EventBus源码解析二

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EventBus的post过程

首先看post(Object event)函数实现

    //EventBus.java    /** Posts the given event to the event bus. */    public void post(Object event) {        //取得当前线程的PostingThreadState对象        PostingThreadState postingState = currentPostingThreadState.get();        //取得当前线程的事件序列        List<Object> eventQueue = postingState.eventQueue;        eventQueue.add(event);        //开始分发事件        if (!postingState.isPosting) {            postingState.isMainThread = Looper.getMainLooper() == Looper.myLooper();            postingState.isPosting = true;            if (postingState.canceled) {                throw new EventBusException("Internal error. Abort state was not reset");            }            try {                while (!eventQueue.isEmpty()) {                    //循环发送消息队列中的消息                    postSingleEvent(eventQueue.remove(0), postingState);                }            } finally {                postingState.isPosting = false;                postingState.isMainThread = false;            }        }    }

从上面的代码注释中我们可以发现分发事件的核心代码在 postSingleEvent(Object event, PostingThreadState postingState) 中，下面看一下具体实现

    private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {        Class<?> eventClass = event.getClass();        boolean subscriptionFound = false;        if (eventInheritance) {        //如果允许事件继承，就找出对此事件订阅的父类            List<Class<?>> eventTypes = lookupAllEventTypes(eventClass);            int countTypes = eventTypes.size();            for (int h = 0; h < countTypes; h++) {                Class<?> clazz = eventTypes.get(h);                subscriptionFound |= postSingleEventForEventType(event, postingState, clazz);            }        } else {            subscriptionFound = postSingleEventForEventType(event, postingState, eventClass);        }        if (!subscriptionFound) {            if (logNoSubscriberMessages) {                Log.d(TAG, "No subscribers registered for event " + eventClass);            }            if (sendNoSubscriberEvent && eventClass != NoSubscriberEvent.class &&                    eventClass != SubscriberExceptionEvent.class) {                post(new NoSubscriberEvent(this, event));            }        }    }

这里涉及一个概念：事件继承；那么什么是所谓的事件继承呢？通俗点讲就是事件B继承自事件A，在发送事件B的时候也会发送事件A,默认是事件继承的，如果不需要请自定义EventBus，不要使用默认的
注意这里的sendNoSubscriberEvent变量，如果不需要在没有订阅者的时候发送没有订阅者消息，也是需要自定义EventBus的
打印没有订阅者也是如此

下面我们分析一下 postSingleEventForEventType(Object event, PostingThreadState postingState, Class eventClass)，看一下函数实现

    private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass) {        CopyOnWriteArrayList<Subscription> subscriptions;        synchronized (this) {            //根据事件类型取得订阅者集合            subscriptions = subscriptionsByEventType.get(eventClass);        }        if (subscriptions != null && !subscriptions.isEmpty()) {            for (Subscription subscription : subscriptions) {                postingState.event = event;                postingState.subscription = subscription;                boolean aborted = false;                try {                    //发送事件到订阅者                    postToSubscription(subscription, event, postingState.isMainThread);                    aborted = postingState.canceled;                } finally {                    postingState.event = null;                    postingState.subscription = null;                    postingState.canceled = false;                }                if (aborted) {                    break;                }            }            return true;        }        return false;    }

代码不复杂，可以看出来发送事件的代码是第14行的postToSubscription函数，下面看一下这个函数的实现

    private void postToSubscription(Subscription subscription, Object event, boolean isMainThread) {        switch (subscription.subscriberMethod.threadMode) {            case POSTING:                invokeSubscriber(subscription, event);                break;            case MAIN:                if (isMainThread) {                    invokeSubscriber(subscription, event);                } else {                    mainThreadPoster.enqueue(subscription, event);                }                break;            case BACKGROUND:                if (isMainThread) {                    backgroundPoster.enqueue(subscription, event);                } else {                    invokeSubscriber(subscription, event);                }                break;            case ASYNC:                asyncPoster.enqueue(subscription, event);                break;            default:                throw new IllegalStateException("Unknown thread mode: " + subscription.subscriberMethod.threadMode);        }    }

上面的代码就是具体的事件分发过程了，可以看到事件会根据threadMode的类型决定使用哪种方式分发事件，如果忘了threadMode，请看以前的文章EventBus简单使用
1. 先说POSTING模式，在这个模式下，事件处理函数是和事件发送线程中执行的，所以就直接调用invokeSubscriber函数处理事件
2. 在MAIN模式下，如果发送事件函数是在主线程中，那么就直接调用invokeSubscriber函数处理事件，如果不是主线程，需要切换线程到主线程，切换线程用的是Handler
3. 在BACKGROUND模式下，如果线程是主线程，就需要切换线程到background线程中，如果不是主线程，就直接调用invokeSubscriber处理事件
4. 在ASYNC模式下，切换线程到其他非主线程中处理事件

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