ViewGroup事件分发

Myth丶恋晨 2022-08-21 02:28 156阅读 0赞

## ViewGroup事件分发 ##

### ViewGroup点击事件流程 ###

ViewGroup:`dispatchTouchEvent()`\->ViewGroup:`onInterceptTouchEvent()` \- > View:`dispatchTouchEvent()` \- > View:`onTouchEvent()` \- >ViewGroup:`onTouchEvent()`

### ViewGroup:dispatchTouchEvent()源码 ###

*  **因为代码较多，分段思考。首先看ACTION\_DOWN的代码**

//是否超屏幕
     if (!onFilterTouchEventForSecurity(ev)) {  
               return false;  
           }  
    
           final int action = ev.getAction();  
           final float xf = ev.getX();  
           final float yf = ev.getY();  
           final float scrolledXFloat = xf + mScrollX;  
           final float scrolledYFloat = yf + mScrollY;  
           final Rect frame = mTempRect;  
    
           boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;  
    
           if (action == MotionEvent.ACTION_DOWN) {  
                //---------------关键点1
               if (mMotionTarget != null) {  
                   // this is weird, we got a pen down, but we thought it was 
                   // already down! 
                   // XXX: We should probably send an ACTION_UP to the current 
                   // target. 
                   mMotionTarget = null;  
               }  
              //------------关键点2
               if (disallowIntercept || !onInterceptTouchEvent(ev)) {  
                   // reset this event's action (just to protect ourselves) 
                   ev.setAction(MotionEvent.ACTION_DOWN);  
                   // We know we want to dispatch the event down, find a child 
                   // who can handle it, start with the front-most child. 
                   final int scrolledXInt = (int) scrolledXFloat;  
                   final int scrolledYInt = (int) scrolledYFloat;  
                   final View[] children = mChildren;  
                   final int count = mChildrenCount;  
    
                   for (int i = count - 1; i >= 0; i--) {  
                       final View child = children[i];  
                       if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE  
                               || child.getAnimation() != null) {  
                           child.getHitRect(frame);  
                           if (frame.contains(scrolledXInt, scrolledYInt)) {  
                               // offset the event to the view's coordinate system 
                               final float xc = scrolledXFloat - child.mLeft;  
                               final float yc = scrolledYFloat - child.mTop;  
                               ev.setLocation(xc, yc);  
                               child.mPrivateFlags &= ~CANCEL_NEXT_UP_EVENT; 
                                //关键点3 
                               if (child.dispatchTouchEvent(ev))  {  
                                   // Event handled, we have a target now. 
                                   mMotionTarget = child;  
                                   return true;  
                               }  
                               // The event didn't get handled, try the next view. 
                               // Don't reset the event's location, it's not 
                               // necessary here. 
                           }  
                       }  
                   }  
               }  
           }

在关键点1中我们能看到有一个字段`mMotionTarget`，先判断其是否为null, 如果不为null,则将其置为空。该字段主要目的是保存消费触摸事件的View的。

在关键点2中，对`disallowIntercept`和`！onInterceptTouchEvent(ev)`进行了判段，如果两者有一个为true就可以向下分发。`disallowIntercept`的默认为false，可以通过在子View的`dispatchTouchEvent()`中调用`getParent().requestDisallowInterceptTouchEvent(true)`方法修改其值。`onInterceptTouchEvent()`方法在当前类中进行实现，返回false表示不拦截，返回true表示拦截。

在关键点3中，首先在之前判断了该View是否包含触摸点，其次if中判断childView的`dispatchTouchEvent()`是否返回true，如果返回true，表示childView捕获了本次触摸，则将childView 赋值给`mMotionTarget`，同时结束ViewGroup的`disaptchTouchEvent()`方法，并返回true。

*  **ACTION\_MOVE的代码**

public boolean dispatchTouchEvent(MotionEvent ev) {  
           final int action = ev.getAction();  
           final float xf = ev.getX();  
           final float yf = ev.getY();  
           final float scrolledXFloat = xf + mScrollX;  
           final float scrolledYFloat = yf + mScrollY;  
           final Rect frame = mTempRect;  
    
           boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;  
    
          //...ACTION_DOWN 
    
          //...ACTIN_UP or ACTION_CANCEL 
    
           // The event wasn't an ACTION_DOWN, dispatch it to our target if 
           // we have one. 
            //-------关键点1
            final View target = mMotionTarget;  
    
    
           // if have a target, see if we're allowed to and want to intercept its 
           // events 
           if (!disallowIntercept && onInterceptTouchEvent(ev)) {  
               //.... 
           }  
           // finally offset the event to the target's coordinate system and 
           // dispatch the event. 
           final float xc = scrolledXFloat - (float) target.mLeft;  
           final float yc = scrolledYFloat - (float) target.mTop;  
           ev.setLocation(xc, yc);  
    
           return target.dispatchTouchEvent(ev);  
       }

在关键点1中，直接将在ACTION\_DOWN中保存的`mMotionTarget`赋值给target，并`return target.dispatchTouchEvent(ev);`。由此可见，之所以在childView中当ACTION\_DOWN没有捕获，就无法捕获后续的ACTION\_MOVE和ACTION\_UP触摸，是因为此时在ViewGroup中直接将事件交给了在ACTION\_DOWN中保存的处理触摸的View对象。

*  **ACTION\_UP的代码**

public boolean dispatchTouchEvent(MotionEvent ev) {  
           if (!onFilterTouchEventForSecurity(ev)) {  
               return false;  
           }  
    
           final int action = ev.getAction();  
           final float xf = ev.getX();  
           final float yf = ev.getY();  
           final float scrolledXFloat = xf + mScrollX;  
           final float scrolledYFloat = yf + mScrollY;  
           final Rect frame = mTempRect;  
    
           boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;  
    
           if (action == MotionEvent.ACTION_DOWN) {...}  
    
        boolean isUpOrCancel = (action == MotionEvent.ACTION_UP) ||  
                       (action == MotionEvent.ACTION_CANCEL);  
    
        if (isUpOrCancel) {  
                   mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;  
               }  
        final View target = mMotionTarget;  
        if(target ==null ){...}  
        if (!disallowIntercept && onInterceptTouchEvent(ev)) {...}  
    
           if (isUpOrCancel) {  
               mMotionTarget = null;  
           }  
    
           // finally offset the event to the target's coordinate system and 
           // dispatch the event. 
           final float xc = scrolledXFloat - (float) target.mLeft;  
           final float yc = scrolledYFloat - (float) target.mTop;  
           ev.setLocation(xc, yc);  
    
           return target.dispatchTouchEvent(ev);  
       }

在ACTION\_UP实现没有太大的难点，清楚一下字段属性，调用之前保存的childView对象的分发方法。

*  **当没有childView来处理触摸时**

final View target = mMotionTarget;  
           if (target == null) {  
               // We don't have a target, this means we're handling the 
               // event as a regular view. 
               ev.setLocation(xf, yf);  
               if ((mPrivateFlags & CANCEL_NEXT_UP_EVENT) != 0) {  
                   ev.setAction(MotionEvent.ACTION_CANCEL);  
                   mPrivateFlags &= ~CANCEL_NEXT_UP_EVENT;  
               }  
               return super.dispatchTouchEvent(ev);  
           }

当没有触摸时，会调用父类的`dispatchTouchEvent(ev)`，因为ViewGroup继承自View，那么一切就很简单了。就会调用ViewGroup本身的`onTouchEvent()`。

### 总结 ###

1.  在ACTION\_DOWN中，如果ViewGroup中有能处理该次触摸的childView，则将会调用childView的`dispatchTouchEvent`，同时保存childView对象。当本次触摸的ACTION\_MOVE和ACTION\_UP事件，也将有此次childView来处理。
2.  当ViewGroup中没有能够处理此次触摸的childView,则会调用其父类View的`dispatchTouchEvent()`方法，那么就类似childView的调用了。