上一篇讲了view的事件分发，比较简单。接下来看看稍微复杂一点的ViewGroup。
我们还是先用log看一下主要方法是如何执行的：

ViewGroup事件分发的核心内容主要在dispatchTouchEvent（）这个方法中。点进去看一下源码,代码有点长，在关键部分注释一下：

@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;
        //1.没有被遮挡
        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.
                //2.清空异常及已有状态 给所有之前选择出的Target发送Cancel事件，
                //确保之前Target能收到完整的事件周期； 
                cancelAndClearTouchTargets(ev);
                resetTouchState();
            }

            // Check for interception.
            final boolean intercepted;
            //3.如果是ACTION_DOWN事件，或者已经将之前的ACTION_DOWN事件分发给childview
            //注：当ViewGroup的childview成功处理时，会将mFirstTouchTarget 指向该childview
            if (actionMasked == MotionEvent.ACTION_DOWN
                    || mFirstTouchTarget != null) {
                //4.是否禁止拦截
                final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
                //5.如果没有禁止拦截
                if (!disallowIntercept) {
                    //6.判断是否拦截，也就是onInterceptTouchEvent的返回值
                    intercepted = onInterceptTouchEvent(ev);
                    ev.setAction(action); // restore action in case it was changed
                }
                  //7.如果禁止拦截，当然就不拦截了
                   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.
                // 8.在这种情况下，actionMasked != ACTION_DOWN && mFirstTouchTarget == null
                // 第一次的down事件没有被此ViewGroup的children处理掉（要么是它们自己不处理，要么是ViewGroup从一
                // 开始的down事件就开始拦截），则接下来的所有事件
                // 也没它们的份，即不处理down事件的话，那表示你对后面接下来的事件也不感兴趣
                //意味着上一次ACTION_DOWN 事件没有分发下去，所以后续事件不再询问，直接拦截
                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.
            //9.此touch事件是否cancel
            final boolean canceled = resetCancelNextUpFlag(this)
                    || actionMasked == MotionEvent.ACTION_CANCEL;

            // Update list of touch targets for pointer down, if needed.
            //10.是否拆分事件，与多点触控有关，默认拆分
            final boolean split = (mGroupFlags & FLAG_SPLIT_MOTION_EVENTS) != 0;
            //11.找到的childview，接下来ViewGroup判断要将此touch事件交给他处理
            TouchTarget newTouchTarget = null;
            //12.down或pointer_down事件已经被处理
            boolean alreadyDispatchedToNewTouchTarget = false;
            //13.没有cancel也没有拦截，即有效事件
            if (!canceled && !intercepted) {

                // 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//第一个手指按下
                        || (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN)//更多手指按下
                        || actionMasked == MotionEvent.ACTION_HOVER_MOVE) {//鼠标指针在屏幕上
                    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 = buildTouchDispatchChildList();
                        final boolean customOrder = preorderedList == null
                                && isChildrenDrawingOrderEnabled();
                        final View[] children = mChildren;
                        //14.遍历childview
                        for (int i = childrenCount - 1; i >= 0; i--) {
                            final int childIndex = getAndVerifyPreorderedIndex(
                                    childrenCount, i, customOrder);
                            final View child = getAndVerifyPreorderedView(
                                    preorderedList, children, 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.
                            //15.不可见而且没有正在执行的动画，跳过
                            if (childWithAccessibilityFocus != null) {
                                if (childWithAccessibilityFocus != child) {
                                    continue;
                                }
                                childWithAccessibilityFocus = null;
                                i = childrenCount - 1;
                            }
                            //16.手指不在该childview范围中，跳过
                            if (!canViewReceivePointerEvents(child)
                                    || !isTransformedTouchPointInView(x, y, child, null)) {
                                ev.setTargetAccessibilityFocus(false);
                                continue;
                            }
                            //17.寻找childview对应的TouchTarget
                            newTouchTarget = getTouchTarget(child);
                            //18.已经有对应的TouchTarget，比如在同一个child上按下了多跟手指
                            if (newTouchTarget != null) {
                                // Child is already receiving touch within its bounds.
                                // Give it the new pointer in addition to the ones it is handling.
                                //19. newTouchTarget已经有了，跳出for循环
                                newTouchTarget.pointerIdBits |= idBitsToAssign;
                                break;
                            }
                           
                            //20.复位标志位,避免上次操作带来的影响
                            resetCancelNextUpFlag(child);
                            //21.重点：dispatchTransformedTouchEvent方法当child为空时调用自身dispatchTouchEvent
                            //否则调用child的dispatchTouchEvent方法，在这child不为空，所以将事件分发下去
                            // 需要注意的是，有可能用户一个手指按在了child1上，另一个手指按在了child2上
                            // 这样TouchTarget的链就形成了（TouchTarget内部类似单链表）
                            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();
                                //22. 如果处理掉了的话，将此child添加到touch链的头部
                                // 注意这个方法内部会更新 mFirstTouchTarget
                                newTouchTarget = addTouchTarget(child, idBitsToAssign);
                                // 23.down或pointer_down事件已经被处理了
                                alreadyDispatchedToNewTouchTarget = true;
                                //24.退出循环
                                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;
                    }
                }
            }
            // 25.非down事件直接从这里开始处理，不会走上面的一大堆寻找TouchTarget的逻辑

            // Dispatch to touch targets.
            //26.如果没有找到合适的childview，交给自己处理，调用super.dispatchTouchEvent(event);
            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;
                //27.上面说了，用户有可能不同的手指按在不同的childview上，所以要遍历TouchTarget的链表
                while (target != null) {
                    final TouchTarget next = target.next;
                    //28.已经处理过，不再处理
                    if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) {
                        handled = true;
                    } else {
                        //检查是否需要拦截
                        final boolean cancelChild = resetCancelNextUpFlag(target.child)
                                || intercepted;
                        // 如果ViewGroup从半路拦截(cancelChild为true)了touch事件则给touch链上的child发送cancel事件
                        if (dispatchTransformedTouchEvent(ev, cancelChild,
                                target.child, target.pointerIdBits)) {
                            // TouchTarget链中任意一个处理了则设置handled为true
                            handled = true;
                        }
                        // 如果cancel的话，则回收此target节点
                        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.
            // 取消或up事件时resetTouchState
            if (canceled
                    || actionMasked == MotionEvent.ACTION_UP
                    || actionMasked == MotionEvent.ACTION_HOVER_MOVE) {
                resetTouchState();
            } 
            // 当多个手指按下时抬起某个手指，将其相关的信息移除
            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);
        }
        // 返回处理的结果
        return handled;
    }

主要的功能，上面注释了一部分。这里总结一下：

ACTION_DOWN

首先，ACTION_DOWN 事件的任务就是找到分发对象（找到接收事件分发的childview）

1.在viewgroup没有被遮罩的情况下，先清空异常及已有状态，给所有之前选择出的Target发送Cancel事件，确保之前Target能收到完整的事件周期； 清除已有Target，复位所有标志位（如PFLAG_CANCEL_NEXT_UP_EVENT、FLAG_DISALLOW_INTERCEPT等） 。
对应编号1-2
2.判断是否需要拦截，某个 view 一旦开始处理事件，如果不消费 ACTION_DOWN 事件，则同一事件序列中的其他事件不会再交给它来处理,直接拦截。如果是ACTION_DOWN 事件，或者向下分发了ACTION_DOWN 事件，则调用onInterceptTouchEvent以确定当前ViewGroup是否拦截该Down事件。与此同时，还要关注禁止拦截属性。禁止拦截由public void requestDisallowInterceptTouchEvent(boolean disallowIntercept)方法设置，当disallowIntercept为true时，禁止拦截。
对应编号3-8
3.如果事件没有被拦截也没有被cancel，遍历childview寻找合适的分发对象。｛如果该childview不可见而且没有正在执行的动画，跳过。如果手指不在该childview范围中，也跳过。找到合适的childview之后，判断是否已经有一个手指按在此childview上（对应编号18），如果是直接跳出循环，因为已经找到了合适的分发对象（即此newTouchTarget.child）。如果否，先复位该childview的状态，然后调用childview的dispatchTouchEvent方法把事件分发下去，并把该childview添加到TouchTarget 链表中，对mFirstTouchTarget赋值（编号3处用到，如果mFirstTouchTarget！=null，说明已经将事件分发下去了）。然后退出循环｝（花括号内为循环体）。如果没有合适的childview，则交给自己处理，调用super.dispatchTouchEvent(event);（编号26）
对应编号13-26 。 至此，找到了合适的childview，

非ACTION_DOWN 事件

上面我们得到了TouchTarget，其中的链表记录了接收事件的childview（可能是多个，多点触控）。所以我们先遍历target，检查是否被ViewGroup从半路拦截，如果是，分发cancel，否者正常的把事件分发下去。

图片说明

最后一图以蔽之。