EventBus 开源库学习（二）

整体流程阅读

EventBus在使用的时候基本分为以下几步：
1、注册订阅者

EventBus.getDefault().register(this);

2、订阅者解注册，否者会导致内存泄漏

EventBus.getDefault().unregister(this);

3、在订阅者中编写注解为Subscribe的事件处理函数

@Subscribe(threadMode = ThreadMode.MAIN, sticky = true, priority = 1)
public void onMsgEventReceived(MsgEvent event) {Toast.makeText(this, event.getMsg(), Toast.LENGTH_LONG).show();
}

4、事件发送

EventBus.getDefault().post("msg1 - coming！！！");

我们先按使用的流程大体看下源码逻辑，源码版本3.3.1：

注册源码逻辑

    public static EventBus getDefault() {EventBus instance = defaultInstance;if (instance == null) {synchronized (EventBus.class) {instance = EventBus.defaultInstance;if (instance == null) {instance = EventBus.defaultInstance = new EventBus();}}}return instance;}

EventBus 使用了双重校验锁的单例设计模式，保证用到的对象是唯一的，首次使用对象为空的时候通过下面构造创建一个。

    public EventBus() {this(DEFAULT_BUILDER);}

DEFAULT_BUILDER是一个final常量，在加载的时候就进行初始化，赋一个EventBusBuilder对象如下面代码所示。

private static final EventBusBuilder DEFAULT_BUILDER = new EventBusBuilder();

EventBusBuilder是EventBus的建造类，里面参数在加载的时候进行了初始化。

public class EventBusBuilder {private final static ExecutorService DEFAULT_EXECUTOR_SERVICE = Executors.newCachedThreadPool();boolean logSubscriberExceptions = true;boolean logNoSubscriberMessages = true;boolean sendSubscriberExceptionEvent = true;boolean sendNoSubscriberEvent = true;boolean throwSubscriberException;boolean eventInheritance = true;boolean ignoreGeneratedIndex;boolean strictMethodVerification;ExecutorService executorService = DEFAULT_EXECUTOR_SERVICE;List<Class<?>> skipMethodVerificationForClasses;List<SubscriberInfoIndex> subscriberInfoIndexes;Logger logger;MainThreadSupport mainThreadSupport;...
}

如果有需要的话，我们也可以通过配置EventBusBuilder来更改EventBus的属性，在EventBus中有一个静态方法直接返回一直新的EventBusBuilder对象，设置完参数后调用build()来以新的配置来新建一个EventBus对象。

#EventBuspublic static EventBusBuilder builder() {return new EventBusBuilder();}#EventBusBuilder/** Builds an EventBus based on the current configuration. */public EventBus build() {return new EventBus(this);}

然后通过下面的调用来设置：

EventBus.builder().eventInheritance(false).logSubscriberExceptions(false).build().register(this);

拿到EventBus对象以后，我们可以调用其register方法进行订阅者注册了。

    public void register(Object subscriber) {Class<?> subscriberClass = subscriber.getClass();List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);synchronized (this) {for (SubscriberMethod subscriberMethod : subscriberMethods) {subscribe(subscriber, subscriberMethod);}}}

首先获取订阅者类subscriber，然后通过findSubscriberMethods方法获取该类中以@Subscribe注解的函数，由于一个类中可能监听多个事件，因此获取的方法可能是多个，所有的方法赋值到一个List列表中，然后遍历这个列表进行注册。

    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);....}

上面就是注册最主要的代码，步骤解析如下：

• 根据注解方法获取监听事件的类型eventType，并对订阅者subscriber和订阅函数subscriberMethod建立一个订阅关系对象newSubscription；

• 根据eventType在subscriptionsByEventType（HashMap）中获取所有该事件类型的订阅关系列表subscriptions；

• 如果订阅关系列表subscriptions为空就新建一个，然后以key为eventType，value为newSubscription添加进去；

• 如果订阅关系列表subscriptions不为空，判断是否存在newSubscription，如果存在，说明之前已经注册过，抛出异常；

• 如果订阅关系列表subscriptions不为空，列表页没有订阅关系newSubscription，我们遍历添加进去，这里通过订阅函数的priority 来决定存放在列表中的位置，从这里也能看出priority 越大，存放位置越靠前，和上一篇中分析的：值越大，优先级越高，越优先接收到事件。我们可以猜出是通过遍历这个表来进行事件发送的，在表里的位置越靠前，越先收到事件。

• 然后通过订阅者subscriber在另一个HashMap - typesBySubscriber中获取该订阅者订阅的所有事件，因为一个订阅者可以订阅多个不同的事件，因此获取的是个List列表subscribedEvents；

• 首先判断subscribedEvents列表是不是空的，如果是空说明以前没有订阅过任何事件，新建一个List，然后以key为subscriber，value为subscribedEvents添加到typesBySubscriber；

• 然后在新建的subscribedEvents中添加我们订阅的事件eventType。

这里出现了两个HashMap：subscriptionsByEventType与typesBySubscriber，通过上面的解析可以知道：
subscriptionsByEventType：一个事件可能有多个订阅者，key是事件，value是所有订阅该事件的所有的订阅者；
typesBySubscriber：一个订阅者可能订阅多个事件，key是订阅者，value是订阅者订阅的所有事件；
看命名就知道By后面的是key值，前面的是value值，一个好的命名就是这样吧。

解注册源码逻辑

在使用EventBus时，注册完后我习惯接着去写解注册的代码，怕后面会忘，因此按照写代码习惯在讲下解注册的源码实现。

    /** Unregisters the given subscriber from all event classes. */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());}}

如果是我们自己写这部分代码，根据前面注册的过程，我们肯定是要把添加到两个HashMap中的值移除掉。好了，看上面源码，首先在typesBySubscriber中获取该订阅者订阅的事件列表subscribedTypes；

如果为空说明该订阅者没有订阅任何事件，无任何操作；如果不为空，遍历所有事件调用unsubscribeByEventType方法进行解注册，然后在typesBySubscriber中移除这个订阅者subscriber。

    /** Only updates subscriptionsByEventType, not typesBySubscriber! Caller must update typesBySubscriber. */private void unsubscribeByEventType(Object subscriber, Class<?> eventType) {List<Subscription> subscriptions = subscriptionsByEventType.get(eventType);if (subscriptions != null) {int size = subscriptions.size();for (int i = 0; i < size; i++) {Subscription subscription = subscriptions.get(i);if (subscription.subscriber == subscriber) {subscription.active = false;subscriptions.remove(i);i--;size--;}}}}

在上一个方法中，已经把typesBySubscriber这个订阅者移除了，那么unsubscribeByEventType函数就是遍历事件所有的订阅者，然后把解注册的订阅者在subscriptionsByEventType中给移除掉。上面的逻辑就是干这个事情。不过一边遍历一边移除是有风险的，这个大家要注意，index需要也跟着进行减少。

总结下：注册和解注册就是往两个HashMap添加和移除数据的过程。

事件发送post源码逻辑

    /** 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);····}

上面是部分post代码，第一行先解释这个变量currentPostingThreadState及内部类PostingThreadState。

    private final ThreadLocal<PostingThreadState> currentPostingThreadState = new ThreadLocal<PostingThreadState>() {@Overrideprotected PostingThreadState initialValue() {return new PostingThreadState();}};/** For ThreadLocal, much faster to set (and get multiple values). */final static class PostingThreadState {final List<Object> eventQueue = new ArrayList<>();boolean isPosting;boolean isMainThread;Subscription subscription;Object event;boolean canceled;}

currentPostingThreadState是一个ThreadLocal，相当于是线程的私有财产，里面维护的变量只属于当前线程，线程间不会共享。维护的变量是一个自定义类PostingThreadState，用来保存发送线程的发送状态信息：当前线程是否为主线程，是否在发送事件，发送的事件列表、接收事件的订阅者等。

    /** 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 = 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;}}}

现在继续看post逻辑。首先，获取到当前线程的发送状态postingState，然后拿到事件列表，并把需要post的事件加入到列表中。判断是否启动了事件发送流程，如果已经启动了，不在做处理，加入列表中的事件会轮到处理。如果没有启动就启动处理流程，并将isPosting赋值为true。循环从事件列表中获取事件，通过postSingleEvent进行处理。

    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) {logger.log(Level.FINE, "No subscribers registered for event " + eventClass);}if (sendNoSubscriberEvent && eventClass != NoSubscriberEvent.class &&eventClass != SubscriberExceptionEvent.class) {post(new NoSubscriberEvent(this, event));}}}

在postSingleEvent中，有一个变量的判断eventInheritance，这个变量表示是否查找发送事件的父类或接口的订阅者，默认是true。例如，发送事件MsgEvent这个对象，MsgEvent继承至Event这个抽象类，发送后，不光所有订阅MsgEvent的订阅者可以收到事件，所有订阅Event的这个事件的订阅者也会收到事件。前面有分析这个字段的值是可以重新配置的，代码如下。

EventBus.builder().eventInheritance(false)   //发送的时候不考虑事件父类.logSubscriberExceptions(false).build().register(this);

eventInheritance为true，通过lookupAllEventTypes，向上找到所有父类事件类，然后遍历找到所有事件的订阅者，并发送事件，如果为false，直接将当前事件发送给订阅者。

postSingleEvent函数主要是找到需要post的所有相关事件，然后进一步调用postSingleEventForEventType发送给订阅者，因此函数后面的逻辑是判断postSingleEventForEventType的返回值subscriptionFound，即判断当前事件有没有订阅者进行处理，如果没有处理的，会发送一个NoSubscriberEvent。例如：如果我在上一节的例子中post一个新的对象student，但是没有订阅者和接收函数，如果监听了NoSubscriberEvent，会收到一个NoSubscriberEvent的事件，告知调用者你post的对象没有订阅者。

@Subscribe()
public void onMsgEventReceived(NoSubscriberEvent event) {Log.i(TAG, "NoSubscriberEvent : " + event);
}Student student = new Student(1,"jane");
EventBus.getDefault().post(student);

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;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;}

从前面分析可知，最后都走到了postSingleEventForEventType这个函数里面，首先在subscriptionsByEventType中找到所有订阅该事件的订阅者subscriptions，subscriptions为空或者个数是0则返回false。有订阅者的话，遍历订阅者，然后通过postToSubscription进行发送，并返回true。

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);}}

这段代码比较简单就是根据订阅者中注解@Subscribe中threadMode值来分别进行处理，上一节也讲过，POSTING表示在哪个线程发送就在哪个线程接收处理，因此直接调用invokeSubscriber通过反射来调用订阅者中的接收事件的方法。

    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);}}

如果threadMode是MAIN，而当前线程是子线程，通过 mainThreadPoster.enqueue(subscription, event)将事件加入主线程队列。

mainThreadSupport = builder.getMainThreadSupport();
mainThreadPoster = mainThreadSupport != null ? mainThreadSupport.createPoster(this) : null;public interface MainThreadSupport {boolean isMainThread();Poster createPoster(EventBus eventBus);
}public class DefaultAndroidMainThreadSupport implements MainThreadSupport {public boolean isMainThread() {return Looper.getMainLooper() == Looper.myLooper();}@Overridepublic Poster createPoster(EventBus eventBus) {return new HandlerPoster(eventBus, Looper.getMainLooper(), 10);}
}

mainThreadPoster是由mainThreadSupport 创建的，mainThreadSupport是MainThreadSupport实例，而MainThreadSupport是一个接口，实现类为DefaultAndroidMainThreadSupport，因此，最终是调用到的DefaultAndroidMainThreadSupport中的createPoster，新建了一个HandlerPoster。

public class HandlerPoster extends Handler implements Poster {private final PendingPostQueue queue;private final int maxMillisInsideHandleMessage;private final EventBus eventBus;private boolean handlerActive;public HandlerPoster(EventBus eventBus, Looper looper, int maxMillisInsideHandleMessage) {super(looper);this.eventBus = eventBus;this.maxMillisInsideHandleMessage = maxMillisInsideHandleMessage;queue = new PendingPostQueue();}public void enqueue(Subscription subscription, Object event) {PendingPost pendingPost = PendingPost.obtainPendingPost(subscription, event);synchronized (this) {queue.enqueue(pendingPost);if (!handlerActive) {handlerActive = true;if (!sendMessage(obtainMessage())) {throw new EventBusException("Could not send handler message");}}}}@Overridepublic void handleMessage(Message msg) {...}
}

从上面的源码可以看到，HandlerPoster就是一个Handler，当执行mainThreadPoster.enqueue(subscription, event)时，会将订阅者和事件封装成一个PendingPost，然后加入到PendingPostQueue这个队列中，如果handlerActive为true表示当前Handler正常处理事件，将入队列的事件等着被处理即可。如果为false则启动处理，调用sendMessage发送消息。

public class HandlerPoster extends Handler implements Poster {private final PendingPostQueue queue;private final int maxMillisInsideHandleMessage;private final EventBus eventBus;private boolean handlerActive;public HandlerPoster(EventBus eventBus, Looper looper, int maxMillisInsideHandleMessage) {super(looper);...}public void enqueue(Subscription subscription, Object event) {...}@Overridepublic void handleMessage(Message msg) {boolean rescheduled = false;try {long started = SystemClock.uptimeMillis();while (true) {PendingPost pendingPost = queue.poll();if (pendingPost == null) {synchronized (this) {// Check again, this time in synchronizedpendingPost = queue.poll();if (pendingPost == null) {handlerActive = false;return;}}}eventBus.invokeSubscriber(pendingPost);long timeInMethod = SystemClock.uptimeMillis() - started;if (timeInMethod >= maxMillisInsideHandleMessage) {if (!sendMessage(obtainMessage())) {throw new EventBusException("Could not send handler message");}rescheduled = true;return;}}} finally {handlerActive = rescheduled;}}
}

在handleMessage()方法将中将PendingPost对象循环出队列，交给invokeSubscriber()方法进一步处理。这样就把线程通过Handler切回了主线程。

backgroundPoster.enqueue()和asyncPoster.enqueue也类似，内部都是先将事件入队列，然后再出队列，但是会通过线程池去进一步处理事件。

粘性事件发送postSticky源码逻辑

    public void postSticky(Object event) {synchronized (stickyEvents) {stickyEvents.put(event.getClass(), event);}// Should be posted after it is putted, in case the subscriber wants to remove immediatelypost(event);}

从上面代码可以看到，先把事件放在了stickyEvents列中中，然后调用了post，也就是上面我们解析过的流程。让我们再来回顾下注册的代码：

// Must be called in synchronized blockprivate void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {Class<?> eventType = subscriberMethod.eventType;Subscription newSubscription = new Subscription(subscriber, subscriberMethod);CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);...subscribedEvents.add(eventType);if (subscriberMethod.sticky) {if (eventInheritance) {// Existing sticky events of all subclasses of eventType have to be considered.// Note: Iterating over all events may be inefficient with lots of sticky events,// thus data structure should be changed to allow a more efficient lookup// (e.g. an additional map storing sub classes of super classes: Class -> List<Class>).Set<Map.Entry<Class<?>, Object>> entries = stickyEvents.entrySet();for (Map.Entry<Class<?>, Object> entry : entries) {Class<?> candidateEventType = entry.getKey();if (eventType.isAssignableFrom(candidateEventType)) {Object stickyEvent = entry.getValue();checkPostStickyEventToSubscription(newSubscription, stickyEvent);}}} else {Object stickyEvent = stickyEvents.get(eventType);checkPostStickyEventToSubscription(newSubscription, stickyEvent);}}}

注册源码分析的时候，就分析到subscriberMethod.sticky这个句上面，现在让我们看看下面的逻辑，同样先判断eventInheritance的值，然后将之前放在stickyEvents中的事件拿出来，执行checkPostStickyEventToSubscription。

private void checkPostStickyEventToSubscription(Subscription newSubscription, Object stickyEvent) {if (stickyEvent != null) {// If the subscriber is trying to abort the event, it will fail (event is not tracked in posting state)// --> Strange corner case, which we don't take care of here.postToSubscription(newSubscription, stickyEvent, isMainThread());}}

checkPostStickyEventToSubscription中的代码比较水，又调用了postToSubscription，这个方法上面贴出来了，这里方面看再贴一遍，熟悉的味道：

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);}}

所以，在注册的时候，如果你订阅者的接收方法加了sticky注解，那么在注册的时候就会看下订阅的事件之前有没有通过postSticky发送过，如果有就会立马收到这个事件。

以上是我们平时使用过程的源码解析，码字不易，喜欢就点赞收藏啊。

参考文章：
EventBus 原理解析