android View的事件分发源码解析

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前面提到过，在view中涉及到事件分发，有俩个方法dispatchTouchEvent和onTouchEvent现在来了解下view的分发流程。

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现在事件从activity传到viewgroup，再传递到view上面了，来看view上dispatchTouchEvent的源码

 public boolean dispatchTouchEvent(MotionEvent event) {        // If the event should be handled by accessibility focus first.        if (event.isTargetAccessibilityFocus()) {            // We don't have focus or no virtual descendant has it, do not handle the event.            if (!isAccessibilityFocusedViewOrHost()) {                return false;            }            // We have focus and got the event, then use normal event dispatch.            event.setTargetAccessibilityFocus(false);        }        boolean result = false;        if (mInputEventConsistencyVerifier != null) {            mInputEventConsistencyVerifier.onTouchEvent(event, 0);        }        final int actionMasked = event.getActionMasked();        if (actionMasked == MotionEvent.ACTION_DOWN) {            // Defensive cleanup for new gesture            stopNestedScroll();        }        if (onFilterTouchEventForSecurity(event)) {            if ((mViewFlags & ENABLED_MASK) == ENABLED && handleScrollBarDragging(event)) {                result = true;            }            //noinspection SimplifiableIfStatement            ListenerInfo li = mListenerInfo;            if (li != null && li.mOnTouchListener != null                    && (mViewFlags & ENABLED_MASK) == ENABLED                    && li.mOnTouchListener.onTouch(this, event)) {                result = true;            }            if (!result && onTouchEvent(event)) {                result = true;            }        }        if (!result && mInputEventConsistencyVerifier != null) {            mInputEventConsistencyVerifier.onUnhandledEvent(event, 0);        }        // Clean up after nested scrolls if this is the end of a gesture;        // also cancel it if we tried an ACTION_DOWN but we didn't want the rest        // of the gesture.        if (actionMasked == MotionEvent.ACTION_UP ||                actionMasked == MotionEvent.ACTION_CANCEL ||                (actionMasked == MotionEvent.ACTION_DOWN && !result)) {            stopNestedScroll();        }        return result;    }

view里面dispatchTouchEvent代码比viewgroup里面少了很多，虽然代码少，但是做的事情不是那么简单。
mInputEventConsistencyVerifier 这个是调试相关的不用理会，onFilterTouchEventForSecurity()这个是分发的过滤条件，过滤掉view不在顶层，和设置属性使该View不在顶部，返回false，那么将不会走消费事件的onTouch和onTouchEvent，这个时候事件交给父viewgroup处理。

  public boolean onFilterTouchEventForSecurity(MotionEvent event) {        //noinspection RedundantIfStatement        if ((mViewFlags & FILTER_TOUCHES_WHEN_OBSCURED) != 0                && (event.getFlags() & MotionEvent.FLAG_WINDOW_IS_OBSCURED) != 0) {            // Window is obscured, drop this touch.            return false;        }        return true;    }

FILTER_TOUCHES_WHEN_OBSCURED是android:filterTouchesWhenObscured属性所对应的位。android:filterTouchesWhenObscured是true的话，则表示其他视图在该视图之上，导致该视图被隐藏时，该视图就不再响应触摸事件。
MotionEvent.FLAG_WINDOW_IS_OBSCURED为true的话，则表示该视图的窗口是被隐藏的。

 if (li != null && li.mOnTouchListener != null && (mViewFlags & ENABLED_MASK) == ENABLED                    && li.mOnTouchListener.onTouch(this, event)) {                result = true;   }

这里mOnTouchListener是用户自己设置的监听，当我们对控件设置有监听，并且该控件是可点击的，默认是不可点击的，根据返回值是否消耗掉该事件，如果true，事件消耗，不再走onTouchEvent。返回false才会进入 if (!result && onTouchEvent(event)) {｝处理事件。

public boolean onTouchEvent(MotionEvent event) {        final float x = event.getX();        final float y = event.getY();        final int viewFlags = mViewFlags;        final int action = event.getAction();        if ((viewFlags & ENABLED_MASK) == DISABLED) {            if (action == MotionEvent.ACTION_UP && (mPrivateFlags & PFLAG_PRESSED) != 0) {                setPressed(false);            }            // A disabled view that is clickable still consumes the touch            // events, it just doesn't respond to them.            return (((viewFlags & CLICKABLE) == CLICKABLE                    || (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)                    || (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE);        }        if (mTouchDelegate != null) {            if (mTouchDelegate.onTouchEvent(event)) {                return true;            }        }        if (((viewFlags & CLICKABLE) == CLICKABLE ||                (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE) ||                (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE) {            switch (action) {                case MotionEvent.ACTION_UP:                    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)) {                                    performClick();                                }                            }                        }                        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:                    mHasPerformedLongPress = false;                    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:                    setPressed(false);                    removeTapCallback();                    removeLongPressCallback();                    mInContextButtonPress = false;                    mHasPerformedLongPress = false;                    mIgnoreNextUpEvent = false;                    break;                case MotionEvent.ACTION_MOVE:                    drawableHotspotChanged(x, y);                    // Be lenient about moving outside of buttons                    if (!pointInView(x, y, mTouchSlop)) {                        // Outside button                        removeTapCallback();                        if ((mPrivateFlags & PFLAG_PRESSED) != 0) {                            // Remove any future long press/tap checks                            removeLongPressCallback();                            setPressed(false);                        }                    }                    break;            }            return true;        }        return false;    }

哇，好多代码，和刚才的dispatchTouchEvent，不是同一个级别的。这里需要注意，如果在onTouchEvent中MotionEvent.ACTION_DOWN的时候返回false，后续的事件就不会触发了，这是由于dispatchTouchEvent里面的 if (!result && onTouchEvent(event)) {｝代码。一旦false，事件就抛出去了，和咱没关系了。但是在onTouch里面如果返回false，onTouch还会再次调用呢，这是因为在onTouchEvent里面switch最后一直都是true，默认就消费事件，所以只要满足条件，都会一直调用。

好了就到这里吧，来句总结
View中的dispatchTouchEvent()会将事件传递给”自己的onTouch()”和”自己的onTouchEvent()”进行处理。而且onTouch()的优先级比onTouchEvent()的优先级要高。
onTouch()与onTouchEvent()都是View中用户处理触摸事件的API。onTouch是OnTouchListener接口中的函数，OnTouchListener接口需要用户自己实现。onTouchEvent()是View自带的接口，Android系统提供了默认的实现；当然，用户可以重载该API。
onTouch()与onTouchEvent()有两个不同之处：
onTouch()是View提供给用户，让用户自己处理触摸事件的接口。而onTouchEvent()是Android系统自己实现的接口。
onTouch()的优先级比onTouchEvent()的优先级更高。dispatchTouchEvent()中分发事件的时候，会先将事件分配给onTouch()进行处理，然后才分配给onTouchEvent()进行处理。如果onTouch()对触摸事件进行了处理，并且返回true；那么，该触摸事件就不会分配在分配给onTouchEvent()进行处理了。只有当onTouch()没有处理，或者处理了但返回false时，才会分配给onTouchEvent()进行处理。onTouchEvent里面MotionEvent.ACTION_DOWN必须返回true才能处理事件。

参考文章
http://blog.csdn.net/guolin_blog/article/details/9097463
http://wangkuiwu.github.io/2015/01/03/TouchEvent-View/

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