View事件分发

注：本文基于自己的理解，水平比较有限，欢迎指出不对的地方，不喜勿喷。
上文讲了ViewGroup事件分发，http://blog.csdn.net/zxd_android/article/details/78019600，本文讲下View的事件分发。

我们先来看下View的dispatchTouchEvent方法：

  /**     * Pass the touch screen motion event down to the target view, or this     * view if it is the target.     *     * @param event The motion event to be dispatched.     * @return True if the event was handled by the view, false otherwise.     */    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;    }

方法不长，我们看主要的代码。

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

如果我们设置了onTouhListener，并且当前View处于ENABLED状态，并且onTouch 返回true，设置result 为true，看上面的代码，如果result为true,onTouchEvent就不会执行，我们也能看到，onTouch监听事件或者onTouchEvent其中之一返回true，就表示该view处理事件分发。

接下来，我们看下View的onTouchEnent方法：

 /**     * Implement this method to handle touch screen motion events.     * <p>     * If this method is used to detect click actions, it is recommended that     * the actions be performed by implementing and calling     * {@link #performClick()}. This will ensure consistent system behavior,     * including:     * <ul>     * <li>obeying click sound preferences     * <li>dispatching OnClickListener calls     * <li>handling {@link AccessibilityNodeInfo#ACTION_CLICK ACTION_CLICK} when     * accessibility features are enabled     * </ul>     *     * @param event The motion event.     * @return True if the event was handled, false otherwise.     */    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;    }

1、第6行 if ((viewFlags & ENABLED_MASK) == DISABLED) 如果是DISABLED状态，直接return。
2、第13行 mTouchDelegate是代理事件处理的类，这里我们不做讨论。
3、 if (((viewFlags & CLICKABLE) == CLICKABLE ||
(viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE) ||
(viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE) {

这个if判断有点长，满足CLICLABLE或者LONG_CLICKABLE以及CONTEXT_CLICKABLE其中之一就进入方法内部，我们一般自定义View往往是不可以CLICKABLE 的，所以Touch事件接收不到的原因就在这里，像我们的原生控件Button是因为主题中设置android:clickable为true，所以我们自定义View时候必要的时候加上。

4、先看ACTION_DOWN事件，首先设置mHasPerformedLongPress标志位，表示是否长按，然后postDelayed一个runnable，延迟100ms执行，我们看下这个CheckForTap实现。

 private final class CheckForTap implements Runnable {        public float x;        public float y;        @Override        public void run() {            mPrivateFlags &= ~PFLAG_PREPRESSED;            setPressed(true, x, y);            checkForLongClick(ViewConfiguration.getTapTimeout(), x, y);        }    }

设置setPressed 为true，调用checkForLongClick方法。

 private void checkForLongClick(int delayOffset, float x, float y) {        if ((mViewFlags & LONG_CLICKABLE) == LONG_CLICKABLE) {            mHasPerformedLongPress = false;            if (mPendingCheckForLongPress == null) {                mPendingCheckForLongPress = new CheckForLongPress();            }            mPendingCheckForLongPress.setAnchor(x, y);            mPendingCheckForLongPress.rememberWindowAttachCount();            postDelayed(mPendingCheckForLongPress,                    ViewConfiguration.getLongPressTimeout() - delayOffset);        }    }

首先new 一个Runnable，又推迟一个任务执行。我们再看下

private final class CheckForLongPress implements Runnable {        private int mOriginalWindowAttachCount;        private float mX;        private float mY;        @Override        public void run() {            if (isPressed() && (mParent != null)                    && mOriginalWindowAttachCount == mWindowAttachCount) {                if (performLongClick(mX, mY)) {                    mHasPerformedLongPress = true;                }            }        }

如果手指没有松开，isPressed()一直 为true，最终会触发onLongClick回调。

5、我们再看下ACTION_UP事件， if (!mHasPerformedLongPress && !mIgnoreNextUpEvent) 会remove刚才的长按的runnable。

if (mPerformClick == null) {    mPerformClick = new PerformClick();  }

这里面又是一个runnable，最终触发performClick方法，回调onclick。