Android事件的分發機制，android事件分發

Android事件的分發機制，android事件分發

　　在分析Android事件分發機制前，明確android的兩大基礎控件類型:View和ViewGroup。View即普通的控件，沒有子布局的，如Button、TextView. ViewGroup繼承自View，表示可以有子控件，如Linearlayout、Listview這些。今天我們先來了解View的事件分發機制。

        先看下代碼，非常簡單，只有一個Button，分別給它注冊了OnClick和OnTouch的點擊事件。

 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         });

運行一下項目，結果如下：

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是有先於onClick執行的，因此事件的傳遞順序是先onTouch，在到OnClick。具體為什麼這樣，下面會通過源碼來說明。這時，我們可能注意到了，onTouch的方法是有返回值，這裡是返回false，我們將它改為true再運行一次，結果如下：

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

　　對比兩次結果，我們發現onClick方法不再執行，為什麼會這樣，下面我將通過源碼給大家一步步理清這個思路。 　　查看源碼時，首先要知道所有View類型控件事件入口都是dispatchTouchEvent()，所以我們直接進入到View這個類裡面的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     }

 　　從源碼第25行處可以看到，mOnTouchListener.onTouch()的方法首先被執行，如果li != null && li.mOnTouchListener != null&& (mViewFlags & ENABLED_MASK) == ENABLED&& li.mOnTouchListener.onTouch(this, event)都為真的話，result賦值為true，否則就執行onTouchEvent(event)方法。 　　從上面可以看到要符合條件有四個， 1、ListenerInfo li，它是view中的一個靜態類，裡面定義view的事件的監聽等等，所以有涉及到view的事件，ListenerInfo都會被實例化，因此li不為null 2、mOnTouchiListener是在setOnTouchListener方法裡面賦值的，只要touch事件被注冊，mOnTouchiListener一定不會null 3、 (mViewFlags & ENABLED_MASK) == ENABLED，是判斷當前點擊的控件是否是enable的，button默認為enable，這個條件也恆定為true， 4、重點來了，li.mOnTouchListener.onTouch(this, event)就是回調控件onTouch方法，當這個條件也為true時，result=true,onTouchEvent（event）將不會被執行。如果onTouch返回false，就會再執行onTouchEvent(event)方法。 　　我們接著再進入到onTouchEvent方法查看源碼。

  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     }

        從源碼的21行我們可以看出，該控件可點擊就會進入到switch判斷中，當我們觸發了手指離開的實際，則會進入到MotionEvent.ACTION_UP這個case當中。我們接著往下看，在源碼的50行，調用到了mPerformClick()方法，我們繼續進入到這個方法的源碼看看。

 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     }

    現在我們可以看到，只要ListenerInfo和mOnClickListener不為null就會調用onClick這個方法，之前說過，只要有監聽事件，ListenerInfo就不為null，帶mOnClickListener又是在哪裡賦值呢？我們再繼續看下它的源碼。

1     public void setOnClickListener(@Nullable OnClickListener l) {
2         if (!isClickable()) {
3             setClickable(true);
4         }
5         getListenerInfo().mOnClickListener = l;
6     }

　　看到這裡一切就清楚了，當我們調用setOnClickListener方法來給按鈕注冊一個點擊事件時，就會給mOnClickListener賦值。整個分發事件的順序是onTouch()-->onTouchEvent(event)-->performClick()-->OnClick()。

　　現在我們可以解決之前的問題。

1、onTouch方法是優先於OnClick，所以是執行了onTouch，再執行onClick。

2、無論是dispatchTouchEvent還是onTouchEvent，如果返回true表示這個事件已經被消費、處理了，不再往下傳了。在dispathTouchEvent的源碼裡可以看到，如果onTouchEvent返回了true，那麼它也返回true。如果dispatchTouchEvent在執行onTouch監聽的時候，onTouch返回了true，那麼它也返回true，這個事件提前被onTouch消費掉了。就不再執行onTouchEvent了，更別說onClick監聽了。