Android ViewGroup 事件分发

上文已经分析了 View 的事件分发，View 事件分发的流程为 onTouch -> onTouchEvent -> onClick，而且一定会执行 dispatchTouchEvent 方法

下面还是通过例子来分析

public class TouchViewGroup extends RelativeLayout {
    public TouchViewGroup(Context context) {
        super(context);
    }
    
    public TouchViewGroup(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    public TouchViewGroup(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        Log.d("V10Activity", "onTouchEvent -> TouchViewGroup" + event.getAction());
        return super.onTouchEvent(event);
    }

    @Override
    public boolean dispatchTouchEvent(MotionEvent event) {
        Log.d("V10Activity", "dispatchTouchEvent -> TouchViewGroup" + event.getAction());
        return super.dispatchTouchEvent(event);
    }
}

在 Activity 中注册监听事件

v10Binding.touchGroup.setOnTouchListener(new View.OnTouchListener() {
    @Override
    public boolean onTouch(View v, MotionEvent event) {
        Log.d("V10Activity", "onTouch -> TouchGroup" + event.getAction());
        return false;
    }
});

v10Binding.btn1.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View v) {
        Log.d("V10Activity", "btn1 onClick");
    }
});

v10Binding.btn2.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View v) {
        Log.d("V10Activity", "btn2 onClick");
    }
});

• 点击 Button1

  
  

• 点击 Button2

  
  

• 点击空白区域

  
  空白点击.png

虽然执行了 ViewGroup 的 dispatchTouchEvent 方法，但是并没有执行 onTouch 方法，所以咋们从 dispatchTouchEvent 源码分析

ViewGroup -> dispatchTouchEvent

@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
    ...
    //当执行 DOWN 事件会去清除重置 Touch 状态
    if (actionMasked == MotionEvent.ACTION_DOWN) {
        // Throw away all previous state when starting a new touch gesture.
        // The framework may have dropped the up or cancel event for the previous gesture
        // due to an app switch, ANR, or some other state change.
        cancelAndClearTouchTargets(ev);
        resetTouchState();
    }
    
    final boolean intercepted;
    //DOWN 会进行拦截
    if (actionMasked == MotionEvent.ACTION_DOWN
                    || mFirstTouchTarget != null) {
        final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
        if (!disallowIntercept) {
            //onInterceptTouchEvent 默认返回 false,不拦截
            intercepted = onInterceptTouchEvent(ev);
            ev.setAction(action); // restore action in case it was changed
        } else {
            //不拦截
            intercepted = false;
        }
    } else {
        intercepted = true;
    }
    
    final boolean canceled = resetCancelNextUpFlag(this)
                    || actionMasked == MotionEvent.ACTION_CANCEL;
    final boolean split = (mGroupFlags & FLAG_SPLIT_MOTION_EVENTS) != 0;
    //初始化新的 Touch 状态
    TouchTarget newTouchTarget = null;
    boolean alreadyDispatchedToNewTouchTarget = false;
    //没有取消也没有拦截，则分发给子View
    if (!canceled && !intercepted) {
        if (actionMasked == MotionEvent.ACTION_DOWN
                        || (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN)
                        || actionMasked == MotionEvent.ACTION_HOVER_MOVE) {
            //如果 Touch 状态为空并且子 View 个数不为 0
            if (newTouchTarget == null && childrenCount != 0) {
                final float x = ev.getX(actionIndex);
                final float y = ev.getY(actionIndex);
                final ArrayList<View> preorderedList = buildTouchDispatchChildList();
                final boolean customOrder = preorderedList == null
                                && isChildrenDrawingOrderEnabled();  
                //循环查找
                for (int i = childrenCount - 1; i >= 0; i--) {
                    //分发事件，记录状态
                    if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {
                        mLastTouchDownTime = ev.getDownTime();
                        if (preorderedList != null) {
                            // childIndex points into presorted list, find original index
                            for (int j = 0; j < childrenCount; j++) {
                                if (children[childIndex] == mChildren[j]) {
                                    mLastTouchDownIndex = j;
                                    break;
                                }
                            }
                        } else {
                            mLastTouchDownIndex = childIndex;
                        }
                        mLastTouchDownX = ev.getX();
                        mLastTouchDownY = ev.getY();
                        newTouchTarget = addTouchTarget(child, idBitsToAssign);
                        alreadyDispatchedToNewTouchTarget = true;
                        break;
                    }
                }
            }
        }
    }
    ...
}

dispatchTransformedTouchEvent

private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,
            View child, int desiredPointerIdBits) {
    ...
    if (child == null) {
        handled = super.dispatchTouchEvent(transformedEvent);
    } else {
        final float offsetX = mScrollX - child.mLeft;
        final float offsetY = mScrollY - child.mTop;
        transformedEvent.offsetLocation(offsetX, offsetY);
        if (! child.hasIdentityMatrix()) {
            transformedEvent.transform(child.getInverseMatrix());
        }

        handled = child.dispatchTouchEvent(transformedEvent);
    }
    // Done.
    transformedEvent.recycle();
    return handled;
    ...
}

ViewGroup 的 dispatchTouchEven 首先会判断事件类型，如果为 DOWN 事件则不进行拦截，将事件分发给子 View。如果子View 为 0 则自己处理，否则通过循环便利。

所以我们修改 onInterceptTouchEvent 返回值

@Override
public boolean onInterceptTouchEvent(MotionEvent ev) {
    return true;
}

• 点击 Button1

  
  

• 点击 Button2

  
  

• 点击空白区域

  
  

最终都被 ViewGroup 消费了

ViewGroup 事件分发流程图

ViewGroup 事件分发总结

1. 事件分发首先会调用 ViewGroup 的 dispatchTouchEvent 方法
2. 接着会判断 onInterceptTouchEvent 的返回值
3. 当 onInterceptTouchEvent 返回 false 时，会分发给子View，当 onInterceptTouchEvent 返回 true 时，会将事件拦截交给 ViewGroup 处理

ViewGroup 事件分发大致就介绍到这里了，如果有什么写得不对的，可以在下方评论留言，我会第一时间改正。