在網上搜到了很有意思的一道題，仔細想一下這道題。
答案是如果有ACTION_MOVE操作，那麼ACTION_DOWN傳遞給C，ACTION_MOVE和ACTION_UP都傳遞給B，A沒有獲得任何事件。如果沒有執行滑動操作，那麼ACTION_DOWN和ACTION_UP都傳遞給C。
因爲事件會先從A-》B依次執行
onInterceptTouchEvent，如果那一邊返回了true，就說明該層viewgrou攔截了事件傳遞.
而現在B只對ACTION_MOVE這個事件做攔截，如果B攔截了ACTION_MOVE這個事件，那麼後續所有的事件都由B來消費。
如果B沒有對ACTION_MOVE這個事件做攔截，那麼後續所有的事件還是由C來消費。
這裏追加個問題。
也就是當一層viewgrou決定攔截一個事件，那麼後續的事件都會由他來消費嗎？
不一定，這裏B View的onInterceptTouchEvent改一下，如下:
@Override
public boolean onInterceptTouchEvent(MotionEvent ev) {
if(ev.getAction()== MotionEvent.ACTION_DOWN){
return true;
}
return super.onInterceptTouchEvent(ev);
}
攔截的是第一個ACTION_DOWN事件。
運行一下程序，原本以爲結果是ACTION_DOWN和ACTION_UP還有ACTION_MOVE都會由B來執行，但是結果卻是B只執行了ACTION_DOWN，那麼ACTION_UP和ACTION_MOVE去哪裏了？
這裏需要理解下onTouchEvent方法執行了什麼？因爲onTouchEvent如果返回true的情況後續事件纔會繼續消費，而在View的
onTouchEvent源碼
public boolean onTouchEvent(MotionEvent event) {
final float x = event.getX();
final float y = event.getY();
final int viewFlags = mViewFlags;
final int action = event.getAction();
final boolean clickable = ((viewFlags & CLICKABLE) == CLICKABLE
|| (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)
|| (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE;
if ((viewFlags & ENABLED_MASK) == DISABLED) {
if (action == MotionEvent.ACTION_UP && (mPrivateFlags & PFLAG_PRESSED) != 0) {
setPressed(false);
}
mPrivateFlags3 &= ~PFLAG3_FINGER_DOWN;
// A disabled view that is clickable still consumes the touch
// events, it just doesn't respond to them.
return clickable;
}
if (mTouchDelegate != null) {
if (mTouchDelegate.onTouchEvent(event)) {
return true;
}
}
if (clickable || (viewFlags & TOOLTIP) == TOOLTIP) {
switch (action) {
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) {
// 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();
}
}
}
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;
case MotionEvent.ACTION_DOWN:
if (event.getSource() == InputDevice.SOURCE_TOUCHSCREEN) {
mPrivateFlags3 |= PFLAG3_FINGER_DOWN;
}
mHasPerformedLongPress = false;
if (!clickable) {
checkForLongClick(0, x, y);
break;
}
if (performButtonActionOnTouchDown(event)) {
break;
}
// Walk up the hierarchy to determine if we're inside a scrolling container.
boolean isInScrollingContainer = isInScrollingContainer();
// For views inside a scrolling container, delay the pressed feedback for
// a short period in case this is a scroll.
if (isInScrollingContainer) {
mPrivateFlags |= PFLAG_PREPRESSED;
if (mPendingCheckForTap == null) {
mPendingCheckForTap = new CheckForTap();
}
mPendingCheckForTap.x = event.getX();
mPendingCheckForTap.y = event.getY();
postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout());
} else {
// Not inside a scrolling container, so show the feedback right away
setPressed(true, x, y);
checkForLongClick(0, x, y);
}
break;
case MotionEvent.ACTION_CANCEL:
if (clickable) {
setPressed(false);
}
removeTapCallback();
removeLongPressCallback();
mInContextButtonPress = false;
mHasPerformedLongPress = false;
mIgnoreNextUpEvent = false;
mPrivateFlags3 &= ~PFLAG3_FINGER_DOWN;
break;
case MotionEvent.ACTION_MOVE:
if (clickable) {
drawableHotspotChanged(x, y);
}
// Be lenient about moving outside of buttons
if (!pointInView(x, y, mTouchSlop)) {
// Outside button
// Remove any future long press/tap checks
removeTapCallback();
removeLongPressCallback();
if ((mPrivateFlags & PFLAG_PRESSED) != 0) {
setPressed(false);
}
mPrivateFlags3 &= ~PFLAG3_FINGER_DOWN;
}
break;
}
return true;
}
return false;
}
仔細看下 case MotionEvent.ACTION_DOWN
當我們ACTION_DOWN事件執行的時候，CLICKABLE如果爲false的話，會執行break掉代碼段，而直接去執行return fasle的操作。所以後面的事件就接受不到了，所以我們把這個CLICKABLE設置爲true，是不是後面的事件就可以接收到了。
這個設置在xml佈局裏面給B加上 android:clickable="true" 這個屬性，再跑一下程序，發現後面的ACTION_UP和ACTION_MOVE都被B接受到了。
如有需要，demo源碼如下：