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），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 trueresetCancelNextUpFlag(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 TouchTargetwhile (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。