Android触摸事件源码分析:Activity->ViewGroup->View

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Activity中

当屏幕有touch事件时，首先调用Activity的dispatchTouchEvent方法

 /**
     * Called to process touch screen events.  You can override this to
     * intercept all touch screen events before they are dispatched to the
     * window.  Be sure to call this implementation for touch screen events
     * that should be handled normally.
     *
     * @param ev The touch screen event.
     *
     * @return boolean Return true if this event was consumed.
     */
    public boolean dispatchTouchEvent(MotionEvent ev) {
        if (ev.getAction() == MotionEvent.ACTION_DOWN) {
            onUserInteraction();
        }
        if (getWindow().superDispatchTouchEvent(ev)) {
            return true;
        }
        return onTouchEvent(ev);
}

首先分析Activity的attach方法可以发现getWindow()返回的就是PhoneWindow对象（PhoneWindow为抽象Window的实现子类），那就简单了，也就相当于PhoneWindow类的方法，而PhoneWindow类实现于Window抽象类，所以先看下Window类中抽象方法的定义，如下：

<span style="font-size:24px;">/**
     * Used by custom windows, such as Dialog, to pass the touch screen event
     * further down the view hierarchy. Application developers should
     * not need to implement or call this.
     *用户不需要重写实现的方法，实质也不能，在Activity中没有提供重写的机会，因为Window是以组</span><pre name="code" class="java"><span style="font-size:24px;">*</span><span style="font-family: Arial, Helvetica, sans-serif;"><span style="font-size:18px;">合模式与Activity建立关系的</span></span>

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

在PhoneWindow类里发现，mDecor是DecorView类的实例，同时DecorView是PhoneWindow的内部类。最惊人的发现是DecorView extends FrameLayout implements RootViewSurfaceTaker，看见没有？它是一个真正Activity的root view,它继承了FrameLayout。
不知道大家是不是熟悉Android App开发技巧中关于UI布局优化使用的SDK工具Hierarchy Viewer，打开的时候在最上面会有个DecorView$PhoneWindow的框框

Activity中setContentView时，把我们编写的xmlLayout文件放置在一个id为content的FrameLayout的布局（DecorView）中，这也就是为啥Activity的setContentView方法叫set content view了，就是把我们的xml放入了这个id为content的FrameLayout中

讲完了DecorView，我们在来看看mDecor.superDispatchTouchEvent(event)：

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

Activity的dispatchTouchEvent方法的if (getWindow().superDispatchTouchEvent(ev))本质执行的是一个ViewGroup的dispatchTouchEvent方法（这个ViewGroup是Activity特有的root view，也就是id为content的FrameLayout布局）

在Activity的触摸屏事件派发中：Activity,PhoneWindow,DecorView,ViewGroup
1,首先会触发Activity的dispatchTouchEvent方法。
2,dispatchTouchEvent方法中如果是ACTION_DOWN的情况下会接着触发onUserInteraction方法。
3,接着在dispatchTouchEvent方法中会通过Activity的root View（id为content的FrameLayout），实质是ViewGroup，通过super.dispatchTouchEvent把touchevent派发给各个activity的子view，也就是我们再Activity.onCreat方法中setContentView时设置的view。
4，若Activity下面的子view拦截了touchevent事件(返回true)则Activity.onTouchEvent方法就不会执行。

ViewGroup中

既然Activity中的DecorView是ViewGroup的子类调用了dispatchTouchEvent方法，来看看这个方法：

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

        // If the event targets the accessibility focused view and this is it, start
        // normal event dispatch. Maybe a descendant is what will handle the click.
        if (ev.isTargetAccessibilityFocus() && isAccessibilityFocusedViewOrHost()) {
            ev.setTargetAccessibilityFocus(false);
        }

        boolean handled = false;
        if (onFilterTouchEventForSecurity(ev)) {
            final int action = ev.getAction();
            final int actionMasked = action & MotionEvent.ACTION_MASK;
/*清除以往的Touch状态然后开始新的手势。在这里你会发现cancelAndClearTouchTargets(ev)方法中有一个非常重要的操作就是将mFirstTouchTarget设置为了null（刚开始分析大眼瞄一眼没留意，结果越往下看越迷糊，所以这个是分析ViewGroup的dispatchTouchEvent方法第一步中重点要记住的一个地方），接着在resetTouchState()方法中重置Touch状态标识。*/
            // 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.检查拦截
            final boolean intercepted;
            if (actionMasked == MotionEvent.ACTION_DOWN
                    || mFirstTouchTarget != null) {// 说明当事件为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，否则if不成立，然后将intercepted设置为true，也即拦截事件                
final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
                if (!disallowIntercept) {  // 如果没有禁止拦截，就调用onInterceptTouchEvent方法，touch事件就继续传递给子View，默认不拦截
                    intercepted = onInterceptTouchEvent(ev);
                    ev.setAction(action); // restore action in case it was changed存储动作以防止它改变
                } else {
                    intercepted = false; // 如果禁止拦截，intercepted就是false，touch事件就继续传递给子View
                }
            } else {
                // There are no touch targets and this action is not an initial down
                // so this view group continues to intercept touches.如果没有touch目标组件和down事件，这个viewgroup就是继续拦截touch
                intercepted = true;
            }

            // If intercepted, start normal event dispatch. Also if there is already
            // a view that is handling the gesture, do normal event dispatch.
            if (intercepted || mFirstTouchTarget != null) {
                ev.setTargetAccessibilityFocus(false);
            }

            // Check for cancelation.检查取消，然后将结果赋值给局部boolean变量canceled
            final boolean canceled = resetCancelNextUpFlag(this)
                    || actionMasked == MotionEvent.ACTION_CANCEL;

            // Update list of touch targets for pointer down, if needed. 默认是true，作用是是否把事件分发给多个子View
            final boolean split = (mGroupFlags & FLAG_SPLIT_MOTION_EVENTS) != 0;
            TouchTarget newTouchTarget = null;
            boolean alreadyDispatchedToNewTouchTarget = false;
            if (!canceled && !intercepted) { // 如果没有被取消也没有被拦截，就开始进行分发事件了

                // If the event is targeting accessiiblity focus we give it to the
                // view that has accessibility focus and if it does not handle it
                // we clear the flag and dispatch the event to all children as usual.
                // We are looking up the accessibility focused host to avoid keeping
                // state since these events are very rare.
                View childWithAccessibilityFocus = ev.isTargetAccessibilityFocus()
                        ? findChildWithAccessibilityFocus() : null;

                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) { // childrenCount个数是否不为0且新的touch target是空，目的就是为了找到touch target
                        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();//子View的list集合preorderedList
                        final boolean customOrder = preorderedList == null
                                && isChildrenDrawingOrderEnabled();
                        final View[] children = mChildren;
                        for (int i = childrenCount - 1; i >= 0; i--) {//for循环i从childrenCount - 1开始遍历到0，倒序遍历所有的子view
                            final int childIndex = customOrder
                                    ? getChildDrawingOrder(childrenCount, i) : i;
                            final View child = (preorderedList == null)
                                    ? children[childIndex] : preorderedList.get(childIndex);

                            // If there is a view that has accessibility focus we want it
                            // to get the event first and if not handled we will perform a
                            // normal dispatch. We may do a double iteration but this is
                            // safer given the timeframe.
                            if (childWithAccessibilityFocus != null) {
                                if (childWithAccessibilityFocus != child) {
                                    continue;
                                }
                                childWithAccessibilityFocus = null;
                                i = childrenCount - 1;
                            }

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

                            newTouchTarget = getTouchTarget(child); //这一句很重要，通过getTouchTarget去查找当前子View是否在mFirstTouchTarget.next这条target链中的某一个target中，如果在则返回这个target，否则返回null
                            if (newTouchTarget != null) {//找到了接收Touch事件的子View，即newTouchTarget，那么，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);
                         /**
                          *调用方法dispatchTransformedTouchEvent()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()。dispatchTransformedTouchEvent方法如果返回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；
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，既然已经找到了就跳出该外层for循环；
                          */
                            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;
                            }

                            // The accessibility focus didn't handle the event, so clear
                            // the flag and do a normal dispatch to all children.
                            ev.setTargetAccessibilityFocus(false);
                        }
                        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;
                    }
                }
            }
           /**
  *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事件的返回值决定了dispatchTransformedTouchEvent()的返回值，从而决定mFirstTouchTarget是否为null，进一步决定了ViewGroup   是否处理Touch事件
*/
            // 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;
    }

/**
     * Implement this method to intercept all touch screen motion events.  This
     * allows you to watch events as they are dispatched to your children, and
     * take ownership of the current gesture at any point.
     *实现这个方法拦截屏幕的所有触摸事件，这就允许你观察这些事件被分发给你的孩子，在任何一个点掌控当前手势
     * <p>Using this function takes some care, as it has a fairly complicated
     * interaction with {@link View#onTouchEvent(MotionEvent)
     * View.onTouchEvent(MotionEvent)}, and using it requires implementing
     * that method as well as this one in the correct way.  Events will be
     * received in the following order:
     *使用这个函数要小心，它和View的onTouchEvent有十分复杂的交互，事件能够按照下列的顺序被收到:
     * <ol>
     * <li> You will receive the down event here.1,你将先收到down事件
     * <li> The down event will be handled either by a child of this view
     * group, or given to your own onTouchEvent() method to handle; this means
     * you should implement onTouchEvent() to return true, so you will
     * continue to see the rest of the gesture (instead of looking for
     * a parent view to handle it).  Also, by returning true from
     * onTouchEvent(), you will not receive any following
     * events in onInterceptTouchEvent() and all touch processing must
     * happen in onTouchEvent() like normal.
     *  2，down事件要么被子View的handle,要么被这个viewgroup的onTouchEvent方法处理
     * <li> For as long as you return false from this function, each following
     * event (up to and including the final up) will be delivered first here
     * and then to the target's onTouchEvent().
* 3，只要onInterceptTouchEvent返回false，后面的每个事件都会继续分发到touch target执行target's onTouchEvent()
     * <li> If you return true from here, you will not receive any
     * following events: the target view will receive the same event but
     * with the action {@link MotionEvent#ACTION_CANCEL}, and all further
     * events will be delivered to your onTouchEvent() method and no longer
     * appear here.
     * 4，onInterceptTouchEvent返回true, 交给这个ViewGroup的onTouchEvent处理。
     * </ol>
     *
     * @param ev The motion event being dispatched down the hierarchy.
     * @return Return true to steal motion events from the children and have
     * them dispatched to this ViewGroup through onTouchEvent().
     * The current target will receive an ACTION_CANCEL event, and no further
     * messages will be delivered here.
     */
    public boolean onInterceptTouchEvent(MotionEvent ev) {
        return false;
    }

如上就是所有ViewGroup关于触摸屏事件的传递机制源码分析。具体总结如下：
1,Android事件派发是先传递到最顶级的ViewGroup，再由ViewGroup递归传递到View的。
2,在ViewGroup中可以通过onInterceptTouchEvent方法对事件传递进行拦截，3,onInterceptTouchEvent方法

   1）返回true   代表不允许事件继续向子View传递，则交给这个ViewGroup的onTouchEvent处理

   2）返回false 代表不对事件进行拦截，默认返回false，则交给子View的dispatchTouchEvent方法处理
4,事件传递到子view 的 dispatchTouchEvent方法中，通过方法传递到当前View的onTouchEvent方法中：
（1）如果返回true，那么这个事件就会止于该view。
（2）如果返回 false ，那么这个事件会从这个子view 往上传递，而且都是传递到父View的onTouchEvent 来接收。

（3）如果传递到ViewGroup的 onTouchEvent 也返回 false 的话，则继续传递到Activity的onTouchEvent中，如果还是false,则这个事件就会“消失“；事件向上传递到中间的任何onTouchEvent方法中，如果返回true，则事件被消费掉，不会再传递。

Android触摸事件源码分析:Activity->ViewGroup->View

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原文地址：http://blog.csdn.net/tuke_tuke/article/details/52206303