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~
