本文主要是介绍EventBus 流程解析，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！
先介绍控件使用方法，然后再从基本的使用方法断点调试，整体了解一下流程。
EventBus 基本使用
在 module 的 build.gradle添加
implementation 'org.greenrobot:eventbus:3.1.1'
在接收消息的地方注册 eventBus
EventBus.getDefault().register(this);
然后创建一个事件类
public class FirstEvent {public String message;public FirstEvent(String message){this.message = message;}
}
在注册 EventBus 的类中创建接收事件方法
// 这里的threadMode可以设置不同的值，在不同的线程中处理事件接收
@Subscribe(threadMode = ThreadMode.MAIN)
public void setText(FirstEvent event){textView.setText(event.message);
}
然后在任意一个地方发送一个消息
EventBus.getDefault().post(new FirstEvent("来自第某个地方的消息"));
EventBus一个基本的流程就这样完成了,接下来看每一步的源码
注册流程
EventBus.getDefault()
/** Convenience singleton for apps using a process-wide EventBus instance. */
public static EventBus getDefault() {if (defaultInstance == null) {synchronized (EventBus.class) {if (defaultInstance == null) {defaultInstance = new EventBus();}}}return defaultInstance;
}
这是一个单例方法，用的是 DoubleCheck 的方式，创建了一个 EventBus 的实例。
register()
public void register(Object subscriber) {Class<?> subscriberClass = subscriber.getClass();List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);synchronized (this) {for (SubscriberMethod subscriberMethod : subscriberMethods) {subscribe(subscriber, subscriberMethod);}}
}
简单来看，register 方法是将注册的类与该类中事件处理的方法进行一个绑定。
SubscriberMethod 类是一个实体类，将订阅相关的数据进行一个封装：
然后我们看一下 subscriberMethodFinder.findSubscriberMethods() 方法
List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);if (subscriberMethods != null) {return subscriberMethods;}if (ignoreGeneratedIndex) {subscriberMethods = findUsingReflection(subscriberClass);} else {subscriberMethods = findUsingInfo(subscriberClass);}if (subscriberMethods.isEmpty()) {throw new EventBusException("Subscriber " + subscriberClass+ " and its super classes have no public methods with the @Subscribe annotation");} else {METHOD_CACHE.put(subscriberClass, subscriberMethods);return subscriberMethods;}
}
findSubscriberMethods() 方法会先从 METHOD_CACHE 中查找，没有找到则会根据 ignoreGeneratedIndex 标志选择是采用反射的方式寻找还是用 findUsingInfo 方法，这里这个标志和 findUsingInfo 方法目前都不知道是什么。找到后会将结果加入到缓存中。
接下来就是 subscribe() 方法
private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {Class<?> eventType = subscriberMethod.eventType;Subscription newSubscription = new Subscription(subscriber, subscriberMethod);CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);if (subscriptions == null) {subscriptions = new CopyOnWriteArrayList<>();subscriptionsByEventType.put(eventType, subscriptions);} else {if (subscriptions.contains(newSubscription)) {throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event "+ eventType);}}int size = subscriptions.size();for (int i = 0; i <= size; i++) {if (i == size || subscriberMethod.priority > subscriptions.get(i).subscriberMethod.priority) {subscriptions.add(i, newSubscription);break;}}List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);if (subscribedEvents == null) {subscribedEvents = new ArrayList<>();typesBySubscriber.put(subscriber, subscribedEvents);}subscribedEvents.add(eventType);// 后面是粘性事件相关处理，先不管...
}
subscribe 方法主要做两件事，一个是 subscriptionsByEventType.put(eventType, subscriptions) ,subscriptionsByEventType 是以事件的类为 key，订阅者的回调方法为 value 的映射关系表。
另一个是 typesBySubscriber.put(subscriber, subscribedEvents) , typesBySubscriber 是以订阅者为 key, 订阅者订阅的事件类为 value 的映射关系表。
到这里一个 EventBus 的注册流程就结束了，从这个流程来看，主要就是建立事件类和订阅者之间的映射关系。
注销流程
unregister()
public synchronized void unregister(Object subscriber) {List<Class<?>> subscribedTypes = typesBySubscriber.get(subscriber);if (subscribedTypes != null) {for (Class<?> eventType : subscribedTypes) {unsubscribeByEventType(subscriber, eventType);}typesBySubscriber.remove(subscriber);} else {logger.log(Level.WARNING, "Subscriber to unregister was not registered before: " + subscriber.getClass());}
}
注销流程就很简单了，先取出该订阅者对应的订阅事件类，然后遍历订阅的事件类，再取出该类所有的订阅者，把我们注销的订阅者移除，最后再把注销的这个订阅者从 typesBySubscriber 中移除。
Post 流程
先看下post的流程图
然后解析每个步骤，看下都做了哪些操作
post ()
public void post(Object event) {PostingThreadState postingState = currentPostingThreadState.get();List<Object> eventQueue = postingState.eventQueue;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()) {postSingleEvent(eventQueue.remove(0), postingState);}} finally {postingState.isPosting = false;postingState.isMainThread = false;}}
}
PostingThreadState 的实例存储在 ThreadLocal 中，是一个内部静态类，将事件队列，post 状态，事件和订阅者等信息封装在一起。post 方法取出当前线程的PostingThreadState，将事件添加到事件队列中，设置一些状态信息，然后循环把事件队列(用一个 ArrayList 实现)的事件取出来分发。
postSingleEvent（）
private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {Class<?> eventClass = event.getClass();boolean subscriptionFound = false;// 标志位，决定是否要post事件所有父类和实现接口if (eventInheritance) {/* lookupAllEventTypes 方法是找到该事件的所有父类和所有实现的接口*/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) {...// 事件没有订阅者就打打日志啥的}
}
postSingleEvent 方法就两个逻辑，一个是决定是否要post事件所有父类和实现接口，另一个就是事件没有订阅者，会判断一些条件，然后重新 post 一个 NoSubscriberEvent 事件。
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;
}
没啥多说的，根据传过来的事件，拿到对应的订阅关系，PostingThreadState 中记录信息，然后交给 postToSubscription 方法。
postToSubscription ()
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 subscriberinvokeSubscriber(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);}
}
最后根据订阅者的回调方法设置的线程，决定再什么线程上反射调用该回调方法，反射调用通过 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);}
}
这里我们重点关注 EventBus 是如何让事件回调在不同线程中执行的：
- POSTTING: 不用多说，再哪个线程抛出来的事件，就再哪个线程直接反射调用回调方法,不涉及线程切换
- MAIN: 在主线程中反射调用回调事件，如果当前线程不是主线程，则通过 mainThreadPoster 插入一个事件， mainThreadPoster 是通过主线程 Looper 创建的一个Handler，调用 enqueue() 方法，会通过自身发一个消息，然后在自己的 handleMessage() 方法中反射调用事件的回调方法，这样就完成了在主线程回调。
- MAIN_ORDERED: 也是在主线程中反射调用回调方法，但是有顺序，不会发生第二个事件早于第一个事件执行完？不太确定。
- BACKGROUND: 如果当前线程是主线程，则将事件添加到 backgroundPoster 中，backgroundPoster的 enqueue() 方法会通过线程池执行一个任务，反射调用回调方法。
- ASYNC: 无论在哪个线程，都交给线程池去处理这个事件。
到这，Post 流程分析基本完成。
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