Android中触摸事件分发机制(3):ViewGroup篇

在上一篇博客中，我们讲解了View的事件分发过程，这稿博客将分析ViewGroup的事件分发过程，并且在最后给出相应的流程图。

在”Android触摸事件分发机制(一):Activity篇”的最后，我们说到ViewGroup的触摸事件分发是从dispatchTouchEvent()方法开始的，下面我们就看一下这个方法:

dispatchTouchEvent()

@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
    if (mInputEventConsistencyVerifier != null) {
        mInputEventConsistencyVerifier.onTouchEvent(ev, 1);
    }

    boolean handled = false;
    if (onFilterTouchEventForSecurity(ev)) {
        final int action = ev.getAction();
        final int actionMasked = action & MotionEvent.ACTION_MASK;

        // Handle an initial down.
        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();
        }

        // Check for interception.
        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;
        }

        // Check for cancelation.
        final boolean canceled = resetCancelNextUpFlag(this)
                || actionMasked == MotionEvent.ACTION_CANCEL;

        // Update list of touch targets for pointer down, if needed.
        final boolean split = (mGroupFlags & FLAG_SPLIT_MOTION_EVENTS) != 0;
        TouchTarget newTouchTarget = null;
        boolean alreadyDispatchedToNewTouchTarget = false;
        if (!canceled && !intercepted) {
            if (actionMasked == MotionEvent.ACTION_DOWN
                    || (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN)
                    || actionMasked == MotionEvent.ACTION_HOVER_MOVE) {
                final int actionIndex = ev.getActionIndex(); // always 0 for down
                final int idBitsToAssign = split ? 1 << ev.getPointerId(actionIndex)
                        : TouchTarget.ALL_POINTER_IDS;

                // Clean up earlier touch targets for this pointer id in case they
                // have become out of sync.
                removePointersFromTouchTargets(idBitsToAssign);

                final int childrenCount = mChildrenCount;
                if (newTouchTarget == null && childrenCount != 0) {
                    final float x = ev.getX(actionIndex);
                    final float y = ev.getY(actionIndex);
                    // Find a child that can receive the event.
                    // Scan children from front to back.
                    final ArrayList<View> preorderedList = buildOrderedChildList();
                    final boolean customOrder = preorderedList == null
                            && isChildrenDrawingOrderEnabled();
                    final View[] children = mChildren;
                    for (int i = childrenCount - 1; i >= 0; i--) {
                        final int childIndex = customOrder
                                ? getChildDrawingOrder(childrenCount, i) : i;
                        final View child = (preorderedList == null)
                                ? children[childIndex] : preorderedList.get(childIndex);
                        if (!canViewReceivePointerEvents(child)
                                || !isTransformedTouchPointInView(x, y, child, null)) {
                            continue;
                        }

                        newTouchTarget = getTouchTarget(child);
                        if (newTouchTarget != null) {
                            // Child is already receiving touch within its bounds.
                            // Give it the new pointer in addition to the ones it is handling.
                            newTouchTarget.pointerIdBits |= idBitsToAssign;
                            break;
                        }

                        resetCancelNextUpFlag(child);
                        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;
                        }
                    }
                    if (preorderedList != null) preorderedList.clear();
                }

                if (newTouchTarget == null && mFirstTouchTarget != null) {
                    // Did not find a child to receive the event.
                    // Assign the pointer to the least recently added target.
                    newTouchTarget = mFirstTouchTarget;
                    while (newTouchTarget.next != null) {
                        newTouchTarget = newTouchTarget.next;
                    }
                    newTouchTarget.pointerIdBits |= idBitsToAssign;
                }
            }
        }

        // Dispatch to touch targets.
        if (mFirstTouchTarget == null) {
            // No touch targets so treat this as an ordinary view.
            handled = dispatchTransformedTouchEvent(ev, canceled, null,
                    TouchTarget.ALL_POINTER_IDS);
        } else {
            // Dispatch to touch targets, excluding the new touch target if we already
            // dispatched to it.  Cancel touch targets if necessary.
            TouchTarget predecessor = null;
            TouchTarget target = mFirstTouchTarget;
            while (target != null) {
                final TouchTarget next = target.next;
                if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) {
                    handled = true;
                } else {
                    final boolean cancelChild = resetCancelNextUpFlag(target.child)
                            || intercepted;
                    if (dispatchTransformedTouchEvent(ev, cancelChild,
                            target.child, target.pointerIdBits)) {
                        handled = true;
                    }
                    if (cancelChild) {
                        if (predecessor == null) {
                            mFirstTouchTarget = next;
                        } else {
                            predecessor.next = next;
                        }
                        target.recycle();
                        target = next;
                        continue;
                    }
                }
                predecessor = target;
                target = next;
            }
        }

        // Update list of touch targets for pointer up or cancel, if needed.
        if (canceled
                || actionMasked == MotionEvent.ACTION_UP
                || actionMasked == MotionEvent.ACTION_HOVER_MOVE) {
            resetTouchState();
        } else if (split && actionMasked == MotionEvent.ACTION_POINTER_UP) {
            final int actionIndex = ev.getActionIndex();
            final int idBitsToRemove = 1 << ev.getPointerId(actionIndex);
            removePointersFromTouchTargets(idBitsToRemove);
        }
    }

    if (!handled && mInputEventConsistencyVerifier != null) {
        mInputEventConsistencyVerifier.onUnhandledEvent(ev, 1);
    }
    return handled;
}

由于ViewGroup继承自View,而View中也有dispatchTouchEvent()方法，所以ViewGroup中的dispatchTouchEvent()是覆写了View中的dispatchTouchEvent()，也说明了它们对触摸事件的处理有所不同。

为了详细地了解整个触摸事件传递过程，这里从ACTION_DOWN开始进行分析。

首先是onFilterTouchEventForSecurity()，这个在View的事件分发中已经讲过，就不再赘述;还有由于MotionEvent.ACTION_DOWN是触摸事件的开始，所以如果是ACTION_DOWN事件的话就需要重置状态。开始时是ACTION_DOWN状态，所以进入cancelAndClearTouchTargets()中，下面是该方法：

cancelAndClearTouchTargets()

/**
  * Cancels and clears all touch targets.
  */
 private void cancelAndClearTouchTargets(MotionEvent event) {
     if (mFirstTouchTarget != null) {
         boolean syntheticEvent = false;
         if (event == null) {
             final long now = SystemClock.uptimeMillis();
             event = MotionEvent.obtain(now, now,
                     MotionEvent.ACTION_CANCEL, 0.0f, 0.0f, 0);
             event.setSource(InputDevice.SOURCE_TOUCHSCREEN);
             syntheticEvent = true;
         }

         for (TouchTarget target = mFirstTouchTarget; target != null; target = target.next) {
             resetCancelNextUpFlag(target.child);
             dispatchTransformedTouchEvent(event, true, target.child, target.pointerIdBits);
         }
         clearTouchTargets();

         if (syntheticEvent) {
             event.recycle();
         }
     }
 }

resetCancelNextUpFlag()和dispatchTransformedTouchEvent()到后面再讲，这里暂时不影响我们理解，就先看clearTouchTargets();该方法如下：

clearTouchTargets()

/**
    * Clears all touch targets.
    */
   private void clearTouchTargets() {
       TouchTarget target = mFirstTouchTarget;
       if (target != null) {
           do {
               TouchTarget next = target.next;
               target.recycle();
               target = next;
           } while (target != null);
           mFirstTouchTarget = null;
       }
   }

显然，会将所有的TouchTarget都回收，并且使mFirstTouchTarget为null.

回到dispatchTouchEvent()方法中，虽然此时mFirstTouchTarget为null,但是actionMasked==MotionEvent.ACTION_DOWN成立，所以会进入到第一个分支中。
下面首先看mGroupFlags的值。我们看一下它在哪里被赋值，找了一下，发现是在requestDisallowInterceptTouchEvent()方法中，如下所示：

requestDisallowInterceptTouchEvent()

public void requestDisallowInterceptTouchEvent(boolean disallowIntercept) {

        if (disallowIntercept == ((mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0)) {
            // We're already in this state, assume our ancestors are too
            return;
        }

        if (disallowIntercept) {
            mGroupFlags |= FLAG_DISALLOW_INTERCEPT;
        } else {
            mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;
        }

        // Pass it up to our parent
        if (mParent != null) {
            mParent.requestDisallowInterceptTouchEvent(disallowIntercept);
        }
    }

显然，如果disallowIntercept为true的话，mGroupFlags的相应位就与FLAG_DISALLOW_INTERCEPT一致,否则与FLAG_DISALLOW_INTERCEPT相反。假设在ViewGroup的外部调用了requestDisallowInterceptTouchEvent(true);则这里intercepted始终为false,否则进入到onInterceptTouchEvent()中，该方法如下：

onInterceptTouchEvent()

public boolean onInterceptTouchEvent(MotionEvent ev){
	return false;
}

这个方法异常的简单，即默认情况下返回false,如果用户重写了该方法，比如返回true，则ViewGroup会截取之后的事件，在ViewGroup中的子View就接收不到后面的事件了。

还有一个分支就是如果不是ACTION_DOWN事件，并且mFirstTouchTarget==null,则表示没有相应的事件接收目标，自然也就没必要再往下传递事件了，所以此时intercept=true;

接着往下看，是确认事件是否被取消，如果是的话就不会进行后续的处理。

然后是获取一个boolean变量split来标记是否把事件分发给多个子View，它的赋值其实跟上面的requestDisallowInterceptTouchEvent()类似，是在setMotionEventSplittingEnabled()中，该方法如下所示：

setMotionEventSplittingEnabled()

/**
 * Enable or disable the splitting of MotionEvents to multiple children during touch event
 * dispatch. This behavior is enabled by default for applications that target an
 * SDK version of {@link Build.VERSION_CODES#HONEYCOMB} or newer.
 *
 * <p>When this option is enabled MotionEvents may be split and dispatched to different child
 * views depending on where each pointer initially went down. This allows for user interactions
 * such as scrolling two panes of content independently, chording of buttons, and performing
 * independent gestures on different pieces of content.
 *
 * @param split <code>true</code> to allow MotionEvents to be split and dispatched to multiple
 *              child views. <code>false</code> to only allow one child view to be the target of
 *              any MotionEvent received by this ViewGroup.
 * @attr ref android.R.styleable#ViewGroup_splitMotionEvents
 */
public void setMotionEventSplittingEnabled(boolean split) {
    // TODO Applications really shouldn't change this setting mid-touch event,
    // but perhaps this should handle that case and send ACTION_CANCELs to any child views
    // with gestures in progress when this is changed.
    if (split) {
        mGroupFlags |= FLAG_SPLIT_MOTION_EVENTS;
    } else {
        mGroupFlags &= ~FLAG_SPLIT_MOTION_EVENTS;
    }
}

如果事件既没有被取消，也没有被当前ViewGroup截取，那就会进入下面复杂的处理了。

由于现在是ACTION_DOWN事件，那么actionIndex为0，后面那些idBitsToAssign和removePointersFromTouchTargets()不影响我们理解，暂时先不讲。

现在newTouchTarget为null,如果ViewGroup中有子View的话，则进入该分支中。

首先是获取到x和y坐标，然后是通过前序遍历的方法来构建所有的子View，之所以采用前序遍历，是因为preorderedList中的顺序是按照addView或者XML布局文件中的顺序而来的，后addView添加的子View，会因为Android的UI后刷新机制显示在上层。假如点击的地方有两个子View都包含在其中，那么后添加进去的那个子View会先响应事件，这样其实也是符合人的思维方式的，因为后被添加的子View会在图层的上层，自然我们点击时也是希望上层的View最先响应。

结合上面的分析，就能理解for循环采用倒序遍历的原因了。之后通过canViewReceivePointerEvents()和isTransformedTouchPointInVieww()方法来判断当前child是否能够接受事件，并且点击位置是否在child的范围内，如果不满足则continue.

一直遍历到找到第一个符合上述条件的child为止，找到之后利用进入getTouchTarget()方法中，如下所示：

getTouchTarget()

/**
 * Gets the touch target for specified child view.
 * Returns null if not found.
 */
private TouchTarget getTouchTarget(View child) {
    for (TouchTarget target = mFirstTouchTarget; target != null; target = target.next) {
        if (target.child == child) {
            return target;
        }
    }
    return null;
}

显然，getTouchTarget()是遍历以mFirstTouchTarget为首的TouchTarget链条,看child是否存在某个TouchTarget中。

前面分析过，此时为ACTION_DOWN事件，所以mFirstTouchTarget为null,从而getTouchTarget()返回null.所以暂时不会break,而是执行到dispatchTransformedTouchEvent()处，这个方法非常重要。下面是该方法体：

dispatchTransformedTouchEvent()

/**
 * Transforms a motion event into the coordinate space of a particular child view,
 * filters out irrelevant pointer ids, and overrides its action if necessary.
 * If child is null, assumes the MotionEvent will be sent to this ViewGroup instead.
 */
private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,
        View child, int desiredPointerIdBits) {
    final boolean handled;

    // Canceling motions is a special case.  We don't need to perform any transformations
    // or filtering.  The important part is the action, not the contents.
    final int oldAction = event.getAction();
    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;
    }

    // Calculate the number of pointers to deliver.
    final int oldPointerIdBits = event.getPointerIdBits();
    final int newPointerIdBits = oldPointerIdBits & desiredPointerIdBits;

    // If for some reason we ended up in an inconsistent state where it looks like we
    // might produce a motion event with no pointers in it, then drop the event.
    if (newPointerIdBits == 0) {
        return false;
    }

    // If the number of pointers is the same and we don't need to perform any fancy
    // irreversible transformations, then we can reuse the motion event for this
    // dispatch as long as we are careful to revert any changes we make.
    // Otherwise we need to make a copy.
    final MotionEvent transformedEvent;
    if (newPointerIdBits == oldPointerIdBits) {
        if (child == null || child.hasIdentityMatrix()) {
            if (child == null) {
                handled = super.dispatchTouchEvent(event);
            } else {
                final float offsetX = mScrollX - child.mLeft;
                final float offsetY = mScrollY - child.mTop;
                event.offsetLocation(offsetX, offsetY);

                handled = child.dispatchTouchEvent(event);

                event.offsetLocation(-offsetX, -offsetY);
            }
            return handled;
        }
        transformedEvent = MotionEvent.obtain(event);
    } else {
        transformedEvent = event.split(newPointerIdBits);
    }

    // Perform any necessary transformations and dispatch.
    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;
}

首先判断是否要取消事件，如果要取消的话则进行判断，如果child为null则执行super.dispatchTouchEvent()，即当前ViewGroup作为View的dispatchTouchEvent()方法，关于View的dispatchTouchEvent()方法，在上一篇博客中详细讲过，此处不再赘述;如果child不为null,则调用child.dispatchTouchEvent()，注意这里其实就进入递归了，因为dispatchTransformedTouchEvent()是在ViewGroup#dispatchTouchEvent()中被调用,而这里又调用child.dispatchTouchEvent()方法，如果child也是ViewGroup，则进入ViewGroup的dispatchTouchEvent()，否则调用的是View的dispatchTouchEvent()方法。

如果不取消事件或者不是ACTION_CANCEL事件，则分两种情况，一种是newPointerIdBits==oldPointerIdBits,此时说明当前的事件和之前的事件相同，因而不需要变形，可直接调用super.dispatchTouchEvent(event)或者child.dispatchTouchEvent(event),否则需要调用MotionEvent.obtain(event)获得transformedEvent,之后对变形后的事件进行类似的处理。

如果事件被child或其子View消费掉，则dispatchTransformedTouchEvent()会返回true,从而进入下面的分支：先获取到mLastTouchDownTime,mLastTouchDownIndex和mLastTouchDownX,mLastTouchDownY,之后通过addTouchTarget()得到一个TouchTarget对象，下面是该方法：

addTouchTarget()

/**
  * Adds a touch target for specified child to the beginning of the list.
  * Assumes the target child is not already present.
  */
 private TouchTarget addTouchTarget(View child, int pointerIdBits) {
     TouchTarget target = TouchTarget.obtain(child, pointerIdBits);
     target.next = mFirstTouchTarget;
     mFirstTouchTarget = target;
     return target;
 }

之后跳出循环，后面的处理其实主要是针对非ACTION_DOWN的情况，可以看到后续还有dispatchTransformedTouchEvent()方法的调用，即为非ACTION_DOWN情况下也进行类似的递归调用，只要有一个child消费了该事件，就会使handled=true;从而返回true.

非ACTION_DOWN情形要简单得多，主要就是160行之后的处理，上面已经分析过，就不再赘述了。

至此，我们可以得到ViewGroup的触摸事件分发机制的流程图：

实例

在工作中遇到的两个触摸事件冲突的问题，我觉得是很好的实例。

1)RecyclerView中第一个item是ViewPager,在下拉这个item时,会将下拉变成点击ViewPager中的点击事件，从而进入电影详情页。

显然这是触摸事件冲突导致的，本来应该是RecyclerView的下拉事件，由于被ViewPager消费，从而变成点击事件，那知道了原因，解决的方法也就很简单了。

首先定义一个手势识别对象:

private void initGestureDetector() {
   gestureDetector = new GestureDetector(null, new GestureDetector.OnGestureListener() {
     @Override public boolean onDown(MotionEvent e) {
       return false;
     }

     @Override public void onShowPress(MotionEvent e) {

     }

     @Override public boolean onSingleTapUp(MotionEvent e) {
       return false;
     }

     @Override
     public boolean onScroll(MotionEvent e1, MotionEvent e2, float distanceX, float distanceY) {
       return Math.abs(distanceY) > Math.abs(distanceX);
     }

     @Override public void onLongPress(MotionEvent e) {

     }

     @Override
     public boolean onFling(MotionEvent e1, MotionEvent e2, float velocityX, float velocityY) {
       return false;
     }
   });
 }

即如果发现Y反向的移动量大于X方向的移动量，就返回true.

之后，在AutoScrollViewPager的dispatchTouchEvent中的代码如下:

@Override public boolean dispatchTouchEvent(MotionEvent ev) {
    int action = MotionEventCompat.getActionMasked(ev);

   ...

    if (null == gestureDetector) {
      initGestureDetector();
    }
    if (gestureDetector.onTouchEvent(ev)) {
      getParent().requestDisallowInterceptTouchEvent(false);
      return false;
    }
    
    ... 

    getParent().requestDisallowInterceptTouchEvent(true);
    return super.dispatchTouchEvent(ev);
  }

上面将无关的代码省略了，可见发现Y方向的移动量大于X方向的移动量时，就调用getParent().requestDisallowInterceptTouchEvent(false);让parent(即RecyclerView)截获事件。

2)图片查看器,单击图片放大，双击退出

类似的，需要一个GestureDetector,其初始化如下:

mGestureDetector = new GestureDetector(imageView.getContext(),
               new GestureDetector.SimpleOnGestureListener() {

                   // forward long click listener
                   @Override
                   public void onLongPress(MotionEvent e) {
                       if (null != mLongClickListener) {
                           mLongClickListener.onLongClick(getImageView());
                       }
                   }
               });

       mGestureDetector.setOnDoubleTapListener(this);

而onDoubleTap方法如下:

@Override
 public final boolean onDoubleTap(MotionEvent ev) {
     try {
         float scale = getScale();
         float x = ev.getX();
         float y = ev.getY();

         if (scale < mMidScale) {
             setScale(mMidScale, x, y, true);
         } else if (scale >= mMidScale && scale < mMaxScale) {
             setScale(mMaxScale, x, y, true);
         } else {
             setScale(mMinScale, x, y, true);
         }
     } catch (ArrayIndexOutOfBoundsException e) {
         // Can sometimes happen when getX() and getY() is called
     }

     return true;
 }

即双击时放大。

那单击退出是如何实现的呢？还记得我们在前面分析过，触摸事件是从Activity开始分发的吗!那就简单了，重写Activity中的dispatchTouchEvent()方法，在里面进行手势判断，但是需要注意不要消费事件，否则PhotoViewAttacher接收不到触摸事件!

故代码如下: