EventBus源码研读(下) - 张涛

摘要

本文原创，转载请注明地址： http://kymjs.com/code/2015/12/16/01

EventBus 是一款针对Android优化的发布/订阅事件总线。主要功能是替代Intent, Handler, BroadCast 在 Fragment，Activity，Service，线程之间传递消息.优点是开销小，使用方便,可以很大程度上降低它们之间的耦合，使得我们的代码更加简洁，耦合性更低，提升我们的代码质量。 类似的库还有 Otto ,今天就带大家一起研读 EventBus 的源码.

• post()方法调用流程
  • postSingleEventForEventType()方法
• unregister()
• EventBus工作原理
  • 订阅的逻辑
  • 发送事件的逻辑
  • 响应事件的逻辑
  • 取消注册的逻辑
  • 工作原理图示

在写这篇文章之前，我已经将本文相关的中文注释代码上传到了GitHub： https://github.com/kymjs/EventBus

post()方法调用流程

我们继续来看 EventBus 类，的另一个入口方法 post()

//已省略部分代码 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");         }         while (!eventQueue.isEmpty()) {             postSingleEvent(eventQueue.remove(0), postingState);         }         postingState.isPosting = false;         postingState.isMainThread = false;     } }

post() 方法首先从 currentPostingThreadState 对象中取了一个 PostingThreadState ，我们来看看这个 currentPostingThreadState 对象的创建代码。

private final ThreadLocal<PostingThreadState> currentPostingThreadState = new ThreadLocal<PostingThreadState>() {     @Override     protected PostingThreadState initialValue() {         return new PostingThreadState();     } };

final static class PostingThreadState {     //通过post方法参数传入的事件集合     final List<Object> eventQueue = new ArrayList<Object>();      boolean isPosting; //是否正在执行postSingleEvent()方法     boolean isMainThread;     Subscription subscription;     Object event;     boolean canceled;     }

回到 post() 方法，我们看到其核心代码是这句：

while (!eventQueue.isEmpty()) {     postSingleEvent(eventQueue.remove(0), postingState); }

private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {     Class<?> eventClass = event.getClass();     boolean subscriptionFound = false;     if (eventInheritance) {         //获取到eventClass所有父类的集合         List<Class<?>> eventTypes = lookupAllEventTypes(eventClass);         int countTypes = eventTypes.size();         for (int h = 0; h < countTypes; h++) {             Class<?> clazz = eventTypes.get(h);             //左或右只要有一个为真则为真,并赋值给左             subscriptionFound |= postSingleEventForEventType(event, postingState, clazz);         }     } else {         subscriptionFound = postSingleEventForEventType(event, postingState, eventClass);     }     if (!subscriptionFound) {         if (logNoSubscriberMessages) {             Log.d(TAG, "No subscribers registered for event " + eventClass);         }          //参考sendNoSubscriberEvent注释         if (sendNoSubscriberEvent && eventClass != NoSubscriberEvent.class &&                 eventClass != SubscriberExceptionEvent.class) {             post(new NoSubscriberEvent(this, event));         }     } }

postSingleEventForEventType()方法

然后我们继续往下，看发送方法 postSingleEventForEventType()

private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass) {     CopyOnWriteArrayList<Subscription> subscriptions;     synchronized (this) {         //所有订阅了eventClass的事件集合         subscriptions = subscriptionsByEventType.get(eventClass);     }     if (subscriptions != null && !subscriptions.isEmpty()) {         //回调subscription的响应方法         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; }

它首先通过这一句

subscriptions = subscriptionsByEventType.get(eventClass);

unregister()

我们继续来看 EventBus 类，的最后一个入口方法 unregister()

public synchronized void unregister(Object subscriber) {     List<Class<?>> subscribedTypes = typesBySubscriber.get(subscriber);     if (subscribedTypes != null) {         for (Class<?> eventType : subscribedTypes) {             //取消注册subscriber对eventType事件的响应             unsubscribeByEventType(subscriber, eventType);         }         //当subscriber对所有事件都不响应以后,移除订阅者         typesBySubscriber.remove(subscriber);     } }

EventBus工作原理

最后我们再来从设计者的角度看一看 EventBus 的工作原理。

订阅的逻辑

发送事件的逻辑

响应事件的逻辑

取消注册的逻辑

工作原理图示