Android事件散发详解(三)——ViewGroup的dispatchTouchEvent()源码学习

package cc.aa;import android.os.Environment;import android.view.MotionEvent;import android.view.View;public class UnderstandDispatchTouchEvent {    /**     * dispatchTouchEvent()源码学习及其注释     * 常说事件传递中的流程是:dispatchTouchEvent->onInterceptTouchEvent->onTouchEvent     * 在这个链条中dispatchTouchEvent()是处在链首的位置当然也是最重要的.     * 在dispatchTouchEvent()决定了Touch事件是由自己的onTouchEvent()处理     * 还是分发给子View处理让子View调用其自身的dispatchTouchEvent()处理.     *      *      * 其实dispatchTouchEvent()和onInterceptTouchEvent()以及onTouchEvent()的关系     * 在dispatchTouchEvent()方法的源码中体现得很明显.     * 比如dispatchTouchEvent()会调用onInterceptTouchEvent()来判断是否要拦截.     * 比如dispatchTouchEvent()会调用dispatchTransformedTouchEvent()方法且在该方法中递归调用     * dispatchTouchEvent();从而会在dispatchTouchEvent()里最终调用到onTouchEvent()     *      *      *      * 重点关注:     * 1 子View对于ACTION_DOWN的处理十分重要!!!!!     *   ACTION_DOWN是一系列Touch事件的开端,如果子View对于该ACTION_DOWN事件在onTouchEvent()中返回了false即未消费.     *   那么ViewGroup就不会把后续的ACTION_MOVE和ACTION_UP派发给该子View.在这种情况下ViewGroup就和普通的View一样了,     *   调用该ViewGroup自己的dispatchTouchEvent()从而调用自己的onTouchEvent();即不会将事件分发给子View.     *   详细代码请参见如下代码分析.     *        * 2 为什么子view对于Touch事件处理返回true那么其上层的ViewGroup就无法处理Touch事件了?????     *   这个想必大家都知道了,因为该Touch事件被子View消费了其上层的ViewGroup就无法处理该Touch事件了.     *   那么在源码中的依据是什么呢??请看下面的源码分析     *        * 参考资料:     * 0 http://blog.csdn.net/sahadev_/article/details/23839039     * 1 http://www.cnblogs.com/xiaoweiz/p/3838682.html     * 2 http://hukai.me/android-deeper-touch-event-dispatch-process/     * 3 http://blog.csdn.net/hdxiaoyu2/article/details/25563453     * 4 http://www.eoeandroid.com/thread-542296-1-1.html     * 5 http://www.eoeandroid.com/forum.php?mod=viewthread&tid=319301     * 6 http://stackvoid.com/details-dispatch-onTouch-Event-in-Android/     *   Thank you very much     */	    @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;            /**             * 第一步:对于ACTION_DOWN进行处理(Handle an initial down)             * 因为ACTION_DOWN是一系列事件的开端,当是ACTION_DOWN时进行一些初始化操作.             * 从源码的注释也可以看出来:清除以往的Touch状态(state)开始新的手势(gesture)             * cancelAndClearTouchTargets(ev)中有一个非常重要的操作:             * 将mFirstTouchTarget设置为null!!!!             * 随后在resetTouchState()中重置Touch状态标识             */            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();            }            			/**			 * 第二步:检查是否要拦截(Check for interception)			 * 在dispatchTouchEvent(MotionEventev)这段代码中			 * 使用变量intercepted来标记ViewGroup是否拦截Touch事件的传递.			 * 该变量在后续代码中起着很重要的作用.			 */            final boolean intercepted;			// 事件为ACTION_DOWN或者mFirstTouchTarget不为null(即已经找到能够接收touch事件的目标组件)时if成立            if (actionMasked == MotionEvent.ACTION_DOWN || mFirstTouchTarget != null) {            	//判断disallowIntercept(禁止拦截)标志位				//因为在其他地方可能调用了requestDisallowInterceptTouchEvent(boolean disallowIntercept)				//从而禁止执行是否需要拦截的判断(有点拗口~其实看requestDisallowInterceptTouchEvent()方法名就可明白)                final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;                //当没有禁止拦截判断时(即disallowIntercept为false)调用onInterceptTouchEvent(ev)方法                if (!disallowIntercept) {                	//既然disallowIntercept为false那么就调用onInterceptTouchEvent()方法将结果赋值给intercepted                	//常说事件传递中的流程是:dispatchTouchEvent->onInterceptTouchEvent->onTouchEvent                	//其实在这就是一个体现,在dispatchTouchEvent()中调用了onInterceptTouchEvent()                    intercepted = onInterceptTouchEvent(ev);                    ev.setAction(action); // restore action in case it was changed                } else {                	 //当禁止拦截判断时(即disallowIntercept为true)设置intercepted = false                    intercepted = false;                }            } else {            	//当事件不是ACTION_DOWN并且mFirstTouchTarget为null(即没有Touch的目标组件)时            	//设置 intercepted = true表示ViewGroup执行Touch事件拦截的操作。                //There are no touch targets and this action is not an initial down                //so this view group continues to intercept touches.                intercepted = true;            }                        /**             * 第三步:检查cancel(Check for cancelation)             *              */            final boolean canceled = resetCancelNextUpFlag(this) || actionMasked == MotionEvent.ACTION_CANCEL;                        /**             * 第四步:事件分发(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;            //不是ACTION_CANCEL并且ViewGroup的拦截标志位intercepted为false(不拦截)            if (!canceled && !intercepted) {            	//处理ACTION_DOWN事件.这个环节比较繁琐.                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 (childrenCount != 0) {                    	// 依据Touch坐标寻找子View来接收Touch事件                        // Find a child that can receive the event.                        // Scan children from front to back.                        final View[] children = mChildren;                        final float x = ev.getX(actionIndex);                        final float y = ev.getY(actionIndex);                        final boolean customOrder = isChildrenDrawingOrderEnabled();                        // 遍历子View判断哪个子View接受Touch事件                        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) {                            	// 找到接收Touch事件的子View!!!!!!!即为newTouchTarget.                            	// 既然已经找到了,所以执行break跳出for循环                                // 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);                            /**                             * 如果上面的if不满足,当然也不会执行break语句.                             * 于是代码会执行到这里来.                             *                              * 调用方法dispatchTransformedTouchEvent()将Touch事件传递给子View做                             * 递归处理(也就是遍历该子View的View树)                             * 该方法很重要,看一下源码中关于该方法的描述:                             * 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.                             * 将Touch事件传递给特定的子View.                             * 该方法十分重要!!!!在该方法中为一个递归调用,会递归调用dispatchTouchEvent()方法！！！！！！！！！！！！！！                             * 在dispatchTouchEvent()中:                             * 如果子View为ViewGroup并且Touch没有被拦截那么递归调用dispatchTouchEvent()                             * 如果子View为View那么就会调用其onTouchEvent(),这个就不再赘述了.                             *                              *                              * 该方法返回true则表示子View消费掉该事件,同时进入该if判断.                             * 满足if语句后重要的操作有:                             * 1 给newTouchTarget赋值                             * 2 给alreadyDispatchedToNewTouchTarget赋值为true.                             *   看这个比较长的英语名字也可知其含义:已经将Touch派发给新的TouchTarget                             * 3 执行break.                             *   因为该for循环遍历子View判断哪个子View接受Touch事件,既然已经找到了                             *   那么就跳出该for循环.                             * 4 注意:                             *   如果dispatchTransformedTouchEvent()返回false即子View                             *   的onTouchEvent返回false(即Touch事件未被消费)那么就不满足该if条件,也就无法执行addTouchTarget()                             *   从而导致mFirstTouchTarget为null.那么该子View就无法继续处理ACTION_MOVE事件                             *   和ACTION_UP事件!!!!!!!!!!!!!!!!!!!!!!                             * 5 注意:                             *   如果dispatchTransformedTouchEvent()返回true即子View                             *   的onTouchEvent返回true(即Touch事件被消费)那么就满足该if条件.                             *   从而mFirstTouchTarget不为null!!!!!!!!!!!!!!!!!!!                             * 6 小结:                             *   对于此处ACTION_DOWN的处理具体体现在dispatchTransformedTouchEvent()                             *   该方法返回boolean,如下:                             *   true---->事件被消费----->mFirstTouchTarget!=null                             *   false--->事件未被消费---->mFirstTouchTarget==null                             *   因为在dispatchTransformedTouchEvent()会调用递归调用dispatchTouchEvent()和onTouchEvent()                             *   所以dispatchTransformedTouchEvent()的返回值实际上是由onTouchEvent()决定的.                             *   简单地说onTouchEvent()是否消费了Touch事件(true or false)的返回值决定了dispatchTransformedTouchEvent()                             *   的返回值!!!!!!!!!!!!!从而决定了mFirstTouchTarget是否为null!!!!!!!!!!!!!!!!从而进一步决定了ViewGroup是否                             *   处理Touch事件.这一点在下面的代码中很有体现.                             *                                *                              */                            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;                            }                        }                    }                                        /**                     * 该if条件表示:                     * 经过前面的for循环没有找到子View接收Touch事件并且之前的mFirstTouchTarget不为空                     */                    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指向了最初的TouchTarget                        newTouchTarget.pointerIdBits |= idBitsToAssign;                    }                }            }                                    /**             * 分发Touch事件至target(Dispatch to touch targets)             *              * 经过上面对于ACTION_DOWN的处理后mFirstTouchTarget有两种情况:             * 1 mFirstTouchTarget为null             * 2 mFirstTouchTarget不为null             *              * 当然如果不是ACTION_DOWN就不会经过上面较繁琐的流程             * 而是从此处开始执行,比如ACTION_MOVE和ACTION_UP             */            if (mFirstTouchTarget == null) {            	/**            	 * 情况1：mFirstTouchTarget为null            	 *             	 * 经过上面的分析mFirstTouchTarget为null就是说Touch事件未被消费.            	 * 即没有找到能够消费touch事件的子组件或Touch事件被拦截了，            	 * 则调用ViewGroup的dispatchTransformedTouchEvent()方法处理Touch事件则和普通View一样.            	 * 即子View没有消费Touch事件,那么子View的上层ViewGroup才会调用其onTouchEvent()处理Touch事件.            	 * 在源码中的注释为:No touch targets so treat this as an ordinary view.            	 * 也就是说此时ViewGroup像一个普通的View那样调用dispatchTouchEvent(),且在dispatchTouchEvent()            	 * 中会去调用onTouchEvent()方法.            	 * 具体的说就是在调用dispatchTransformedTouchEvent()时第三个参数为null.            	 * 第三个参数View child为null会做什么样的处理呢?            	 * 请参见下面dispatchTransformedTouchEvent()的源码分析            	 *             	 * 这就是为什么子view对于Touch事件处理返回true那么其上层的ViewGroup就无法处理Touch事件了!!!!!!!!!!            	 * 这就是为什么子view对于Touch事件处理返回false那么其上层的ViewGroup才可以处理Touch事件!!!!!!!!!!            	 *             	 */                handled = dispatchTransformedTouchEvent(ev, canceled, null,TouchTarget.ALL_POINTER_IDS);            } else {            	/**            	 * 情况2：mFirstTouchTarget不为null即找到了可以消费Touch事件的子View且后续Touch事件可以传递到该子View            	 * 在源码中的注释为:            	 * 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;                        //对于非ACTION_DOWN事件继续传递给目标子组件进行处理,依然是递归调用dispatchTransformedTouchEvent()                        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;                }            }            /**             * 处理ACTION_UP和ACTION_CANCEL             * Update list of touch targets for pointer up or cancel, if needed.             * 在此主要的操作是还原状态             */            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;    }                //=====================以上为dispatchTouchEvent()源码分析======================                //===============以下为dispatchTransformedTouchEvent()源码分析=================        /**     * 在dispatchTouchEvent()中调用dispatchTransformedTouchEvent()将事件分发给子View处理     *      * 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.     *      * 在此请着重注意第三个参数:View child     * 在dispatchTouchEvent()中多次调用了dispatchTransformedTouchEvent(),但是有时候第三个参数为null,有时又不是.     * 那么这个参数是否为null有什么区别呢？     * 在如下dispatchTransformedTouchEvent()源码中可见多次对于child是否为null的判断,并且均做出如下类似的操作:     * if (child == null) {     *       handled = super.dispatchTouchEvent(event);     *    } else {     *       handled = child.dispatchTouchEvent(event);     * }     * 这个代码是什么意思呢??     * 当child == null时会将Touch事件传递给该ViewGroup自身的dispatchTouchEvent()处理.     * 即super.dispatchTouchEvent(event)正如源码中的注释描述的一样:     * If child is null, assumes the MotionEvent will be sent to this ViewGroup instead.     * 当child != null时会调用该子view(当然该view可能是一个View也可能是一个ViewGroup)的dispatchTouchEvent(event)处理.     * 即child.dispatchTouchEvent(event);     *      *      */    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;    }                        }