Android事件分发与责任链模式

一、责任链模式

责任链模式是一种行为模式,为请求创建一个接收者的对象链.这样就避免,一个请求链接多个接收者的情况.进行外部解耦.类似于单向链表结构。

优点：

1. 降低耦合度。它将请求的发送者和接收者解耦。

2. 简化了对象。使得对象不需要知道链的结构。

3. 增强给对象指派职责的灵活性。通过改变链内的成员或者调动它们的次 序，允许动态地新增或者删除责任。

4. 增加新的请求处理类很方便。

缺点：

1. 不能保证请求一定被接收。

2. 系统性能将受到一定影响，而且在进行代码调试时不太方便，可能会造成循环调用。

3. 可能不容易观察运行时的特征，有碍于除错。

责任链的模式在事件分发场景方面的原理：

一般我们理解的事件分发的模式如下（传统模式）：

使用责任链模式直接将message丢到链中,让他们自己匹配.

二、android 事件分发传递机制

1. view事件传递分发层级结构

 a). 事件收集之后最先传递给 activity， 然后依次向下传递，大致如下：

activity －> phonewindow －> decorview －> viewgroup －> ... －> view

这样的事件分发机制逻辑非常清晰，可是，你是否注意到一个问题？如果最后分发到view，如果这个view也没有处理事件怎么办，就这样让事件浪费掉？当然不会啦。

 b). 如果没有任何view消费掉事件，那么这个事件会按照反方向回传，最终传回给activity，如果最后 activity 也没有处理，本次事件才会被抛弃:

activity <－ phonewindow <－ decorview <－ viewgroup <－ ... <－ view

可以看到，这是一个非常经典的责任链模式，如果我能处理就拦截下来自己干，如果自己不能处理或者不确定就交给责任链中下一个对象。 这种设计是非常精巧的，上层view既可以直接拦截该事件，自己处理，也可以先询问(分发给)子view，如果子view需要就交给子view处理，如果子view不需要还能继续交给上层view处理。既保证了事件的有序性，又非常的灵活。

view点击事件分发有三个关键流程方法:

1.dispatchtouchevent：事件下发 --- view和viewgroup都有的方法

2.onintercepttouchevent：拦截下发的事件,并交给自己ontouchevent处理处理 ---viewgroup才有的方法

3.ontouchevent：事件上报 --- view和viewgroup都有的方法

以下是不同层级对事件的分发、拦截和消费的功能表：

可以看到 activity 和 view 都是没有事件拦截的：

a). activity 作为原始的事件分发者，如果 activity 拦截了事件会导致整个屏幕都无法响应事件，这肯定不是我们想要的效果。

b). view最为事件传递的最末端，要么消费掉事件，要么不处理进行回传，根本没必要进行事件拦截。

下图是点击view，事件传递但是都没有被处理，生成的一个完整的事件分发流程图：

如果事件被view处理了，那么事件分发流程图应该如下：

如果事件被viewgroup拦截处理了， 那么事件分发流程图应该如下：

从上面的流程，我们可以概括android的事件分发机制为：责任链模式，事件层层传递，直到被消费。

三、q&a

上面我们讲解了一下android的事件分发机制，可能很多人会有疑惑，下面我们针对部分疑惑进行分析和说明：

1. 为什么 view 会有 dispatchtouchevent ?

答：我们知道 view 可以注册很多事件监听器，例如：单击事件(onclick)、长按事件(onlongclick)、触摸事件(ontouch)，并且view自身也有 ontouchevent 方法，那么问题来了，这么多与事件相关的方法应该由谁管理？毋庸置疑就是 dispatchtouchevent，所以 view 也会有事件分发。

view的dispatchtouchevent源码：

/**
     * pass the touch screen motion event down to the target view, or this
     * view if it is the target.
     *
     * @param event the motion event to be dispatched.
     * @return true if the event was handled by the view, false otherwise.
     */
    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;
            if (li != null && li.montouchlistener != null
                    && (mviewflags & enabled_mask) == enabled
                    && li.montouchlistener.ontouch(this, event)) {
                result = true;
            }

            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;
    }

2. view事件分发时各个方法调用顺序是怎样的？

a). 单击事件(onclicklistener) 需要两个两个事件(action_down 和 action_up )才能触发，如果先分配给onclick判断，等它判断完再交由其他相应时间显然是不合理的，会造成 view 无法响应其他事件，应该最后调用。(所以此调用顺序最后)

b). 长按事件(onlongclicklistener) 同理，也是需要长时间等待才能出结果，肯定不能排到前面，但因为不需要action_up，应该排在 onclick 前面。(onlongclicklistener > onclicklistener)

c). 触摸事件(ontouchlistener) 如果用户注册了触摸事件，说明用户要自己处理触摸事件了，这个应该排在最前面。(最前)

d). view自身处理(ontouchevent) 提供了一种默认的处理方式，如果用户已经处理好了，也就不需要了，所以应该排在 onclicklistener 后面。(ontouchlistener > onclicklistener)

所以事件的调度顺序应该是 ontouchlistener > ontouchevent > onlongclicklistener > onclicklistener。

3. viewgroup 的事件分发流程又是如何的呢？

在默认的情况下 viewgroup 事件分发流程是这样的。

a). 判断自身是否需要(询问 onintercepttouchevent 是否拦截)，如果需要，调用自己的 ontouchevent。

b). 自身不需要或者不确定，则询问 childview ，一般来说是调用手指触摸位置的 childview。

c). 如果子 childview 不需要则调用自身的 ontouchevent。

viewgroup的dispatchtouchevent源码：

@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;

            // 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.
            final boolean intercepted;
            if (actionmasked == motionevent.action_down
                    || mfirsttouchtarget != null) {
                final boolean disallowintercept = (mgroupflags & flag_disallow_intercept) != 0;
                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 split = (mgroupflags & flag_split_motion_events) != 0;
            touchtarget newtouchtarget = null;
            boolean alreadydispatchedtonewtouchtarget = false;
            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;

                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) {
                        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;
                        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;
                            }

                            if (!canviewreceivepointerevents(child)
                                    || !istransformedtouchpointinview(x, y, child, null)) {
                                ev.settargetaccessibilityfocus(false);
                                continue;
                            }

                            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);
                            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;
                    }
                }
            }

            // 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;
    }

4. viewgroup将事件分发给childview的机制


viewgroup分发事件时会遍历 childview，如果手指触摸的点在 childview 区域内就分发给这个view。当 childview 重叠时，一般会分配给显示在最上面的 childview。

5. viewgroup 和 childview 同时注册了事件监听器(onclick等)，哪个会执行?

事件优先给 childview，会被 childview消费掉，viewgroup 不会响应。

 

附：参考资料：

1. android事件传递机制分析

2. android 事件分发机制详解

3. 安卓自定义view进阶-事件分发机制原理