Android View事件传播机制

Android上View是最常用的一个类，很多控件都是继承它的，接下来主要介绍一些view的传播机制，这个对于处理各种点击事件还是很重要的，下面就拿android N的代码看一下view上面touch event的传播过程

 

一、View中的dispatchTouchEvent方法 

public boolean dispatchTouchEvent(MotionEvent event) {
        if (mInputEventConsistencyVerifier != null) {
            mInputEventConsistencyVerifier.onTouchEvent(event, 0);
        }
 
        if (onFilterTouchEventForSecurity(event)) {
            //noinspection SimplifiableIfStatement
            ListenerInfo li = mListenerInfo;
            if (li != null && li.mOnTouchListener != null && (mViewFlags & ENABLED_MASK) == ENABLED
                    && li.mOnTouchListener.onTouch(this, event)) {
                return true;
            }
 
            if (onTouchEvent(event)) {
                return true;
            }
        }
 
        if (mInputEventConsistencyVerifier != null) {
            mInputEventConsistencyVerifier.onUnhandledEvent(event, 0);
        }
        return false;
}

dispatchTouchEvent是你触碰屏幕后第一个进入的函数，所以从这边开始分析

首先看到这个条件   

 li != null && li.mOnTouchListener != null && (mViewFlags & ENABLED_MASK) == ENABLED
                    && li.mOnTouchListener.onTouch(this, event)

其中mViewFlags & ENABLED_MASK)代表这个控件是否enable，mOnTouchListener.onTouch(this, event)这个就是我们看到的onTouch函数，你可以覆盖这个函数，默认情况下这个函数返回false，也就是上面的条件如果成立，那么就不会进入onTouchEvent这个函数，这个函数里面就是我们常见的onClick和onLongClick这些事件 

 

二、ViewGroup中的dispatchTouchEvent方法

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 View[] children = mChildren;
 
                        final boolean customOrder = isChildrenDrawingOrderEnabled();
                        for (int i = childrenCount - 1; i >= 0; i--) {
                            final int childIndex = customOrder ?
                                    getChildDrawingOrder(childrenCount, i) : i;
                            final View child = children[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();
                                mLastTouchDownIndex = childIndex;
                                mLastTouchDownX = ev.getX();
                                mLastTouchDownY = ev.getY();
                                newTouchTarget = addTouchTarget(child, idBitsToAssign);
                                alreadyDispatchedToNewTouchTarget = true;
                                break;
                            }
                        }
                    }
 
                    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;
 }

同样来分析一下这个函数，首先你会发现这个函数比View中的同样函数内容多了好多，这是因为涉及到事件传播机制了，继承View的都是像button这种没有子view的控件，所以没有事件传播，但是ViewGroup就不一样了，它需要进行传播，接下来看一下怎么传播吧

1、首先它会判断是否ACTION_DOWN或者是否有目标View了（ACTION_DOWN会产生目标view链表，链表不为空说明有目标view,注意的是在寻找目标view链表过程中针对每个ViewGroup是找到它第一个愿意接收事件的child，一旦找到就不再传播给接下来的child了，也就是只能有一个child接收ACTION_DOWN），如果是的话再判断是否拦截，拦截函数是onInterceptTouchEvent，下面是它的代码：

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

可以看到很简单，true代表拦截，false代表不拦截，默认是不拦截的，如果拦截的话就调用View类的dispatchTouchEvent，不再传播事件给子view了

 

2、如果不拦截的话那么就要开始传播这个事件了，选择子view有几个条件，像子view必须可见，而且点击的坐标在子view的范围内等，如果这些条件都满足，就会开始调用dispatchTransformedTouchEvent函数，下面是它的源码：

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;
 }

可以看到它里面主要调用了dispatchTouchEvent函数，这个函数会根据这个View有没有子view来决定调用View里的dispatchTouchEvent还是ViewGroup里的dispatchTouchEvent，这个就回到了上面介绍的方法了，从上面也可以看到如果dispatchTouchEvent返回false，那么接下来的事件都不会传给它了，可以看一下源码：

//add view to TouchTarget if dispatchTransformedTouchEvent return true
resetCancelNextUpFlag(child);
if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {
         // Child wants to receive touch within its bounds.
         mLastTouchDownTime = ev.getDownTime();
         mLastTouchDownIndex = childIndex;
         mLastTouchDownX = ev.getX();
         mLastTouchDownY = ev.getY();
         newTouchTarget = addTouchTarget(child, idBitsToAssign);
         alreadyDispatchedToNewTouchTarget = true;
         break;
}
...

// deliver action to only view of TouchTarget
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;
 }

首先ACTION_DOWN时会检测所有的child view，如果dispatchTransformedTouchEvent返回false，那么这个view就不会加入到TouchTarget链表里，这样后面的ACTION_MOVE等其他事件只传播给TouchTarget里面的view，这样就轮不到那些不在TouchTarget里的view了

 

3、通过分析还有一个地方需要注意，如果ACTION是ACTION_DOWN或者ACTION_CANCEL都会重置状态，这个就是出现在ACTION被前面的view拦截后会收到一个ACTION_CANCEL事件来通知后面的view，这样后面的view就会重置状态。