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。
1 btn.setOnClickListener(new View.OnClickListener() { 2 @Override 3 public void onClick(View v) { 4 Log.i("Tag", "This is button onClick event"); 5 } 6 }); 7 btn.setOnTouchListener(new View.OnTouchListener() { 8 @Override 9 public boolean onTouch(View v, MotionEvent event) { 10 Log.i("Tag", "This is button onTouch action" + event.getAction()); 11 return false; 12 } 13 });
运行一下项目，结果如下：
1 I/Tag: This is button onTouch action0 2 I/Tag: This is button onTouch action2 3 I/Tag: This is button onTouch action2 4 I/Tag: This is button onTouch action1 5 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：
1 I/Tag: This is button onTouch action0 2 I/Tag: This is button onTouch action2 3 I/Tag: This is button onTouch action2 4 I/Tag: This is button onTouch action2 5 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。
1 public boolean dispatchTouchEvent(MotionEvent event) { 2 // If the event should be handled by accessibility focus first. 3 if (event.isTargetAccessibilityFocus()) { 4 // We don't have focus or no virtual descendant has it, do not handle the event. 5 if (!isAccessibilityFocusedViewOrHost()) { 6 return false; 7 } 8 // We have focus and got the event, then use normal event dispatch. 9 event.setTargetAccessibilityFocus(false); 10 } 11 boolean result = false; 12 if (mInputEventConsistencyVerifier != null) { 13 mInputEventConsistencyVerifier.onTouchEvent(event, 0); 14 } 15 final int actionMasked = event.getActionMasked(); 16 if (actionMasked == MotionEvent.ACTION_DOWN) { 17 // Defensive cleanup for new gesture 18 stopNestedScroll(); 19 } 20 if (onFilterTouchEventForSecurity(event)) { 21 //noinspection SimplifiableIfStatement 22 ListenerInfo li = mListenerInfo; 23 if (li != null && li.mOnTouchListener != null 24 && (mViewFlags & ENABLED_MASK) == ENABLED 25 && li.mOnTouchListener.onTouch(this, event)) { 26 result = true; 27 } 28 if (!result && onTouchEvent(event)) { 29 result = true; 30 } 31 } 32 if (!result && mInputEventConsistencyVerifier != null) { 33 mInputEventConsistencyVerifier.onUnhandledEvent(event, 0); 34 } 35 // Clean up after nested scrolls if this is the end of a gesture; 36 // also cancel it if we tried an ACTION_DOWN but we didn't want the rest 37 // of the gesture. 38 if (actionMasked == MotionEvent.ACTION_UP || 39 actionMasked == MotionEvent.ACTION_CANCEL || 40 (actionMasked == MotionEvent.ACTION_DOWN && !result)) { 41 stopNestedScroll(); 42 } 43 return result; 44 }
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。
1 public boolean onTouchEvent(MotionEvent event) { 2 final float x = event.getX(); 3 final float y = event.getY(); 4 final int viewFlags = mViewFlags; 5 final int action = event.getAction(); 6 if ((viewFlags & ENABLED_MASK) == DISABLED) { 7 if (action == MotionEvent.ACTION_UP && (mPrivateFlags & PFLAG_PRESSED) != 0) { 8 setPressed(false); 9 } 10 // A disabled view that is clickable still consumes the touch 11 // events, it just doesn't respond to them. 12 return (((viewFlags & CLICKABLE) == CLICKABLE 13 || (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE) 14 || (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE); 15 } 16 if (mTouchDelegate != null) { 17 if (mTouchDelegate.onTouchEvent(event)) { 18 return true; 19 } 20 } 21 if (((viewFlags & CLICKABLE) == CLICKABLE || 22 (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE) || 23 (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE) { 24 switch (action) { 25 case MotionEvent.ACTION_UP: 26 boolean prepressed = (mPrivateFlags & PFLAG_PREPRESSED) != 0; 27 if ((mPrivateFlags & PFLAG_PRESSED) != 0 || prepressed) { 28 // take focus if we don't have it already and we should in 29 // touch mode. 30 boolean focusTaken = false; 31 if (isFocusable() && isFocusableInTouchMode() && !isFocused()) { 32 focusTaken = requestFocus(); 33 } 34 if (prepressed) { 35 // The button is being released before we actually 36 // showed it as pressed. Make it show the pressed 37 // state now (before scheduling the click) to ensure 38 // the user sees it. 39 setPressed(true, x, y); 40 } 41 if (!mHasPerformedLongPress && !mIgnoreNextUpEvent) { 42 // This is a tap, so remove the longpress check 43 removeLongPressCallback(); 44 // Only perform take click actions if we were in the pressed state 45 if (!focusTaken) { 46 // Use a Runnable and post this rather than calling 47 // performClick directly. This lets other visual state 48 // of the view update before click actions start. 49 if (mPerformClick == null) { 50 mPerformClick = new PerformClick(); 51 } 52 if (!post(mPerformClick)) { 53 performClick(); 54 } 55 } 56 } 57 if (mUnsetPressedState == null) { 58 mUnsetPressedState = new UnsetPressedState(); 59 } 60 if (prepressed) { 61 postDelayed(mUnsetPressedState, 62 ViewConfiguration.getPressedStateDuration()); 63 } else if (!post(mUnsetPressedState)) { 64 // If the post failed, unpress right now 65 mUnsetPressedState.run(); 66 } 67 removeTapCallback(); 68 } 69 mIgnoreNextUpEvent = false; 70 break; 71 case MotionEvent.ACTION_DOWN: 72 mHasPerformedLongPress = false; 73 if (performButtonActionOnTouchDown(event)) { 74 break; 75 } 76 // Walk up the hierarchy to determine if we're inside a scrolling container. 77 boolean isInScrollingContainer = isInScrollingContainer(); 78 // For views inside a scrolling container, delay the pressed feedback for 79 // a short period in case this is a scroll. 80 if (isInScrollingContainer) { 81 mPrivateFlags |= PFLAG_PREPRESSED; 82 if (mPendingCheckForTap == null) { 83 mPendingCheckForTap = new CheckForTap(); 84 } 85 mPendingCheckForTap.x = event.getX(); 86 mPendingCheckForTap.y = event.getY(); 87 postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout()); 88 } else { 89 // Not inside a scrolling container, so show the feedback right away 90 setPressed(true, x, y); 91 checkForLongClick(0); 92 } 93 break; 94 case MotionEvent.ACTION_CANCEL: 95 setPressed(false); 96 removeTapCallback(); 97 removeLongPressCallback(); 98 mInContextButtonPress = false; 99 mHasPerformedLongPress = false; 100 mIgnoreNextUpEvent = false; 101 break; 102 case MotionEvent.ACTION_MOVE: 103 drawableHotspotChanged(x, y); 104 // Be lenient about moving outside of buttons 105 if (!pointInView(x, y, mTouchSlop)) { 106 // Outside button 107 removeTapCallback(); 108 if ((mPrivateFlags & PFLAG_PRESSED) != 0) { 109 // Remove any future long press/tap checks 110 removeLongPressCallback(); 111 setPressed(false); 112 } 113 } 114 break; 115 } 116 return true; 117 } 118 return false; 119 }
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。
1 public boolean performClick() { 2 final boolean result; 3 final ListenerInfo li = mListenerInfo; 4 if (li != null && li.mOnClickListener != null) { 5 playSoundEffect(SoundEffectConstants.CLICK); 6 li.mOnClickListener.onClick(this); 7 result = true; 8 } else { 9 result = false; 10 } 11 sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_CLICKED); 12 return result; 13 }
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。
1 public void setOnClickListener(@Nullable OnClickListener l) { 2 if (!isClickable()) { 3 setClickable(true); 4 } 5 getListenerInfo().mOnClickListener = l; 6 }
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。
