android View的事件分发源码解析

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。

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.youkuaiyun.com/guolin_blog/article/details/9097463
http://wangkuiwu.github.io/2015/01/03/TouchEvent-View/

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