的CANCEL_ACTION。
小结：
当父View不拦截DOWN事件时，拦截MOVE事件时，此次它的子View会收到CANCEl事件，可以自行验证下。
2 ACTION_DOWN初始化操作时
public boolean dispatchTouchEvent(MotionEvent ev) {
if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onTouchEvent(ev, 1);
}
// If the event targets the accessibility focused view and this is it, start
// normal event dispatch. Maybe a descendant is what will handle the click.
if (ev.isTargetAccessibilityFocus() && isAccessibilityFocusedViewOrHost()) {
ev.setTargetAccessibilityFocus(false);
}
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. //1
cancelAndClearTouchTargets(ev);
resetTouchState();
}
这个源码第1处注释写的很清楚，系统可能由于切换APP切换，ANR，或者其他一些状态改变导致丢失up,或者cancel事件。 3 在子View处理事件的过程中被父View中移除时
public void removeViews(int start, int count) {
removeViewsInternal(start, count);
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) {
if (mTransition != null) {
mTransition.removeChild(this, view);
}
boolean clearChildFocus = false;
if (view == mFocused) {
view.unFocus(null);
clearChildFocus = true;
}
if (view == mFocusedInCluster) {
clearFocusedInCluster(view);
}
view.clearAccessibilityFocus();
cancelTouchTarget(view);//3.1处
cancelHoverTarget(view);
再看这个3.1处这个方法
@UnsupportedAppUsage
private void cancelTouchTarget(View view) {
TouchTarget predecessor = null;
TouchTarget target = mFirstTouchTarget;
while (target != null) {
final TouchTarget next = target.next;
if (target.child == view) {
if (predecessor == null) {
mFirstTouchTarget = next;
} else {
predecessor.next = next;
}
target.recycle();
final long now = SystemClock.uptimeMillis();
MotionEvent event = MotionEvent.obtain(now, now,
MotionEvent.ACTION_CANCEL, 0.0f, 0.0f, 0);
event.setSource(InputDevice.SOURCE_TOUCHSCREEN);
view.dispatchTouchEvent(event);
event.recycle();
return;
}
predecessor = target;
target = next;
}
}
构造了一个CANCEL事件的event，然后view去分发这个事件。 4 子View设置了PFLAG_CANCEL_NEXT_UP_EVENT标志位时。 View#dispatchTouchEvent方法片段截取
// Dispatch to touch targets.
if (mFirstTouchTarget == null) {
// No touch targets so treat this as an ordinary view.
handled = dispatchTransformedTouchEvent(ev, canceled, null,
TouchTarget.ALL_POINTER_IDS);
} else { //4.1处
// 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) {
handled = true;
} else {
final boolean cancelChild = resetCancelNextUpFlag(target.child)//4.2处
|| intercepted;
if (dispatchTransformedTouchEvent(ev, cancelChild,
target.child, target.pointerIdBits)) {
handled = true;
}
if (cancelChild) {
if (predecessor == null) {
mFirstTouchTarget = next;
} else {
predecessor.next = next;
}
target.recycle();
target = next;
continue;
}
}
predecessor = target;
target = next;
}
}
当执行到4.1时代表有子View处理事件，4.2处即使父View在此过程中没有拦截，但是子View设置了PFLAG_CANCEL_NEXT_UP_EVENT标志位，同样cancelChild为ture，后续逻辑同1了。 现在可以得出结论：当一个按钮被按下时，滑出button区域时，是不会收到CANCEL事件的。只会收到DWON-MOVE-MOVE-MOVE-MOVE-MOVEz最后UP事件，滑出button区域依然可以收到MOVE和UP事件，这点可以自行验证。实践是检验真理的唯一标准。 而且UP事件后也不会触发onclick了， View#onTouchEvent()
case MotionEvent.ACTION_MOVE:
if (clickable) {
drawableHotspotChanged(x, y);
}
final int motionClassification = event.getClassification();
final boolean ambiguousGesture =
motionClassification == MotionEvent.CLASSIFICATION_AMBIGUOUS_GESTURE;
int touchSlop = mTouchSlop;
if (ambiguousGesture && hasPendingLongPressCallback()) {
if (!pointInView(x, y, touchSlop)) {
// The default action here is to cancel long press. But instead, we
// just extend the timeout here, in case the classification
// stays ambiguous.
removeLongPressCallback();
long delay = (long) (ViewConfiguration.getLongPressTimeout()
* mAmbiguousGestureMultiplier);
// Subtract the time already spent
delay -= event.getEventTime() - event.getDownTime();
checkForLongClick(
delay,
x,
y,
TOUCH_GESTURE_CLASSIFIED__CLASSIFICATION__LONG_PRESS);
}
touchSlop *= mAmbiguousGestureMultiplier;
}
// Be lenient about moving outside of buttons
if (!pointInView(x, y, touchSlop)) {//4.3处
// Outside button
// Remove any future long press/tap checks
removeTapCallback();
removeLongPressCallback();
if ((mPrivateFlags & PFLAG_PRESSED) != 0) {
setPressed(false);4.4处
}
mPrivateFlags3 &= ~PFLAG3_FINGER_DOWN;
}
public void setPressed(boolean pressed) {
final boolean needsRefresh = pressed != ((mPrivateFlags & PFLAG_PRESSED) == PFLAG_PRESSED);
if (pressed) {
mPrivateFlags |= PFLAG_PRESSED;
} else { //4.5
mPrivateFlags &= ~PFLAG_PRESSED;
}
if (needsRefresh) {
refreshDrawableState();
}
dispatchSetPressed(pressed);
}
4.3处代码注释判断是否超过view的边界，就会调用4.4处设置setPressed(false)，再执行4.5代码注释其实就是移除一个PFLAG_PREDDED标志位，
case MotionEvent.ACTION_UP:
mPrivateFlags3 &= ~PFLAG3_FINGER_DOWN;
if ((viewFlags & TOOLTIP) == TOOLTIP) {
handleTooltipUp();
}
if (!clickable) {
removeTapCallback();
removeLongPressCallback();
mInContextButtonPress = false;
mHasPerformedLongPress = false;
mIgnoreNextUpEvent = false;
break;
}
boolean prepressed = (mPrivateFlags & PFLAG_PREPRESSED) != 0;
if ((mPrivateFlags & PFLAG_PRESSED) != 0 || prepressed) { //4.6
// take focus if we don't have it already and we should in
// touch mode.
boolean focusTaken = false;
if (isFocusable() && isFocusableInTouchMode() && !isFocused()) {
focusTaken = requestFocus();
}
if (prepressed) {
// The button is being released before we actually
// showed it as pressed. Make it show the pressed
// state now (before scheduling the click) to ensure
// the user sees it.
setPressed(true, x, y);
}
if (!mHasPerformedLongPress && !mIgnoreNextUpEvent) {
// This is a tap, so remove the longpress check
removeLongPressCallback();
// Only perform take click actions if we were in the pressed state
if (!focusTaken) {
// Use a Runnable and post this rather than calling
// performClick directly. This lets other visual state
// of the view update before click actions start.
if (mPerformClick == null) {
mPerformClick = new PerformClick();
}
if (!post(mPerformClick)) {
performClickInternal(); //4.7处
}
}
}
if (mUnsetPressedState == null) {
mUnsetPressedState = new UnsetPressedState();
}
if (prepressed) {
postDelayed(mUnsetPressedState,
ViewConfiguration.getPressedStateDuration());
} else if (!post(mUnsetPressedState)) {
// If the post failed, unpress right now
mUnsetPressedState.run();
}
removeTapCallback();
}
mIgnoreNextUpEvent = false;
break;
当在MOVE事件了View超过了边界了就移除了PFLAG_PRESSED时，那么代码4.6处代码就判断条件进不去，此时就执行不大4.7代码注释处的performClickInternal()也就不会onClcik。 一旦滑出View范围，View就会被移除PRESSED标志位，这个是不可逆的(即使你再滑回来，再UP时候)也不会执行performClick后续逻辑。
