知道了EventBus的注冊(cè)流程后，我們來(lái)了解一下EventBus發(fā)布事件的流程。如果還沒(méi)看過(guò)注冊(cè)流程，建議先瀏覽：
Android必知必會(huì)的EventBus之使用篇
Android必知必會(huì)的EventBus源碼分析之注冊(cè)

我們知道EventBus的發(fā)布事件一行代碼解決
EventBus.getDefault().post(new MessageEvent());
MessageEvent是我們?cè)谄渌胤接嗛喌氖录愋?。接著我們看?code>post方法。

 /** Posts the given event to the event bus. */
    public void post(Object event) {
        //通過(guò)ThreadLocal獲取當(dāng)前線程的狀態(tài)
        PostingThreadState postingState = currentPostingThreadState.get();
        //獲取當(dāng)前線程的事件隊(duì)列
        List<Object> eventQueue = postingState.eventQueue;
        //將我們發(fā)送的類型事件添加到隊(duì)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()) {
                   //當(dāng)隊(duì)列不為空是，不停的發(fā)送單一事件，知道隊(duì)列為空
                    postSingleEvent(eventQueue.remove(0), postingState);
                }
            } finally {
                postingState.isPosting = false;
                postingState.isMainThread = false;
            }
        }
    }

通過(guò)上面代碼，我們知道EventBus的post方法通過(guò)本地線程ThreadLocal去獲取事件隊(duì)列。并將我們發(fā)布的事件類型添加到隊(duì)列中。在隊(duì)列eventQueue不為空的情況，調(diào)用postSingleEvent方法。我們看看postSingleEvent方法。

 private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
        Class<?> eventClass = event.getClass();
        boolean subscriptionFound = false;
        //默認(rèn)情況為true
        if (eventInheritance) {
            //查看父類或者父接口是否有該事件類型
            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));
            }
        }
    }

通過(guò)上面代碼，我們知道最終會(huì)調(diào)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;
    }

通過(guò)上一篇文章，我們知道，subscriptions存放著事件的訂閱類和訂閱方法。讓我們看看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);
        }
    }

通過(guò)代碼可以發(fā)現(xiàn)，EventBus的線程切換在此次。至于不同參考EventBus的使用這里我們先看看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，我們知道，在注冊(cè)過(guò)程中，通過(guò)注解和反射機(jī)制，將相關(guān)的訂閱類和方法包裝到了Subscription。在此次，切換線程后，再調(diào)用Subscription中的訂閱方法。到這里，發(fā)布事件流程就結(jié)束了。
至于黏性事件的發(fā)布，原理應(yīng)該差不多，大家可以自行查閱。