EventBus源码阅读（一）

　　EventBus虽然有诸多缺点，但是作为一个经典的事件总线框架，其代码还是有一些学习价值，从他的代码中可以获得一些启发，运用于开发。

　　EventBus有两个入口，一个是订阅，一个是发送事件。今天从发送事件开始阅读。

        EventBus.getDefault().post("11");

　　进入post方法

    /** Posts the given event to the event bus. */
    public void post(Object event) {
        PostingThreadState postingState = currentPostingThreadState.get();
        List<Object> eventQueue = postingState.eventQueue;
        eventQueue.add(event);

        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()) {
                    postSingleEvent(eventQueue.remove(0), postingState);
                }
            } finally {
                postingState.isPosting = false;
                postingState.isMainThread = false;
            }
        }
    }

　　post方法第一件事，就是获取当前posting的线程状态。并用一个PostingThreadState来保存。

    /** For ThreadLocal, much faster to set (and get multiple values). */
    final static class PostingThreadState {
        final List<Object> eventQueue = new ArrayList<Object>();
        boolean isPosting;
        boolean isMainThread;
        Subscription subscription;
        Object event;
        boolean canceled;
    }

　　PostingThreadState这个类，是保存了一些状态。比如是否正在发送，是否在主线程工作,以及当前的订阅者和事件列表。

　　post第二行就获取了事件列表——eventQueue。将此次post的事件，加入eventQueue。将下来的代码比较简单，只要postingState不是正在发送，就陆续把事件列表中的事件postSingleEvent出去，直到eventQueue为空为止。

　　下面来看看postSingleEvent的方法:

    private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
        Class<?> eventClass = event.getClass();
        boolean subscriptionFound = false;
        if (eventInheritance) {
            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));
            }
        }
    }

　　eventInheritance在源码中说明如下：　

    /**
     * By default, EventBus considers the event class hierarchy (subscribers to super classes will be notified).
     * Switching this feature off will improve posting of events. For simple event classes extending Object directly,
     * we measured a speed up of 20% for event posting. For more complex event hierarchies, the speed up should be
     * >20%.
     * <p/>
     * However, keep in mind that event posting usually consumes just a small proportion of CPU time inside an app,
     * unless it is posting at high rates, e.g. hundreds/thousands of events per second.
     */

　　它决定了订阅者的父类是否接到通知，在默认情况下，是置为true的，我们可以手动设置，暂时不深究。

　　所以我们先看

            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);
            }

　　通过lookupAllEventTypes获取eventClass的所有父类。然后通过轮循，对所有的类及父类进行发送事件。只要有一个类，有被订阅，则subscriptionFound为true。

　　下面可以看到：

        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));
            }
        }

　　如果没有找到订阅者，则直接打出Log。默认情况下，sendNoSubscriberEvent为true。因此，如果没有订阅者，将会发送一个NoSubscriberEvent事件。由此，我们可出postSingleEvent中核心方法是postSingleEventForEventType，因此，我们再去看看这个方法。

　　postSingleEventForEventType代码如下：

    private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass) {
        CopyOnWriteArrayList<Subscription> subscriptions;
        synchronized (this) {
            subscriptions = subscriptionsByEventType.get(eventClass);
        }
        if (subscriptions != null && !subscriptions.isEmpty()) {
            for (Subscription subscription : subscriptions) {
                postingState.event = event;
                postingState.subscription = subscription;
                boolean aborted = false;
                try {
                    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;
    }

　　这个方法，我们可以看到，subscriptionsByEventType中，我们通过eventClass取出相应的订阅源，然后对订阅源调用 postToSubscription(subscription, event, postingState.isMainThread);方法。

　　最后，postToSubscription方法，就会根据不同的线程模式，通过订阅源执行方法：

    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);
        }
    }

　　除了，POSTING情况，其他情况，都各自有一个Poster来执行任务，它们分别是mainThreadPoster、backgroundPoster和asyncPoster。他们的具体实习，留作下一篇进行分析。

　　至此，EventBus的事件发送，基本流程走通，只剩下两个没有深究的问题，一个是各式各样的Poster到底如何工作，另一个是eventInheritance的具体作用。这两个问题，下篇再阅读。

Done~