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[操作系统]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 action02 I/Tag: This is button onTouch action23 I/Tag: This is button onTouch action24 I/Tag: This is button onTouch action15 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 action02 I/Tag: This is button onTouch action23 I/Tag: This is button onTouch action24 I/Tag: This is button onTouch action25 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 gesture18       stopNestedScroll();19     }20     if (onFilterTouchEventForSecurity(event)) {21       //noinspection SimplifiableIfStatement22       ListenerInfo li = mListenerInfo;23       if (li != null && li.mOnTouchListener != null24           && (mViewFlags & ENABLED_MASK) == ENABLED25           && 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 rest37     // 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 buttons105           if (!pointInView(x, y, mTouchSlop)) {106             // Outside button107             removeTapCallback();108             if ((mPrivateFlags & PFLAG_PRESSED) != 0) {109               // Remove any future long press/tap checks110               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。