Android事件的分发机制

　　在分析Android事件分发机制前，明确android的两大基础控件类型:View和ViewGroup。View即普通的控件，没有子布局的，如Button、TextView. ViewGroup继承自View，表示可以有子控件，如Linearlayout、Listview这些。今天我们先来了解View的事件分发机制。

        先看下代码，非常简单，只有一个Button，分别给它注册了OnClick和OnTouch的点击事件。

         btn.setOnClickListener(new View.OnClickListener() {
             @Override
             public void onClick(View v) {
                 Log.i("Tag", "This is button onClick event");
             }
         });
         btn.setOnTouchListener(new View.OnTouchListener() {
             @Override
             public boolean onTouch(View v, MotionEvent event) {
                 Log.i("Tag", "This is button onTouch action" + event.getAction());
                 return false;
             }
         });

运行一下项目，结果如下：

 I/Tag: This is button onTouch action0
 I/Tag: This is button onTouch action2
 I/Tag: This is button onTouch action2
 I/Tag: This is button onTouch action1
 I/Tag: This is button onClick event

可以看到，onTouch是有先于onClick执行的，因此事件的传递顺序是先onTouch，在到OnClick。具体为什么这样，下面会通过源码来说明。这时，我们可能注意到了，onTouch的方法是有返回值，这里是返回false，我们将它改为true再运行一次，结果如下：

 I/Tag: This is button onTouch action0
 I/Tag: This is button onTouch action2
 I/Tag: This is button onTouch action2
 I/Tag: This is button onTouch action2
 I/Tag: This is button onTouch action1

　　对比两次结果，我们发现onClick方法不再执行，为什么会这样，下面我将通过源码给大家一步步理清这个思路。

　　查看源码时，首先要知道所有View类型控件事件入口都是dispatchTouchEvent()，所以我们直接进入到View这个类里面的dispatchTouchEvent（）方法看一下。

  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)) {
             //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;
     }

 　　从源码第25行处可以看到，mOnTouchListener.onTouch()的方法首先被执行，如果li != null && li.mOnTouchListener != null&& (mViewFlags & ENABLED_MASK) == ENABLED&& li.mOnTouchListener.onTouch(this, event)都为真的话，result赋值为true，否则就执行onTouchEvent(event)方法。

　　从上面可以看到要符合条件有四个，

1、ListenerInfo li，它是view中的一个静态类，里面定义view的事件的监听等等，所以有涉及到view的事件，ListenerInfo都会被实例化，因此li不为null

2、mOnTouchiListener是在setOnTouchListener方法里面赋值的，只要touch事件被注册，mOnTouchiListener一定不会null

3、 (mViewFlags & ENABLED_MASK) == ENABLED，是判断当前点击的控件是否是enable的，button默认为enable，这个条件也恒定为true，

4、重点来了，li.mOnTouchListener.onTouch(this, event)就是回调控件onTouch方法，当这个条件也为true时，result=true,onTouchEvent（event）将不会被执行。如果onTouch返回false，就会再执行onTouchEvent(event)方法。

　　我们接着再进入到onTouchEvent方法查看源码。

     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);
                     }
                     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;
     }

从源码的21行我们可以看出，该控件可点击就会进入到switch判断中，当我们触发了手指离开的实际，则会进入到MotionEvent.ACTION_UP这个case当中。我们接着往下看，在源码的50行，调用到了mPerformClick()方法，我们继续进入到这个方法的源码看看。

     public boolean performClick() {
         final boolean result;
         final ListenerInfo li = mListenerInfo;
         if (li != null && li.mOnClickListener != null) {
             playSoundEffect(SoundEffectConstants.CLICK);
             li.mOnClickListener.onClick(this);
             result = true;
         } else {
             result = false;
         }
         sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_CLICKED);
         return result;
     }

现在我们可以看到，只要ListenerInfo和mOnClickListener不为null就会调用onClick这个方法，之前说过，只要有监听事件，ListenerInfo就不为null，带mOnClickListener又是在哪里赋值呢？我们再继续看下它的源码。

     public void setOnClickListener(@Nullable OnClickListener l) {
         if (!isClickable()) {
             setClickable(true);
         }
         getListenerInfo().mOnClickListener = l;
     }

　　看到这里一切就清楚了，当我们调用setOnClickListener方法来给按钮注册一个点击事件时，就会给mOnClickListener赋值。整个分发事件的顺序是onTouch()-->onTouchEvent(event)-->performClick()-->OnClick()。

　　现在我们可以解决之前的问题。

1、onTouch方法是优先于OnClick，所以是执行了onTouch，再执行onClick。

2、无论是dispatchTouchEvent还是onTouchEvent，如果返回true表示这个事件已经被消费、处理了，不再往下传了。在dispathTouchEvent的源码里可以看到，如果onTouchEvent返回了true，那么它也返回true。如果dispatchTouchEvent在执行onTouch监听的时候，onTouch返回了true，那么它也返回true，这个事件提前被onTouch消费掉了。就不再执行onTouchEvent了，更别说onClick监听了。