Android必知必会EventBus源码分析之发布事件

知道了EventBus的注册流程后，我们来了解一下EventBus发布事件的流程。如果还没看过注册流程，建议先浏览：
Android必知必会的EventBus之使用篇
Android必知必会的EventBus源码分析之注册

我们知道EventBus的发布事件一行代码解决
EventBus.getDefault().post(new MessageEvent());
MessageEvent是我们在其他地方订阅的事件类型。接着我们看看post方法。

 /** Posts the given event to the event bus. */
    public void post(Object event) {
        //通过ThreadLocal获取当前线程的状态
        PostingThreadState postingState = currentPostingThreadState.get();
        //获取当前线程的事件队列
        List<Object> eventQueue = postingState.eventQueue;
        //将我们发送的类型事件添加到队列中
        eventQueue.add(event);

        if (!postingState.isPosting) {
            postingState.isMainThread = isMainThread();
            postingState.isPosting = true;
            if (postingState.canceled) {
                throw new EventBusException("Internal error. Abort state was not reset");
            }
            try {
                while (!eventQueue.isEmpty()) {
                   //当队列不为空是，不停的发送单一事件，知道队列为空
                    postSingleEvent(eventQueue.remove(0), postingState);
                }
            } finally {
                postingState.isPosting = false;
                postingState.isMainThread = false;
            }
        }
    }

通过上面代码，我们知道EventBus的post方法通过本地线程ThreadLocal去获取事件队列。并将我们发布的事件类型添加到队列中。在队列eventQueue不为空的情况，调用postSingleEvent方法。我们看看postSingleEvent方法。

 private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
        Class<?> eventClass = event.getClass();
        boolean subscriptionFound = false;
        //默认情况为true
        if (eventInheritance) {
            //查看父类或者父接口是否有该事件类型
            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) {
                logger.log(Level.FINE, "No subscribers registered for event " + eventClass);
            }
            if (sendNoSubscriberEvent && eventClass != NoSubscriberEvent.class &&
                    eventClass != SubscriberExceptionEvent.class) {
                post(new NoSubscriberEvent(this, event));
            }
        }
    }

通过上面代码，我们知道最终会调用postSingleEventForEventType。

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;
    }

通过上一篇文章，我们知道，subscriptions存放着事件的订阅类和订阅方法。让我们看看postToSubscription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 MAIN_ORDERED:
                if (mainThreadPoster != null) {
                    mainThreadPoster.enqueue(subscription, event);
                } else {
                    // temporary: technically not correct as poster not decoupled from subscriber
                    invokeSubscriber(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);
        }
    }

通过代码可以发现，EventBus的线程切换在此次。至于不同参考EventBus的使用这里我们先看看invokeSubscriber方法。

  void invokeSubscriber(Subscription subscription, Object event) {
        try {
            subscription.subscriberMethod.method.invoke(subscription.subscriber, event);
        } catch (InvocationTargetException e) {
            handleSubscriberException(subscription, event, e.getCause());
        } catch (IllegalAccessException e) {
            throw new IllegalStateException("Unexpected exception", e);
        }
    }

因此，我们知道，在注册过程中，通过注解和反射机制，将相关的订阅类和方法包装到了Subscription。在此次，切换线程后，再调用Subscription中的订阅方法。到这里，发布事件流程就结束了。
至于黏性事件的发布，原理应该差不多，大家可以自行查阅。