事件分发中，View和ViewGroup的功能区分

View.dispatchTouchEvent()---->事件的处理逻辑,
    ---> ViewGroup.dispatchTouchEvent()--->实现了分发流程的逻辑。

事件注册流程

事件注册是在ViewRootImpl中实现的，一般的，每一个Activity的Window都有一个ViewRootImpl，这个是所有View的parent，内部会处理事件分发， 刷新，与sf通信，Canvas创建等。

setView@ViewRootImpl.java
--->mInputEventReceiver = new WindowInputEventReceiver(inputChannel, Looper.myLooper());
  --->public void onInputEvent(InputEvent event) # WindowInputEventReceiver
      --->doProcessInputEvents
        --->deliverInputEvent
          --->stage.deliver(q);@InputStage.java](mailto:--->stage.deliver(q);@InputStage.java)
            --->ViewPostImeInputStage
              --->onProcess
                --->processKeyEvent
                  --->mView. dispatchTouchEvent(event) mView就是DecorView
                    --->final Window.Callback cb = mWindow.getCallback() // 进入Activity
final boolean handled = cb != null && mFeatureId < 0 ? cb.dispatchKeyEvent(event): super.dispatchKeyEvent(event);
if (handled) {
    return true;
}
                      --->win.superDispatchTouchEvent(event) // PhoneWindow
                        --->mDecor.superDispatcTouchEvent(event)
                          ---->super.dispatchTouchEvent(event)//mDecorView,       
                             ---->ViewGroup.dispatchTouchEvent() //最终到ViewGroup处理事件
                                ---->View.dispatchTouchEvent()
                                  ---->View.onTouch()
                                  ---->View.onTouchEvent()
                                    ---->onClick()
                                    ---->onLongClick()// 这里是通过postdelay一个runnable实现的。

View.dispatchTouchEvent // 事件处理方法，事件的处理核心。                     
事件首先分发到DecorView，然后再到Activity

InputStage的一些执行，input事件（QueuedInputEvent）分为多阶段处理（InputStage）,每个阶段对应一个stage，一个事件对应多个stage，当前阶段（stage）若是已经被处理，则跳过执行下一个阶段，否则先进行处理然后执行下一阶段

InputStage   
mSyntheticInputStage = new SyntheticInputStage();
InputStage viewPostImeStage = new ViewPostImeInputStage(mSyntheticInputStage);
InputStage nativePostImeStage = new NativePostImeInputStage(viewPostImeStage,
        "aq:native-post-ime:" + counterSuffix);
InputStage earlyPostImeStage = new EarlyPostImeInputStage(nativePostImeStage);
InputStage imeStage = new ImeInputStage(earlyPostImeStage,
        "aq:ime:" + counterSuffix);
InputStage viewPreImeStage = new ViewPreImeInputStage(imeStage);
InputStage nativePreImeStage = new NativePreImeInputStage(viewPreImeStage,
        "aq:native-pre-ime:" + counterSuffix);

事件处理机制的几个重要方法

dispatchTouchEvent， // 只要事件到View，View就会执行这个方法。

onInterceptTouchEvent，// 用于拦截事件。

onTouchEvent:

（1）onTouchEvent在哪里执行的-->在View的dispatchTouchEvent中。

（2）onTouch和onClick 执行的位置，onTouch先执行，onClick后执行。

（3）LongClick  // ACTION_DOWN处理，removeLongPressCallback(); 按压时间<400,action_down处理

（4）按下移出view,为什么onClick不执行，不过long事件依然可以执行。 //  if (!pointInView(x, y, touchSlop)) 原因如下：

if (!pointInView(x, y, touchSlop)) {
    // Outside button
    // Remove any future long press/tap checks
    removeTapCallback();
    removeLongPressCallback();
    if ((mPrivateFlags & PFLAG_PRESSED) != 0) {
        setPressed(false);
    }
    mPrivateFlags3 &= ~PFLAG3_FINGER_DOWN;
}

上边这个这个代码是在Move里边执行的，会实时检测位置。

if (!pointInView(x, y, touchSlop)) {
    // The default action here is to cancel long press. But instead, we
    // just extend the timeout here, in case the classification
    // stays ambiguous.
    removeLongPressCallback();
    long delay = (long) (ViewConfiguration.getLongPressTimeout()
            * mAmbiguousGestureMultiplier);
    // Subtract the time already spent
    delay -= event.getEventTime() - event.getDownTime();
    checkForLongClick(
            delay,
            x,
            y,
            TOUCH_GESTURE_CLASSIFIED__CLASSIFICATION__LONG_PRESS);
}
touchSlop *= mAmbiguousGestureMultiplier;

根据上边的代码，Long事件依然可以响应。

if ((mPrivateFlags & PFLAG_PRESSED) != 0 才能进入onClick,移除了点击与长按事件。

View.dispatchTouchEvent

   --->onTouch // 设置setOnTouchListener()之后就会执行，返回值影响onClick执行，false才能继续执行。
     --->!result && onTouchEvent(event) // 短路与，当result为true，onTouchEvent不执行。onClick在onTouchEvent里边。
        // MotionEvent.ACTION_UP:
        ---->PerformClick
          --->performClickInternal
            --->performClick
              --->onClick  // 表示该事件被这个View消费了。

ViewGroup.dispatchTouchEvent

   第一根手指按下：Action_down，第二根到第n根，action_point_down中间移动 ,Action_move多次调用。    抬起：倒数第n根到倒数第二根：Action_point_up，最后一根手指：Action_up。安卓手机最多能够识别32根手指，和int的位数相关。  位运算。一位表示一个手指：

int idBitsToAssign = split ? 1 << ev.getPointerId(actionIndex) : TouchTarget.ALL_POINTER_IDS;

事件分发解析

整体流程如下

ViewGroup事件分发源码解析

@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.
        // 残障人士的辅助类，智能聊天
        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;

            // Handle an initial down.
            // 多指或者单指，只会执行一次。第二、三等手指按下，是Action_point_down
            // 重置状态，
            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.检测是否拦截，父容器的权利。
            final boolean intercepted;
            if (actionMasked == MotionEvent.ACTION_DOWN
                    || mFirstTouchTarget != null) {
                final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
                // 决定onInterceptTouchEvent是否执行。
                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;
            }

            // 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.
            final boolean canceled = resetCancelNextUpFlag(this)
                    || actionMasked == MotionEvent.ACTION_CANCEL;

            // Update list of touch targets for pointer down, if needed.
            final boolean isMouseEvent = ev.getSource() == InputDevice.SOURCE_MOUSE;
            // split默认为true，是否可以多指。
            final boolean split = (mGroupFlags & FLAG_SPLIT_MOTION_EVENTS) != 0
                    && !isMouseEvent;
            TouchTarget newTouchTarget = null;
            boolean alreadyDispatchedToNewTouchTarget = false;
            // 在if中分发事件。
            if (!canceled && !intercepted) {
                // If the event is targeting accessibility 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;
                // 第一根手指按下时。
                if (actionMasked == MotionEvent.ACTION_DOWN
                        // 只要有手指按下，第二根，第三根...
                        || (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN)
                        // 鼠标
                        || actionMasked == MotionEvent.ACTION_HOVER_MOVE) {
                    // 如果事件是MotionEvent.ACTION_DOWN，actionIndex = 0；用来进行事件的计数,如果是第二根第三个手指，这个相应的就是1、2......
                    final int actionIndex = ev.getActionIndex(); // always 0 for down
                    // 手指的ID。这里是一个位运算，一位表示一根手指，int是32为，因此安卓手机最多能够识别32根手指
                    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);
                    // 有多少个子孩子。
                    final int childrenCount = mChildrenCount;
                    if (newTouchTarget == null && childrenCount != 0) {
                        final float x =
                                isMouseEvent ? ev.getXCursorPosition() : ev.getX(actionIndex);
                        final float y =
                                isMouseEvent ? ev.getYCursorPosition() : ev.getY(actionIndex);
                        // Find a child that can receive the event.
                        // Scan children from front to back.
                        // 将子View进行排序，确定谁来优先处理事件。
                        final ArrayList<View> preorderedList = buildTouchDispatchChildList();
                        final boolean customOrder = preorderedList == null
                                && isChildrenDrawingOrderEnabled();
                        final View[] children = mChildren;
                        // 倒叙遍历。xml写在后边的先处理事件。
                        for (int i = childrenCount - 1; i >= 0; i--) {
                            final int childIndex = getAndVerifyPreorderedIndex(
                                    childrenCount, i, customOrder);
                            final View child = getAndVerifyPreorderedView(
                                    preorderedList, children, childIndex);
                            // 判断这个view是否能够处理点击事件。
                            if (!child.canReceivePointerEvents() // View可见或者Animation不为空
                                    // 点击区域是否在View上边
                                    || !isTransformedTouchPointInView(x, y, child, null)) {
                                continue;
                            }
                            // 单指操作为空，多指不为空。
                            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);
                            // 在询问child是否处理事件，如果child处理，则命中if--递归
                            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();
                                // 创建了newTouchTarget，并且newTouchTarget = mFirstTouchTarget，如果是多指操作，那么addTouchTarget（里边是链表）会将这些target指向下一个目标，
                                newTouchTarget = addTouchTarget(child, idBitsToAssign);
                                // 后边会用到。
                                alreadyDispatchedToNewTouchTarget = true;
                                // 退出循环，事件已经处理完成，不在询问其他Child
                                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();
                    }

                    // 多指处理的情况
                    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.
            // 没有child处理事件的时候，这个值才是空。
            if (mFirstTouchTarget == null) {
                // No touch targets so treat this as an ordinary view.
                // 所有的Child都不处理，就询问自己是否处理
                handled = dispatchTransformedTouchEvent(ev, canceled, null,
                        TouchTarget.ALL_POINTER_IDS);
            } else {
                // 有子View处理了事件
                // 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循环为单指操作时，只会执行一次。
                while (target != null) {
                    // 单指操作next = null;
                    final TouchTarget next = target.next;
                    // 单指操作会直接命中if，直接返回，不做处理。
                    if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) {
                        handled = true;
                    } else {
                        final boolean cancelChild = resetCancelNextUpFlag(target.child)
                                || intercepted;
                        // 询问target.child（前面保存的addTouchTarget（））要不要处理这个事件
                        if (dispatchTransformedTouchEvent(ev, cancelChild,
                                target.child, target.pointerIdBits)) {
                            handled = true;
                        }
                        // 为true，取消child处理事件。
                        if (cancelChild) {
                            if (predecessor == null) {
                                // mFirstTouchTarget置为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;
    }
	
// 对View进行排列，用来处理事件。显示在最上边的优先处理事件。	
ArrayList<View> buildOrderedChildList() {
        final int childrenCount = mChildrenCount;
        if (childrenCount <= 1 || !hasChildWithZ()) return null;

        if (mPreSortedChildren == null) {
            mPreSortedChildren = new ArrayList<>(childrenCount);
        } else {
            // callers should clear, so clear shouldn't be necessary, but for safety...
            mPreSortedChildren.clear();
            mPreSortedChildren.ensureCapacity(childrenCount);
        }

        final boolean customOrder = isChildrenDrawingOrderEnabled();
        for (int i = 0; i < childrenCount; i++) {
            // add next child (in child order) to end of list
            final int childIndex = getAndVerifyPreorderedIndex(childrenCount, i, customOrder);
            final View nextChild = mChildren[childIndex];
            // 默认不设置，则为0
            final float currentZ = nextChild.getZ();

            // insert ahead of any Views with greater Z
            int insertIndex = i;
            while (insertIndex > 0 && mPreSortedChildren.get(insertIndex - 1).getZ() > currentZ) {
                insertIndex--;
            }
            // 谁显示在最上边，xml中，布局靠后的放在集合后边。谁最先处理事件
            mPreSortedChildren.add(insertIndex, nextChild);
        }
        return mPreSortedChildren;
    }
	
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) {
            // Viewgroup的super就是View.dispatchTouchEvent(处理事件)
            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());
            }
            // child是容器，ViewGroup.dispathTouchEvent。Child是View,View.dispatchTouchEvent（处理事件）
            handled = child.dispatchTouchEvent(transformedEvent);
        }

        // Done.
        transformedEvent.recycle();
        return handled;
    }

// 多指操作组合成链表。
private TouchTarget addTouchTarget(@NonNull View child, int pointerIdBits) {
        final TouchTarget target = TouchTarget.obtain(child, pointerIdBits);
        // 单指操作，target0->next为空。
        target.next = mFirstTouchTarget;
        mFirstTouchTarget = target;
        return target;
    }

View.dispatchTouchEvent负责完成事件的处理

public boolean dispatchTouchEvent(MotionEvent event) {
    // If the event should be handled by accessibility focus first.
    if (event.isTargetAccessibilityFocus()) {
        // We don't have focus or no virtual descendant has it, do not handle the event.
        if (!isAccessibilityFocusedViewOrHost()) {
            return false;
        }
        // We have focus and got the event, then use normal event dispatch.
        event.setTargetAccessibilityFocus(false);
    }
    boolean result = false;

    if (mInputEventConsistencyVerifier != null) {
        mInputEventConsistencyVerifier.onTouchEvent(event, 0);
    }

    final int actionMasked = event.getActionMasked();
    if (actionMasked == MotionEvent.ACTION_DOWN) {
        // Defensive cleanup for new gesture
        stopNestedScroll();
    }

    if (onFilterTouchEventForSecurity(event)) {
        if ((mViewFlags & ENABLED_MASK) == ENABLED && handleScrollBarDragging(event)) {
            result = true;
        }
        //noinspection SimplifiableIfStatement
        ListenerInfo li = mListenerInfo;
        // 首先处理onTouch事件。
        if (li != null && li.mOnTouchListener != null
                && (mViewFlags & ENABLED_MASK) == ENABLED
                && li.mOnTouchListener.onTouch(this, event)) {
            result = true;
        }
        
        // 然后处理onTouchEvent事件，这个事件里边会处理长按事件和点击事件。
        if (!result && onTouchEvent(event)) {
            result = true;
        }
    }

    if (!result && mInputEventConsistencyVerifier != null) {
        mInputEventConsistencyVerifier.onUnhandledEvent(event, 0);
    }

    // Clean up after nested scrolls if this is the end of a gesture;
    // also cancel it if we tried an ACTION_DOWN but we didn't want the rest
    // of the gesture.
    if (actionMasked == MotionEvent.ACTION_UP ||
            actionMasked == MotionEvent.ACTION_CANCEL ||
            (actionMasked == MotionEvent.ACTION_DOWN && !result)) {
        stopNestedScroll();
    }

    return result;
}
    
public boolean onTouchEvent(MotionEvent event) {
    // 这里可以实现拓展点击区域的操作
    if (mTouchDelegate != null) {
        if (mTouchDelegate.onTouchEvent(event)) {
            return true;
        }
    }

    if (clickable || (viewFlags & TOOLTIP) == TOOLTIP) {
        switch (action) {
            case MotionEvent.ACTION_UP:
                mPrivateFlags3 &= ~PFLAG3_FINGER_DOWN;
                if ((viewFlags & TOOLTIP) == TOOLTIP) {
                    handleTooltipUp();
                }
                if (!clickable) {
                    removeTapCallback();
                    removeLongPressCallback();
                    mInContextButtonPress = false;
                    mHasPerformedLongPress = false;
                    mIgnoreNextUpEvent = false;
                    break;
                }
                boolean prepressed = (mPrivateFlags & PFLAG_PREPRESSED) != 0;
                if ((mPrivateFlags & PFLAG_PRESSED) != 0 || prepressed) {
                    // take focus if we don't have it already and we should in
                    // touch mode.
                    boolean focusTaken = false;
                    if (isFocusable() && isFocusableInTouchMode() && !isFocused()) {
                        focusTaken = requestFocus();
                    }

                    if (prepressed) {
                        // The button is being released before we actually
                        // showed it as pressed.  Make it show the pressed
                        // state now (before scheduling the click) to ensure
                        // the user sees it.
                        setPressed(true, x, y);
                    }

                    if (!mHasPerformedLongPress && !mIgnoreNextUpEvent) {
                        // This is a tap, so remove the longpress check
                        removeLongPressCallback();

                        // Only perform take click actions if we were in the pressed state
                        if (!focusTaken) {
                            // Use a Runnable and post this rather than calling
                            // performClick directly. This lets other visual state
                            // of the view update before click actions start.
                            if (mPerformClick == null) {
                                mPerformClick = new PerformClick();
                            }
                            // onClick 事件在Action_up才会触发，另外，如果在抬起手指的时候已经离开View的区域了，就不会触发了。
                            if (!post(mPerformClick)) {
                                performClickInternal();
                            }
                        }
                    }

                    if (mUnsetPressedState == null) {
                        mUnsetPressedState = new UnsetPressedState();
                    }

                    if (prepressed) {
                        postDelayed(mUnsetPressedState,
                                ViewConfiguration.getPressedStateDuration());
                    } else if (!post(mUnsetPressedState)) {
                        // If the post failed, unpress right now
                        mUnsetPressedState.run();
                    }

                    removeTapCallback();
                }
                mIgnoreNextUpEvent = false;
                break;

            case MotionEvent.ACTION_DOWN:
                if (event.getSource() == InputDevice.SOURCE_TOUCHSCREEN) {
                    mPrivateFlags3 |= PFLAG3_FINGER_DOWN;
                }
                mHasPerformedLongPress = false;

                if (!clickable) {
                    checkForLongClick(
                            ViewConfiguration.getLongPressTimeout(),
                            x,
                            y,
                            TOUCH_GESTURE_CLASSIFIED__CLASSIFICATION__LONG_PRESS);
                    break;
                }

                if (performButtonActionOnTouchDown(event)) {
                    break;
                }

                // Walk up the hierarchy to determine if we're inside a scrolling container.
                boolean isInScrollingContainer = isInScrollingContainer();

                // For views inside a scrolling container, delay the pressed feedback for
                // a short period in case this is a scroll.
                if (isInScrollingContainer) {
                    mPrivateFlags |= PFLAG_PREPRESSED;
                    if (mPendingCheckForTap == null) {
                        mPendingCheckForTap = new CheckForTap();
                    }
                    mPendingCheckForTap.x = event.getX();
                    mPendingCheckForTap.y = event.getY();
                    postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout());
                } else {
                    // Not inside a scrolling container, so show the feedback right away
                    setPressed(true, x, y);
                    // 在Action_down事件里边检测和触发long事件。
                    checkForLongClick(
                            ViewConfiguration.getLongPressTimeout(),
                            x,
                            y,
                            TOUCH_GESTURE_CLASSIFIED__CLASSIFICATION__LONG_PRESS);
                }
                break;

            case MotionEvent.ACTION_CANCEL:
                if (clickable) {
                    setPressed(false);
                }
                removeTapCallback();
                removeLongPressCallback();
                mInContextButtonPress = false;
                mHasPerformedLongPress = false;
                mIgnoreNextUpEvent = false;
                mPrivateFlags3 &= ~PFLAG3_FINGER_DOWN;
                break;

            case MotionEvent.ACTION_MOVE:
                if (clickable) {
                    drawableHotspotChanged(x, y);
                }

                final int motionClassification = event.getClassification();
                final boolean ambiguousGesture =
                        motionClassification == MotionEvent.CLASSIFICATION_AMBIGUOUS_GESTURE;
                int touchSlop = mTouchSlop;
                if (ambiguousGesture && hasPendingLongPressCallback()) {
                    if (!pointInView(x, y, touchSlop)) {
                        // The default action here is to cancel long press. But instead, we
                        // just extend the timeout here, in case the classification
                        // stays ambiguous.
                        removeLongPressCallback();
                        long delay = (long) (ViewConfiguration.getLongPressTimeout()
                                * mAmbiguousGestureMultiplier);
                        // Subtract the time already spent
                        delay -= event.getEventTime() - event.getDownTime();
                        checkForLongClick(
                                delay,
                                x,
                                y,
                                TOUCH_GESTURE_CLASSIFIED__CLASSIFICATION__LONG_PRESS);
                    }
                    touchSlop *= mAmbiguousGestureMultiplier;
                }

                // Be lenient about moving outside of buttons
                if (!pointInView(x, y, touchSlop)) {
                    // Outside button
                    // Remove any future long press/tap checks
                    removeTapCallback();
                    removeLongPressCallback();
                    if ((mPrivateFlags & PFLAG_PRESSED) != 0) {
                        setPressed(false);
                    }
                    mPrivateFlags3 &= ~PFLAG3_FINGER_DOWN;
                }

                final boolean deepPress =
                        motionClassification == MotionEvent.CLASSIFICATION_DEEP_PRESS;
                if (deepPress && hasPendingLongPressCallback()) {
                    // process the long click action immediately
                    removeLongPressCallback();
                    checkForLongClick(
                            0 /* send immediately */,
                            x,
                            y,
                            TOUCH_GESTURE_CLASSIFIED__CLASSIFICATION__DEEP_PRESS);
                }

                break;
        }

        return true;
    }

    return false;
}

整个流程其实就是在执行这个代码：

public boolean dispatchTouchEvent(MotionEvent ev) {
    if (ev.getAction() == MotionEvent.ACTION_DOWN) {
        onUserInteraction();
    }
    if (getWindow().superDispatchTouchEvent(ev)) {
        return true;
    }
    return onTouchEvent(ev);
}

相比于View，ViewGroup不仅要做时间分发，还可能需要做事件响应处理。

一些问题

1、Down事件谁处理，move事件就由谁处理；

2、单指操作，不需要管链表问题。

3、继承Activity和AppcompatActivity的区别：