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View的事件分发机制

2019.01.22 Grooter Android  热度 ℃

先从Activity源码开始分析，找到dispatchTouchEvent方法

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public boolean dispatchTouchEvent(MotionEvent ev) {
    if (ev.getAction() == MotionEvent.ACTION_DOWN) {
        onUserInteraction();
    }
    if (getWindow().superDispatchTouchEvent(ev)) {
        return true;
    }
    return onTouchEvent(ev);
}

View的事件分发机制

在该段源码中有两个判断一个是判断MotionEvent是否是ACTION_DOWN操作，如果是则调用onUserInteraction方法，该方法是一个空方法，提供给用户重写，源码中对该空方法的说明如下：

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/**
 * Called whenever a key, touch, or trackball event is dispatched to the
 * activity.  Implement this method if you wish to know that the user has
 * interacted with the device in some way while your activity is running.
 * This callback and {@link #onUserLeaveHint} are intended to help
 * activities manage status bar notifications intelligently; specifically,
 * for helping activities determine the proper time to cancel a notfication.
 *
 * <p>All calls to your activity's {@link #onUserLeaveHint} callback will
 * be accompanied by calls to {@link #onUserInteraction}.  This
 * ensures that your activity will be told of relevant user activity such
 * as pulling down the notification pane and touching an item there.
 *
 * <p>Note that this callback will be invoked for the touch down action
 * that begins a touch gesture, but may not be invoked for the touch-moved
 * and touch-up actions that follow.
 *
 * @see #onUserLeaveHint()
 */

第二个判断调用了Window中的superDispatchTouchEvent方法，PhoneWindow实现了Window类的该方法，该方法源码为：

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@Override
public boolean superDispatchTouchEvent(MotionEvent event) {
    return mDecor.superDispatchTouchEvent(event);
}

在以上方法中的DecorView是PhoneWindow对应的布局，该容器包括标题栏（ActionBar）和ContentParent（setContentView设置的布局），在以上方法中又调用了DecorView的superDispatchTouchEvent方法：

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public boolean superDispatchTouchEvent(MotionEvent event) {
    return super.dispatchTouchEvent(event);
}

图片来自Android事件分发机制详解：史上最全面、最易懂
Activity中dispatchTouch流程图

由于DecorView继承于FrameLayout，而FrameLayout又继承于ViewGroup所以从以上源码super.dispatchTouchEvent(event)可知MotonEvent便传递到ViewGroup中来了

ViewGroup中dispatchTouchEvent的部分源码如下：

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final boolean intercepted;
if (actionMasked == MotionEvent.ACTION_DOWN
        || mFirstTouchTarget != null) {
    final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
    if (!disallowIntercept) {
        intercepted = onInterceptTouchEvent(ev);
        ev.setAction(action); // restore action in case it was changed
    } else {
        intercepted = false;
    }
} else {
    // There are no touch targets and this action is not an initial down
    // so this view group continues to intercept touches.
    intercepted = true;
}

当事件被子View消耗时mFirstTouchTarget会被赋值，actionMasked == MotionEvent.ACTION_DOWN || mFirstTouchTarget != null指如果事件不是Down且mFirstTouchTarget为null（即已拦截）时不再需要判断是否拦截，除非新事件为点击事件，为一系列新事件则需要调用onInterceptTouchEvent方法判断是否拦截新事件。

若不拦截该事件则会开始遍历子View，通过以下源码判断当前子View是否具备接收事件的条件：

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if (!canViewReceivePointerEvents(child)
        || !isTransformedTouchPointInView(x, y, child, null)) {
    ev.setTargetAccessibilityFocus(false);
    continue;
}

不满足则继续循环，满足则执行到以下代码段：

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if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {
    // Child wants to receive touch within its bounds.
    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;
}

其中dispatchTransformedTouchEvent方法便是将事件分发到该子View的关键方法：

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//ViewGroup dispatchTransformedTouchEvent
if (cancel || oldAction == MotionEvent.ACTION_CANCEL) {
    event.setAction(MotionEvent.ACTION_CANCEL);
    if (child == null) {
        handled = super.dispatchTouchEvent(event);
    } else {
        handled = child.dispatchTouchEvent(event);
    }
    event.setAction(oldAction);
    return handled;
}

完成分发后，通过newTouchTarget = addTouchTarget(child, idBitsToAssign)对mFirstTouchTarget赋值：

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private TouchTarget addTouchTarget(@NonNull View child, int pointerIdBits) {
    final TouchTarget target = TouchTarget.obtain(child, pointerIdBits);
    target.next = mFirstTouchTarget;
    mFirstTouchTarget = target;
    return target;
}

图片来自Android事件分发机制详解：史上最全面、最易懂
ViewGroup中dispatchTouch流程图

此时事件分发到子View中：

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//View dispatchTouchEvent
if (onFilterTouchEventForSecurity(event)) {
    if ((mViewFlags & ENABLED_MASK) == ENABLED && handleScrollBarDragging(event)) {
        result = true;
    }
    //noinspection SimplifiableIfStatement
    ListenerInfo li = mListenerInfo;
    if (li != null && li.mOnTouchListener != null
            && (mViewFlags & ENABLED_MASK) == ENABLED
            && li.mOnTouchListener.onTouch(this, event)) {
        result = true;
    }

    if (!result && onTouchEvent(event)) {
        result = true;
    }
}

其中onFilterTouchEventForSecurity方法判断是否分发事件给当前控件（通过判断其是否在顶部和有无设置不在顶部也能被分发）

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public boolean onFilterTouchEventForSecurity(MotionEvent event) {
    //noinspection RedundantIfStatement
    if ((mViewFlags & FILTER_TOUCHES_WHEN_OBSCURED) != 0
            && (event.getFlags() & MotionEvent.FLAG_WINDOW_IS_OBSCURED) != 0) {
        // Window is obscured, drop this touch.
        return false;
    }
    return true;
}

由上可知当li.mOnTouchListener.onTouch(this, event)返回true时onTouchEvent(event)将不会得到执行，onTouchEvent源码为：

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switch (action) {
  case MotionEvent.ACTION_UP:
  ...
      // Only perform take click actions if we were in the pressed state
      if (!focusTaken) {
          // Use a Runnable and post this rather than calling
          // performClick directly. This lets other visual state
          // of the view update before click actions start.
          if (mPerformClick == null) {
              mPerformClick = new PerformClick();
          }
          if (!post(mPerformClick)) {
              performClickInternal();
          }
      }
  ...

图片来自Android事件分发机制详解：史上最全面、最易懂
View中dispatchTouch流程图

performClickInternal方法便是OnClickListenner的内部实现方法，由此可知OnTouch > OnTouchEvent > OnClickListnner

可总结出如下准则：

  1. 事件序列从dowm事件开始，经过若干个move事件，最终以up结束

  2. 一个事件序列只能被一个View拦截消耗

  3. 一个View若不消耗ACTION_DOWN事件（onTouchEvent返回false），则其它事件也不再交由其消耗

  4. 一个View若不消耗除ACTION_DOWN的其它事件，该点击事件会消失，但当前View仍然可以持续接收到后续事件，而未消耗的点击事件将交由activity处理

  5. ViewGroup默认不拦截仍然事件（onInterceptTouchEvent方法默认返回false）

  6. 除了View不可点击，否则View的onTouchEvent方法默认都会消耗事件

  7. View的enable属性不影响onTouchEvent的返回值

  8. 事件遵循先传递给父元素然后再传递给子View的由外向内的传递方式

注：当拦截down后面的事件如：move时第一个被拦截的时间不会直接交由该ViewGroup处理而是以cancle的身份正常传到子View，接下来的事件才会被该ViewGroup拦截

滑动冲突

  • 解决方法

    1. 外部拦截法

      重写onInterceptTouchEvent

    2. 内部拦截法

      调用requestDisallowInterceptTouchEvent方法

    当一个可滑动的布局里存在一个也可滑动的控件时会出现滑动冲突，即在控件视图内无法滑动布局，eg：

  • HorizontalScrollView和ListView

    • 当HorizontalScrollView和ListView三个方法均返回super时，不存在滑动冲突都可正常滑动

    当HorizontalScrollView的拦截器方法返回true时HorizontalScrollView滑动正常，ListView无法滑动

    当HorizontalScrollVie的拦截器返回false则出现滑动冲突

    onInterceptTouchEvent返回super.onInterceptTouchEvent(ev)则会自动处理滑动冲突

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