[EventBus源码解析] EventBus.post 方法详述

前情概要

　　上一篇blog我们了解了EventBus中register/unregister的过程，对EventBus如何实现观察者模式有了基本的认识。今天我们来看一下它是如何分发一个特定事件的，即post(Object event)方法。

本篇概述

　　EventBus中事件的分发与响应，post 方法。

post 方法

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;
        }
    }
}

　　postingState（分发状态）具有ThreadLocal属性，包含一个eventQueue，用来保存当前线程分发中的event，在while循环中，逐一调用 postSingleEvent(eventQueue.remove(0), postingState)，清空发送队列。

postSingleEvent 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));
        }
    }
}

　　这个方法用来分发单个事件，如果EventBus支持继承，则分发继承的所有基类事件；若没有找到订阅者，则根据logNoSubscribeMessages、sendNoSubscriberEvent这两个标志位处理相应的逻辑（打日志、发送NoSubscriberEvent事件）。

　　再进一步，我们看看 postSingleEventForEventType 方法。

postSingleEventForEventType 方法

　　在仔细阅读源代码之前，我们应该大致就能够猜想得到，这一步，理应是遍历该事件类型的订阅者列表，找到对应的订阅者后把事件发送出去；结合之前了解过的EventBus响应事件的四种模式（PostThread, MainThread, BackgroundThread, Async），接下来关注的重点是分发事件时如何处理这一段逻辑。

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;
}

　　通过eventType找到对应的subscriptions后，逐一调用postToSubscription方法，其中对四种事件相应方式进行了处理。最终是通过反射进行了对应方法的调用。

private void postToSubscription(Subscription subscription, Object event, boolean isMainThread) {
    switch (subscription.subscriberMethod.threadMode) {
        case PostThread:
            invokeSubscriber(subscription, event);
            break;
        case MainThread:
            if (isMainThread) {
                invokeSubscriber(subscription, event);
            } else {
                mainThreadPoster.enqueue(subscription, event);
            }
            break;
        case BackgroundThread:
            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);
    }
}

总结

　　分发事件的过程并不复杂，真正核心的一句话就可以概括：根据eventType找到对应的订阅者，通过反射进行具体方法调用。

下期预告

　　四种线程模式实现方式解析。