3.4 View的事件分发机制(四)

本文例子参考：陈育 Android View 事件分发机制源码详解(View篇)

1. activity

public class DispatchActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_dispatch);
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        int action = event.getAction();
        switch (action) {
            case MotionEvent.ACTION_DOWN:
                Log.d("aaa", "activity onTouchEvent down");
                break;
            case MotionEvent.ACTION_MOVE:
                Log.d("aaa", "activity onTouchEvent move");
                break;
            case MotionEvent.ACTION_UP:
                Log.d("aaa", "activity onTouchEvent up");
                break;
        }
        boolean result = super.onTouchEvent(event);
        Log.d("aaa", "activity onTouchEvent result = " + result);
        return result;
    }
}

2. 布局文件

3. ViewGroup

public class ViewGroupA extends LinearLayout {

    public ViewGroupA(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        int action = event.getAction();
        switch (action) {
            case MotionEvent.ACTION_DOWN:
                Log.d("aaa", "ViewGroupA onTouchEvent down");
                break;
            case MotionEvent.ACTION_MOVE:
                Log.d("aaa", "ViewGroupA onTouchEvent move");
                break;
            case MotionEvent.ACTION_UP:
                Log.d("aaa", "ViewGroupA onTouchEvent up");
                break;
        }
        boolean result = super.onTouchEvent(event);
        Log.d("aaa", "ViewGroup onTouchEvent result = " + result);
        return result;
    }

    @Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
        switch (ev.getAction()) {
            case MotionEvent.ACTION_DOWN:
                Log.d("aaa", "ViewGroupA dispatchTouchEvent down");
                break;
            case MotionEvent.ACTION_MOVE:
                Log.d("aaa", "ViewGroupA dispatchTouchEvent move");
                break;
            case MotionEvent.ACTION_UP:
                Log.d("aaa", "ViewGroupA dispatchTouchEvent up");
                break;
        }
        return super.dispatchTouchEvent(ev);
    }

    @Override
    public boolean onInterceptTouchEvent(MotionEvent ev) {
        switch (ev.getAction()) {
            case MotionEvent.ACTION_DOWN:
                Log.d("aaa", "ViewGroupA onInterceptTouchEvent down");
                break;
            case MotionEvent.ACTION_MOVE:
                Log.d("aaa", "ViewGroupA onInterceptTouchEvent move");
                break;
            case MotionEvent.ACTION_UP:
                Log.d("aaa", "ViewGroupA onInterceptTouchEvent up");
                break;
        }
        return super.onInterceptTouchEvent(ev);
    }
}

4. View

public class ViewA extends View {
    
    public ViewA(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        switch (event.getAction()){
            case MotionEvent.ACTION_DOWN:
                Log.d("aaa","ViewA onTouchEvent down");
                break;
            case MotionEvent.ACTION_MOVE:
                Log.d("aaa","ViewA onTouchEvent move");
                break;
            case MotionEvent.ACTION_UP:
                Log.d("aaa","ViewA onTouchEvent up");
                break;
        }
        boolean result = super.onTouchEvent(event);
        Log.d("aaa","ViewA onTouchEvent result="+result);
        return result;
    }

    @Override
    public boolean dispatchTouchEvent(MotionEvent event) {
        switch (event.getAction()){
            case MotionEvent.ACTION_DOWN:
                Log.d("aaa","ViewA dispatchTouchEvent down");
                break;
            case MotionEvent.ACTION_MOVE:
                Log.d("aaa","ViewA dispatchTouchEvent move");
                break; 
            case MotionEvent.ACTION_UP:
                Log.d("aaa","ViewA dispatchTouchEvent up");
                break;
        }
        return super.dispatchTouchEvent(event);
    }
}

5. 默认情况

所有事件都用super的方法，日志如下：

ViewGroupA dispatchTouchEvent down
ViewGroupA onInterceptTouchEvent down
ViewA dispatchTouchEvent down
ViewA onTouchEvent down
ViewA onTouchEvent result=false
ViewGroupA onTouchEvent down
ViewGroupA onTouchEvent result = false
activity onTouchEvent down
activity onTouchEvent result = false
activity onTouchEvent move
activity onTouchEvent result = false
activity onTouchEvent move
activity onTouchEvent result = false
activity onTouchEvent up
activity onTouchEvent result = false

可见，ViewGroupA分发事件->拦截事件(默认false)->ViewA分发事件->ViewA的OnTouchEvent方法(super返回false)->ViewGroupA的onTouchEvent(super返回false)->Activity的onTouchEvent。
后续的move和up事件都直接到Activity处。这里的逻辑应该在Activity的dispatchTouchEvent方法中实现。

6. 由于View默认的clickable属性为false，这里改成true

日志如下：

ViewGroupA dispatchTouchEvent down
ViewGroupA onInterceptTouchEvent down
ViewA dispatchTouchEvent down
ViewA onTouchEvent down
ViewA onTouchEvent result=true
ViewGroupA dispatchTouchEvent up
ViewGroupA onInterceptTouchEvent up
ViewA dispatchTouchEvent up
ViewA onTouchEvent up
ViewA onTouchEvent result=true

ViewA中onTouchEvent在Action为down时返回了true，则ViewGroup中是否拦截的第二个条件mFrstTouchTarge就有值，所以up时，还可以继续判断是否拦截。由于事件在ViewA中已经消耗，就不会传到ViewGroup中。

7. ViewGroup拦截ACTION_DOWN事件：

@Override
public boolean onInterceptTouchEvent(MotionEvent ev) {
    switch (ev.getAction()) {
        case MotionEvent.ACTION_DOWN:
            Log.d("aaa", "ViewGroupA onInterceptTouchEvent down");
            return true;
        case MotionEvent.ACTION_MOVE:
            Log.d("aaa", "ViewGroupA onInterceptTouchEvent move");
            break;
        case MotionEvent.ACTION_UP:
            Log.d("aaa", "ViewGroupA onInterceptTouchEvent up");
            break;
    }
    return super.onInterceptTouchEvent(ev);
}

日志如下：

ViewGroupA dispatchTouchEvent down
ViewGroupA onInterceptTouchEvent down
ViewGroupA onTouchEvent down
ViewGroupA onTouchEvent result = false
activity onTouchEvent down
activity onTouchEvent result = false
activity onTouchEvent up
activity onTouchEvent result = false

由于ViewGroupA在down时拦截了事件，则mFrstTouchTarge就没有值，会调用他自己的onTouchEvent，由于他自己的onTouchEvent返回了false，就交给了Activity。尽管Activity的onTouchEvent也返回了false，后序的事件还是会交个他。

8. ViewGroup拦截ACTION_MOVE事件：

@Override
public boolean onInterceptTouchEvent(MotionEvent ev) {
    switch (ev.getAction()) {
        case MotionEvent.ACTION_DOWN:
            Log.d("aaa", "ViewGroupA onInterceptTouchEvent down");
            break;
        case MotionEvent.ACTION_MOVE:
            Log.d("aaa", "ViewGroupA onInterceptTouchEvent move");
            return true;
        case MotionEvent.ACTION_UP:
            Log.d("aaa", "ViewGroupA onInterceptTouchEvent up");
            break;
    }
    return super.onInterceptTouchEvent(ev);
}

日志如下：

 ViewGroupA dispatchTouchEvent down
 ViewGroupA onInterceptTouchEvent down
 ViewA dispatchTouchEvent down
 ViewA onTouchEvent down
 ViewA onTouchEvent result=true
 ViewGroupA dispatchTouchEvent move
 ViewGroupA onInterceptTouchEvent move
 ViewA onTouchEvent result=true // *
 ViewGroupA dispatchTouchEvent move
 ViewGroupA onTouchEvent move
 ViewGroupA onTouchEvent result = false
 activity onTouchEvent move
 activity onTouchEvent result = false
 ViewGroupA dispatchTouchEvent up
 ViewGroupA onTouchEvent up
 ViewGroupA onTouchEvent result = false
 activity onTouchEvent up
 activity onTouchEvent result = false

除去号那一行，都很好理解。ViewGroupA分发事件，down的时候没有拦截，则到ViewA中消耗掉。接着ViewGroup分发move事件，拦截move事件，处莫名处理。后序分发move事件，由于前一个已经拦截，第二个move直接到ViewGroupA的onTouchEvent处。由于他返回的false，所以最后由Activity处理。
那个*处到底是什么呢？debug进入程序，第一个move时：

if (mFirstTouchTarget == null) {
    handled = dispatchTransformedTouchEvent(ev, canceled, null,
            TouchTarget.ALL_POINTER_IDS);
} else {
    // 由于down时，ViewA已经消耗掉，这里mFirstTouchTarget有值，不为空，进入这里
    TouchTarget predecessor = null;
    TouchTarget target = mFirstTouchTarget;
    while (target != null) {
        final TouchTarget next = target.next;
        if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) {
            handled = true;
        } else {
            // 由于第二次拦截了，所以并没有子View消耗move事件，进入这里
            final boolean cancelChild = resetCancelNextUpFlag(target.child)
                    || intercepted;
            // 下面这行代码非常关键，因为此时cancelChild为true，这个会给子View传一个cancle事件，在子View中把事件cancle掉。
            if (dispatchTransformedTouchEvent(ev, cancelChild,
                    target.child, target.pointerIdBits)) {
                handled = true;
            }
            if (cancelChild) {
                if (predecessor == null) {
                    // 注意这里，又把mFirstTouchTarget附成了null，所以第二个move不会拦截
                    mFirstTouchTarget = next;
                } else {
                    predecessor.next = next;
                }
                target.recycle();
                target = next;
                continue;
            }
        }
        predecessor = target;
        target = next;
    }
}

target的next是target链表中的，这里没有next，所以为null.

// The next target in the target list.
public TouchTarget next;

再看dispatchTransformedTouchEvent方法，如下

private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,
            View child, int desiredPointerIdBits) {
    final boolean handled;

    // 保留当前action
    final int oldAction = event.getAction();
    if (cancel || oldAction == MotionEvent.ACTION_CANCEL) {
        // 如果为cancle，则进来
        event.setAction(MotionEvent.ACTION_CANCEL);
        if (child == null) {
            handled = super.dispatchTouchEvent(event);
        } else {
            handled = child.dispatchTouchEvent(event);
        }
        event.setAction(oldAction);
        // 这里返回子View的处理结果
        return handled;
    }
    // 这里是不为cancle时的处理
    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);
    }

}

可见当第一个move到来时，ViewGroupA的事件分发给了子ViewA，并将事件改成了cancle，由于ViewA是clickable为true，所以返回了true。这个事件在ViewA这里消耗掉了，也就没有再到ViewGroupA里。ViewA修改如下：

@Override
public boolean onTouchEvent(MotionEvent event) {
    switch (event.getAction()){
        case MotionEvent.ACTION_DOWN:
            Log.d("aaa","ViewA onTouchEvent down");
            break;
        case MotionEvent.ACTION_MOVE:
            Log.d("aaa","ViewA onTouchEvent move");
            break;
        case MotionEvent.ACTION_UP:
            Log.d("aaa","ViewA onTouchEvent up");
            break;
        case MotionEvent.ACTION_CANCEL:
            Log.d("aaa","ViewA onTouchEvent cancle");
            break;
    }
    boolean result = super.onTouchEvent(event);
    Log.d("aaa","ViewA onTouchEvent result="+result);
    return result;
}

日志如下：

 ViewGroupA dispatchTouchEvent down
 ViewGroupA onInterceptTouchEvent down
 ViewA dispatchTouchEvent down
 ViewA onTouchEvent down
 ViewA onTouchEvent result=true
 ViewGroupA dispatchTouchEvent move
 ViewGroupA onInterceptTouchEvent move
 ViewA onTouchEvent cancle // * 原*号处
 ViewA onTouchEvent result=true // *
 ViewGroupA dispatchTouchEvent move
 ViewGroupA onTouchEvent move
 ViewGroupA onTouchEvent result = false
 activity onTouchEvent move
 activity onTouchEvent result = false

这也说明了一个View拦截了事件后，后续的所有事件都交由这个View处理，并不会再次判断是否需要拦截。