前言

事件传递主要涉及如下重要方法：

• dispatchTouchEvent 负责事件分发
  Activity-->PhoneWindow-->DecorView-->ViewGroup-->View
  若事件不被拦截，最终会被传递到子View，由子View来进行消费。若子View不消费，则层层回朔之上。
• onInterceptTouchEvent 负责事件拦截
  ViewGroup中的方法，是否对事件进行拦截，若拦截，则自己来消费，若不拦截，将事件分发至子View
• onTouchEvent 事件消费

下面从代码角度具体来进行分析

ViewGroup中的dispatchTouchEvent

dispatchTouchEvent方法

@Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
        //验证事件是否连续
        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);
        }
        //标记事件是否处理完，若传递给子view，并且子view消费(返回true)，则为true
        boolean handled = false;
        //过滤掉一些不合法的事件：当前的View的窗口被遮挡了
        if (onFilterTouchEventForSecurity(ev)) {
            //重置前面为0 ,只留下后八位，用于判断相等时候，可以提高性能。
            final int action = ev.getAction();
            final int actionMasked = action & MotionEvent.ACTION_MASK;
            //将DOWN事件定为一连串事件的初始事件，所以要进行一些初始化操作
            // 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.
                //清空掉之前的一些手势状态
                cancelAndClearTouchTargets(ev);
                //清空掉mFirstTouchTarget
                resetTouchState();
            }
            //检测是否拦截
            // Check for interception.
            final boolean intercepted;
            //如果当前的事件是DOWN事件，或者是已经有了触摸事件的目标
            if (actionMasked == MotionEvent.ACTION_DOWN
                    || mFirstTouchTarget != null) {
                //判断是否允许进行拦截，该变量主要由requestDisallowInterceptTouchEvent方法来确定，该方法由子View发起
                final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
                //允许拦截
                if (!disallowIntercept) {
                    //根据onInterceptTouchEvent方法，判断当前是否拦截
                    intercepted = onInterceptTouchEvent(ev);
                    //重置Action，以免被改变
                    ev.setAction(action); // restore action in case it was changed
                } else {
                    intercepted = false;
                }
            } else {//如果说，事件已经初始化过了，并且没有子View被分配处理，那么就说明，这个ViewGroup已经拦截了这个事件，那么就继续拦截
                // 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.
            //如果viewFlag被设置了PFLAG_CANCEL_NEXT_UP_EVENT ,那么就表示，下一步应该是Cancel事件
            //或者如果当前的Action为取消，那么当前事件应该就是取消了。
            final boolean canceled = resetCancelNextUpFlag(this)
                    || actionMasked == MotionEvent.ACTION_CANCEL;

            //如果需要（不是取消，也没有被拦截）的话，那么在触摸down事件的时候更新触摸目标列表
            //split代表，当前的ViewGroup是不是支持分割MotionEvent到不同的View当中
            // Update list of touch targets for pointer down, if needed.
            final boolean split = (mGroupFlags & FLAG_SPLIT_MOTION_EVENTS) != 0;
            //新的触摸对象
            TouchTarget newTouchTarget = null;
            //标记触摸事件有没有传递给新的触摸对象，并且消费了，即调用子View的dispatchTouchEvent并返回true
            boolean alreadyDispatchedToNewTouchTarget = false;
            //如果不是取消事件并且也没有拦截
            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;
              
                //如果是个全新的Down事件
                //或者是有新的触摸点
                //或者是光标来回移动事件
                if (actionMasked == MotionEvent.ACTION_DOWN
                        || (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN)
                        || actionMasked == MotionEvent.ACTION_HOVER_MOVE) {
                    //这个事件的索引，也就是第几个事件，如果是down事件就是0
                    final int actionIndex = ev.getActionIndex(); // always 0 for down
                    //获取分配的ID的bit数量
                    final int idBitsToAssign = split ? 1 << ev.getPointerId(actionIndex)
                            : TouchTarget.ALL_POINTER_IDS;
                    //清理之前触摸这个指针标识,以防他们的目标变得不同步。
                    // Clean up earlier touch targets for this pointer id in case they
                    // have become out of sync.
                    removePointersFromTouchTargets(idBitsToAssign);

                    final int childrenCount = mChildrenCount;
                    //如果新的触摸对象为null（这个不是铁定的吗）并且当前ViewGroup有子元素
                    if (newTouchTarget == null && childrenCount != 0) {
                        final float x = ev.getX(actionIndex);
                        final float y = ev.getY(actionIndex);
                        //遍历子View，找到一个可以接收事件的子View （就是定位到你当前手指触摸的坐标，找到当前该坐标内的子View） 
                        // 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;
                        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.
                            if (childWithAccessibilityFocus != null) {
                                if (childWithAccessibilityFocus != child) {
                                    continue;
                                }
                                childWithAccessibilityFocus = null;
                                i = childrenCount - 1;
                            }

                            //如果child不可以接收这个触摸的事件，或者触摸事件发生的位置不在这个View的范围内
                            if (!canViewReceivePointerEvents(child)
                                    || !isTransformedTouchPointInView(x, y, child, null)) {
                                ev.setTargetAccessibilityFocus(false);
                                continue;
                            }

                            //获取新的触摸对象，如果当前的子View在之前的触摸目标的列表当中就返回touchTarget
                            //子View不在之前的触摸目标列表那么就返回null
                            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.
                                //如果新的触摸目标对象不为空，那么就把这个触摸的ID赋予它，这样子，
                                //这个触摸的目标对象的id就含有了好几个pointer的ID了
                                //已经获得触摸对象，直接跳出循环
                                newTouchTarget.pointerIdBits |= idBitsToAssign;
                                break;
                            }
                            //表示之前没有触摸对象缓存，newTouchTarget=null
                            //如果子View不在之前的触摸目标列表中，先重置childView的标志，去除掉CACEL的标志
                            resetCancelNextUpFlag(child);
                            //调用子View的dispatchTouchEvent,并且把pointer的id 赋予进去
                            //如果说，子View接收并且处理了这个事件，那么就更新上一次触摸事件的信息，
                            //并且为创建一个新的触摸目标对象，并且绑定这个子View和Pointer的ID
                            if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {
                                //子View在范围内接收并消费了这个事件
                                // 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();
                                //将新的触摸目标添加至单链表缓存
                                newTouchTarget = addTouchTarget(child, idBitsToAssign);
                               //此时事件已经成功分发至子View，更改状态，防止后续再次分发
                                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();
                    }

                    //如果newTouchTarget为null，就代表，这个事件没有找到子View去处理它，
                    //那么，如果之前已经有了触摸对象（比如，我点了一张图，另一个手指在外面图的外面点下去）
                    //那么就把这个之前那个触摸目标定为第一个触摸对象，并且把这个触摸（pointer）分配给最近添加的触摸目标
                    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.
            //如果没有触摸目标
            if (mFirstTouchTarget == null) {
                // No touch targets so treat this as an ordinary view.
                //那么就表示我们要自己在这个ViewGroup处理这个触摸事件了
                handled = dispatchTransformedTouchEvent(ev, canceled, null,
                        TouchTarget.ALL_POINTER_IDS);
            } else {
                //遍历TouchTargt树.分发事件，如果我们已经分发给了新的TouchTarget那么我们就不再分发给newTouchTarget
                // 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;
                    //根据上面的标记，防止再次分发
                    if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) {
                        handled = true;
                    } else {
                        //是否让child取消处理事件，如果为true，就会分发给child一个ACTION_CANCEL事件
                        final boolean cancelChild = resetCancelNextUpFlag(target.child)
                                || intercepted;
                        //分发事件
                        if (dispatchTransformedTouchEvent(ev, cancelChild,
                                target.child, target.pointerIdBits)) {
                            handled = true;
                        }
                        //cancelChild也就是说，派发给了当前child一个ACTION_CANCEL事件，
                        //那么就移除这个child
                        if (cancelChild) {
                            //没有父节点，也就是当前是第一个TouchTarget
                            //那么就把头去掉
                            if (predecessor == null) {
                                mFirstTouchTarget = next;
                            } else {
                                //把下一个赋予父节点的上一个，这样当前节点就被丢弃了
                                predecessor.next = next;
                            }
                            target.recycle();
                            target = next;
                            continue;
                        }
                    }
                    predecessor = target;
                    target = next;
                }
            }

             //遇到了取消事件、或者是单点触摸下情况下手指离开，我们就要更新触摸的状态
            // 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) {
                //如果是多点触摸下的手指抬起事件，就要根据idBit从TouchTarget中移除掉对应的Pointer(触摸点)
                final int actionIndex = ev.getActionIndex();
                final int idBitsToRemove = 1 << ev.getPointerId(actionIndex);
                removePointersFromTouchTargets(idBitsToRemove);
            }
        }

        if (!handled && mInputEventConsistencyVerifier != null) {
            mInputEventConsistencyVerifier.onUnhandledEvent(ev, 1);
        }
        return handled;
    }

方法中涉及到涉及到的重要的传递事件方法：

//该方法涉及到是否像子View传递还是调用super传递给自身来处理
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;
    }

ViewGroup中的onInterceptTouchEvent

public boolean onInterceptTouchEvent(MotionEvent ev) {
        if (ev.isFromSource(InputDevice.SOURCE_MOUSE)
                && ev.getAction() == MotionEvent.ACTION_DOWN
                && ev.isButtonPressed(MotionEvent.BUTTON_PRIMARY)
                && isOnScrollbarThumb(ev.getX(), ev.getY())) {
            return true;
        }
        return false;
    }
自定义View时，重写此方法，根据自己的条件来决定是否拦截

View中的dispatchTouchEvent

上面说到，在ViewGroup的dispatchTouchEvent中，若是事件没被拦截，最终会调用View的dispatchTouchEvent，而且若返回true，则表明子View接收了事件并消费了，那么现在来看看View到底是如何消费的？

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);
        }

        //标记是否消费
        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;
            //可以看出，如果设置了ouTouch监听，并且返回true，则代表消费了，此时将不会传递到下面的onTouchEvent
            if (li != null && li.mOnTouchListener != null
                    && (mViewFlags & ENABLED_MASK) == ENABLED
                    && li.mOnTouchListener.onTouch(this, event)) {
                result = true;
            }
        
            //如果上述不成立，就交由onTouchEvent来消费。
            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;
    }

由上面我们可以看出，如果用户设置了onTouch的监听，并且返回true，那么事件就有onTouch消费，即不会有后续。如果没有，那么将交由onTouchEvent方法来消费。
来看下onTouchEvent：

//前后省略了很多，其实主要就是根据事件的类型，来进行各个事件的消费，然后返回true。这边主要来看下onTouchEvent里面的performClick方法
...
public boolean onTouchEvent(MotionEvent event) {
    switch (action) {
                case MotionEvent.ACTION_UP:
                      if (mPerformClick == null) {
                                    mPerformClick = new PerformClick();
                                }
                                if (!post(mPerformClick)) {
                                    performClick();
                                }
                break;
     }
}
...

具体来看下performClick方法：

public boolean performClick() {
        final boolean result;
        final ListenerInfo li = mListenerInfo;
        if (li != null && li.mOnClickListener != null) {
            playSoundEffect(SoundEffectConstants.CLICK);
            li.mOnClickListener.onClick(this);
            result = true;
        } else {
            result = false;
        }

        sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_CLICKED);

        notifyEnterOrExitForAutoFillIfNeeded(true);

        return result;
    }

很熟悉，这就是我们的点击事件，看来它的回调是发生在onTouchEvent里面的UP事件里。
所以简单的得出了onTouch—>onTouchEvent—>onClick三个执行顺序。

总结

最后用一段伪代码来总结下：

TouchTarget mFirstTouchTarget=null;//子控件是否消费

public boolean dispatchTouchEvent(MotionEvent event) {
         boolean isIntercept =false;//是否拦截
         if(onInterceptTouchEvent(event)){//是否拦截
            isIntercept=true；
        }
        if(!isIntercept){//如果没有拦截

            //执行子控件的dispatchTouchEvent
            boolean consume= child.dispatchTouchEvent(event);
            if(!consume){//子控件没有消费事件
                mFirstTouchTarget = null;
            }else{
                //子控件消费了事件，给mFirstTouchTarget赋值
                mFirstTouchTarget = addTouchTarget(child, idBitsToAssign);
            }
        }else{//如果拦截,子控件没有消费事件

            mFirstTouchTarget = null;
        }
        if(mFirstTouchTarget==null){
            //子控件没有消费事件
            return onTouchEvent(event);
        }else{//子控件消费了事件
            return true;
        }

    }