EventBus--基于订阅模式的事件总线库

文章目录

1. 1. 订阅模式
2. 2. EventBus实现原理
   1. 2.1. subscribe注解
   2. 2.2. EventBus主类
   3. 2.3. 注解处理器
3. 3. 一点疑问
4. 4. 参考资料

本篇讲大名鼎鼎的开源库EventBus.

它的名字就解释了它的作用: 事件总线. 当事件发生时, 通过它发放到总线上的各个接收者.
事件总线(Event Bus)其实只是一种概念或者说一种模式, 而GreenRobot的EventBus项目实现了这种模式. 它不是唯一的实现者.
它的开发组织是greenrobot. 这个组织还有个项目挺有名的, 叫greendao.
本文基于EventBus 3.0代码.

订阅模式

在讲解EventBus之前, 我们重点要了解一下订阅者模式, 又叫做观察者模式. 从名字就可以看出来这种模式很适用于”当一件事情发生时, 所有关注这件事情的人都得到通知”这样的情景. 其实我觉得它就是批量地注册回调.
观察者模式在Java中的实现就是Observable接口, 而在EventBus中的实现就是Subscriber.

EventBus实现原理

尽管原理非常简单, 接口方面也可以想象, 但具体怎么实现还是有讲究的.
我们从接口入手, 看EventBus如何实现事件总线的作用.

1. 定义事件:

public static class MessageEvent { /* Additional fields if needed */ }
事件就是一个普通的Java类. 可以是静态的或非静态的.  

2. 准备订阅者. 用注解的方式指定一个订阅者, 其唯一接受的参数是我们所关心的事件. 可以指定其[线程模式](http://greenrobot.org/eventbus/documentation/delivery-threads-threadmode/):

@Subscribe(threadMode = ThreadMode.MAIN)  
public void onMessageEvent(MessageEvent event) {/* Do something */};

然后可以注册和解除订阅者了. 例如在Android上, 注册和解除的方式如下:

 @Override
 public void onStart() {
     super.onStart();
     EventBus.getDefault().register(this); // 将this这个订阅者注册到EventBus上
 }

 @Override
 public void onStop() {
     super.onStop();
     EventBus.getDefault().unregister(this); // 从EventBus上解除this这个订阅者
 }

3. 发布事件:

 EventBus.getDefault().post(new MessageEvent()); // 往EventBus上发布一个新的MessageEvent事件

通过以上接口, 我们可以有一些大胆的猜测:

1. 事件应当是以Map<EventType, Subscribers> 这样的方式存储的, 同时, 事件和订阅者应当是多对多的关系, 所以应当也有以Map<Subscriber, EventTypes>这样的数据结构存在;
2. 注册和解除订阅者时, 维护以上两个数据结构;
3. 发布事件时, 查询以上两个数据结构;
4. 可以添加其他的属性支持: 线程模式, 订阅者优先级, 事件模式.
   有了以上认知, 其实我们自己也可以实现一套简单的EventBus框架.

下面就来看看EventBus是怎么实现各个特性的.

subscribe注解

EventBus的核心就是Subscribe注解. 下面我们来看看它的实现(如果对注解不熟悉请留言告知, 人多的话我考虑出一篇讲Java注解的教程).

@Retention(RetentionPolicy.RUNTIME) // 作用阶段是Runtime
@Target({ElementType.METHOD}) // 作用对象是Method
public @interface Subscribe {
    ThreadMode threadMode() default ThreadMode.POSTING; // 线程模式. 描述了在什么线程上执行订阅者的订阅方法.

    /**
     * If true, delivers the most recent sticky event (posted with
     * {@link EventBus#postSticky(Object)}) to this subscriber (if event available).
     */
    boolean sticky() default false; // 是否sticky

    /** Subscriber priority to influence the order of event delivery.
     * Within the same delivery thread ({@link ThreadMode}), higher priority subscribers will receive events before
     * others with a lower priority. The default priority is 0. Note: the priority does *NOT* affect the order of
     * delivery among subscribers with different {@link ThreadMode}s! */
    int priority() default 0; // 优先级. 仅影响同一线程中subscriber接收到事件的顺序.
}


/**
 * Each subscriber method has a thread mode, which determines in which thread the method is to be called by EventBus.
 * EventBus takes care of threading independently from the posting thread.
 * 
 * @see EventBus#register(Object)
 * @author Markus
 */
public enum ThreadMode {
    /**
     * Subscriber will be called directly in the same thread, which is posting the event. This is the default. Event delivery
     * implies the least overhead because it avoids thread switching completely. Thus this is the recommended mode for
     * simple tasks that are known to complete in a very short time without requiring the main thread. Event handlers
     * using this mode must return quickly to avoid blocking the posting thread, which may be the main thread.
     */
    POSTING, // 直接抛在当前线程

    /**
     * On Android, subscriber will be called in Android's main thread (UI thread). If the posting thread is
     * the main thread, subscriber methods will be called directly, blocking the posting thread. Otherwise the event
     * is queued for delivery (non-blocking). Subscribers using this mode must return quickly to avoid blocking the main thread.
     * If not on Android, behaves the same as {@link #POSTING}.
     */
    MAIN, // Android中直接抛给主线程, 非Android中行为与POSTING一样

    /**
     * On Android, subscriber will be called in Android's main thread (UI thread). Different from {@link #MAIN},
     * the event will always be queued for delivery. This ensures that the post call is non-blocking.
     */
    MAIN_ORDERED, // Android上直接抛给主线程, 总是放到队列里进行, 避免调用post方法的线程阻塞.

    /**
     * On Android, subscriber will be called in a background thread. If posting thread is not the main thread, subscriber methods
     * will be called directly in the posting thread. If the posting thread is the main thread, EventBus uses a single
     * background thread, that will deliver all its events sequentially. Subscribers using this mode should try to
     * return quickly to avoid blocking the background thread. If not on Android, always uses a background thread.
     */
    BACKGROUND, // Android上抛给后台线程或者非UI线程的当前线程执行, 非Android总是在后台线程执行.

    /**
     * Subscriber will be called in a separate thread. This is always independent from the posting thread and the
     * main thread. Posting events never wait for subscriber methods using this mode. Subscriber methods should
     * use this mode if their execution might take some time, e.g. for network access. Avoid triggering a large number
     * of long running asynchronous subscriber methods at the same time to limit the number of concurrent threads. EventBus
     * uses a thread pool to efficiently reuse threads from completed asynchronous subscriber notifications.
     */
    ASYNC // 总是在后台单独的线程执行.
}

上面的代码和注释已经很清楚了. 我们可以用注解, 让指定的方法, 在接收到指定的事件时, 在指定的线程上执行. 这个注解相当于标记了方法信息, 但具体怎么处理这个注解, 还得看其他入口.

EventBus主类

EventBus类就是其中一个入口.

具体代码见: https://github.com/greenrobot/EventBus/blob/master/EventBus/src/org/greenrobot/eventbus/EventBus.java

/**
 * 以下注释解释了EventBus的设计理念
 * EventBus is a central publish/subscribe event system for Android. Events are posted ({@link #post(Object)}) to the
 * bus, which delivers it to subscribers that have a matching handler method for the event type. To receive events,
 * subscribers must register themselves to the bus using {@link #register(Object)}. Once registered, subscribers
 * receive events until {@link #unregister(Object)} is called. Event handling methods must be annotated by
 * {@link Subscribe}, must be public, return nothing (void), and have exactly one parameter
 * (the event).
 *
 * @author Markus Junginger, greenrobot
 */
public class EventBus {

    /** Log tag, apps may override it. */
    public static String TAG = "EventBus";

    static volatile EventBus defaultInstance;

    private static final EventBusBuilder DEFAULT_BUILDER = new EventBusBuilder(); // 建造者模式
    private static final Map<Class<?>, List<Class<?>>> eventTypesCache = new HashMap<>(); // 以作为Event的类的class为Key, 缓存class和interface, 包括其祖先

    private final Map<Class<?>, CopyOnWriteArrayList<Subscription>> subscriptionsByEventType; // 这就是我们期待看到的 Map<EventType, Subscribers>
    private final Map<Object, List<Class<?>>> typesBySubscriber; // 这就是我们期待看到的 Map<Subscriber, EventTypes>
    private final Map<Class<?>, Object> stickyEvents;

    private final ThreadLocal<PostingThreadState> currentPostingThreadState = new ThreadLocal<PostingThreadState>() {
        @Override
        protected PostingThreadState initialValue() {
            return new PostingThreadState();
        }
    }; // 线程独立的变量, 用来存储跟线程有关的状态

    // @Nullable
    private final MainThreadSupport mainThreadSupport;
    // @Nullable
    private final Poster mainThreadPoster;
    private final BackgroundPoster backgroundPoster;
    private final AsyncPoster asyncPoster;
    private final SubscriberMethodFinder subscriberMethodFinder;
    private final ExecutorService executorService;

    private final boolean throwSubscriberException;
    private final boolean logSubscriberExceptions;
    private final boolean logNoSubscriberMessages;
    private final boolean sendSubscriberExceptionEvent;
    private final boolean sendNoSubscriberEvent;
    private final boolean eventInheritance;

    private final int indexCount;
    private final Logger logger;

    /** Convenience singleton for apps using a process-wide EventBus instance. */
    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;
    }

    public static EventBusBuilder builder() {
        return new EventBusBuilder();
    }

    /** For unit test primarily. */
    public static void clearCaches() {
        SubscriberMethodFinder.clearCaches();
        eventTypesCache.clear();
    }

    /**
     * Creates a new EventBus instance; each instance is a separate scope in which events are delivered. To use a
     * central bus, consider {@link #getDefault()}.
     */
    public EventBus() {
        this(DEFAULT_BUILDER);
    }

    EventBus(EventBusBuilder builder) {
        logger = builder.getLogger();
        subscriptionsByEventType = new HashMap<>();
        typesBySubscriber = new HashMap<>();
        stickyEvents = new ConcurrentHashMap<>();
        mainThreadSupport = builder.getMainThreadSupport();
        mainThreadPoster = mainThreadSupport != null ? mainThreadSupport.createPoster(this) : null;
        backgroundPoster = new BackgroundPoster(this);
        asyncPoster = new AsyncPoster(this);
        indexCount = builder.subscriberInfoIndexes != null ? builder.subscriberInfoIndexes.size() : 0;
        subscriberMethodFinder = new SubscriberMethodFinder(builder.subscriberInfoIndexes,
                builder.strictMethodVerification, builder.ignoreGeneratedIndex);
        logSubscriberExceptions = builder.logSubscriberExceptions;
        logNoSubscriberMessages = builder.logNoSubscriberMessages;
        sendSubscriberExceptionEvent = builder.sendSubscriberExceptionEvent;
        sendNoSubscriberEvent = builder.sendNoSubscriberEvent;
        throwSubscriberException = builder.throwSubscriberException;
        eventInheritance = builder.eventInheritance;
        executorService = builder.executorService;
    }

    /**
     * Registers the given subscriber to receive events. Subscribers must call {@link #unregister(Object)} once they
     * are no longer interested in receiving events.
     * <p/>
     * Subscribers have event handling methods that must be annotated by {@link Subscribe}.
     * The {@link Subscribe} annotation also allows configuration like {@link
     * ThreadMode} and priority.
     */
    public void register(Object subscriber) {
        Class<?> subscriberClass = subscriber.getClass();
        List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass); // 在subscriber对象中寻找带Subscribe注解的方法
        synchronized (this) {
            for (SubscriberMethod subscriberMethod : subscriberMethods) {
                subscribe(subscriber, subscriberMethod); // 对Subscribe方法调用subscribe完成注册
            }
        }
    }

    // Must be called in synchronized block
    private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) { // 为什么不把这个方法直接声明为synchronized?
        Class<?> eventType = subscriberMethod.eventType; // 获取到方法注册的EventType
        Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
        CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType); // 先看看这个EventType是不是已经有subscriber了, 如果没有就新建一个List
        if (subscriptions == null) {
            subscriptions = new CopyOnWriteArrayList<>();
            subscriptionsByEventType.put(eventType, subscriptions);
        } else {
            if (subscriptions.contains(newSubscription)) { // 如果List中已经有对应的<EventType, Subscriber>关系了, 则抛出异常, 表示重复注册了
                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) { // 根据优先级插入subscription到列表中
                subscriptions.add(i, newSubscription);
                break;
            }
        }

        List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber); // 把当前处理的EventType记录到subscriber所处理的所有EventType的列表中
        if (subscribedEvents == null) {
            subscribedEvents = new ArrayList<>();
            typesBySubscriber.put(subscriber, subscribedEvents);
        }
        subscribedEvents.add(eventType);

        if (subscriberMethod.sticky) { // 处理sticky属性
            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);
            }
        }
    }

    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());
        }
    }

    /**
     * Checks if the current thread is running in the main thread.
     * If there is no main thread support (e.g. non-Android), "true" is always returned. In that case MAIN thread
     * subscribers are always called in posting thread, and BACKGROUND subscribers are always called from a background
     * poster.
     */
    private boolean isMainThread() {
        return mainThreadSupport != null ? mainThreadSupport.isMainThread() : true;
    }

    public synchronized boolean isRegistered(Object subscriber) {
        return typesBySubscriber.containsKey(subscriber);
    }

    /** Only updates subscriptionsByEventType, not typesBySubscriber! Caller must update typesBySubscriber. */
    private void unsubscribeByEventType(Object subscriber, Class<?> eventType) { // 在EventType的所有subscriber中移除指定的subscriber. 这个只维护<EventType, Subscriber>
        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--;
                }
            }
        }
    }

    /** Unregisters the given subscriber from all event classes. */
    public synchronized void unregister(Object subscriber) {
        List<Class<?>> subscribedTypes = typesBySubscriber.get(subscriber); // 查询指定的subscriber
        if (subscribedTypes != null) {
            for (Class<?> eventType : subscribedTypes) {
                unsubscribeByEventType(subscriber, eventType); // 对每个被订阅的EventType, 维护Map<EventType, Subscribers>
            }
            typesBySubscriber.remove(subscriber); // 从Map<Subscriber, EventTypes>中移除指定的subscriber
        } else {
            logger.log(Level.WARNING, "Subscriber to unregister was not registered before: " + subscriber.getClass());
        }
    }

    /** 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); // 将事件压入队列

        if (!postingState.isPosting) {
            postingState.isMainThread = isMainThread();
            postingState.isPosting = true; // 如果当前不在isPosting状态, 则转换为isPosting状态并开始post循环, 处理事件队列
            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;
            }
        }
    }

    /**
     * Called from a subscriber's event handling method, further event delivery will be canceled. Subsequent
     * subscribers
     * won't receive the event. Events are usually canceled by higher priority subscribers (see
     * {@link Subscribe#priority()}). Canceling is restricted to event handling methods running in posting thread
     * {@link ThreadMode#POSTING}.
     */
    public void cancelEventDelivery(Object event) {
        PostingThreadState postingState = currentPostingThreadState.get();
        if (!postingState.isPosting) {
            throw new EventBusException(
                    "This method may only be called from inside event handling methods on the posting thread");
        } else if (event == null) {
            throw new EventBusException("Event may not be null");
        } else if (postingState.event != event) {
            throw new EventBusException("Only the currently handled event may be aborted");
        } else if (postingState.subscription.subscriberMethod.threadMode != ThreadMode.POSTING) {
            throw new EventBusException(" event handlers may only abort the incoming event");
        }

        postingState.canceled = true;
    }

    /**
     * Posts the given event to the event bus and holds on to the event (because it is sticky). The most recent sticky
     * event of an event's type is kept in memory for future access by subscribers using {@link Subscribe#sticky()}.
     */
    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 immediately
        post(event);
    }

    /**
     * Gets the most recent sticky event for the given type.
     *
     * @see #postSticky(Object)
     */
    public <T> T getStickyEvent(Class<T> eventType) {
        synchronized (stickyEvents) {
            return eventType.cast(stickyEvents.get(eventType));
        }
    }

    /**
     * Remove and gets the recent sticky event for the given event type.
     *
     * @see #postSticky(Object)
     */
    public <T> T removeStickyEvent(Class<T> eventType) {
        synchronized (stickyEvents) {
            return eventType.cast(stickyEvents.remove(eventType));
        }
    }

    /**
     * Removes the sticky event if it equals to the given event.
     *
     * @return true if the events matched and the sticky event was removed.
     */
    public boolean removeStickyEvent(Object event) {
        synchronized (stickyEvents) {
            Class<?> eventType = event.getClass();
            Object existingEvent = stickyEvents.get(eventType);
            if (event.equals(existingEvent)) {
                stickyEvents.remove(eventType);
                return true;
            } else {
                return false;
            }
        }
    }

    /**
     * Removes all sticky events.
     */
    public void removeAllStickyEvents() {
        synchronized (stickyEvents) {
            stickyEvents.clear();
        }
    }

    public boolean hasSubscriberForEvent(Class<?> eventClass) { // 查询指定的事件是否有订阅者
        List<Class<?>> eventTypes = lookupAllEventTypes(eventClass);
        if (eventTypes != null) {
            int countTypes = eventTypes.size();
            for (int h = 0; h < countTypes; h++) {
                Class<?> clazz = eventTypes.get(h);
                CopyOnWriteArrayList<Subscription> subscriptions;
                synchronized (this) {
                    subscriptions = subscriptionsByEventType.get(clazz);
                }
                if (subscriptions != null && !subscriptions.isEmpty()) {
                    return true;
                }
            }
        }
        return false;
    }

    // 发布单个事件
    private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
        Class<?> eventClass = event.getClass();
        boolean subscriptionFound = false;
        if (eventInheritance) {
            List<Class<?>> eventTypes = lookupAllEventTypes(eventClass); // 查找指定事件相关联的所有事件
            int countTypes = eventTypes.size();
            for (int h = 0; h < countTypes; h++) {
                Class<?> clazz = eventTypes.get(h);
                subscriptionFound |= postSingleEventForEventType(event, postingState, clazz); // 对每个事件class调用
            }
        } 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)); // 发送"无人订阅该事件"事件
            }
        }
    }

    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); // 将事件传递给订阅者
                    aborted = postingState.canceled;
                } finally {
                    postingState.event = null;
                    postingState.subscription = null;
                    postingState.canceled = false;
                }
                if (aborted) { // 如果过程中有任一事件调用了cancel, 则后续订阅者不再接收事件
                    break;
                }
            }
            return true;
        }
        return false;
    }

    // 将事件根据属性分发到不同线程上
    private void postToSubscription(Subscription subscription, Object event, boolean isMainThread) {
        switch (subscription.subscriberMethod.threadMode) {
            case POSTING: // 直接抛到当前线程执行
                invokeSubscriber(subscription, event);
                break;
            case MAIN:
                if (isMainThread) { // 如果当前就是主线程则直接调用
                    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);
        }
    }

    /** Looks up all Class objects including super classes and interfaces. Should also work for interfaces. */
    private static List<Class<?>> lookupAllEventTypes(Class<?> eventClass) {
        synchronized (eventTypesCache) {
            List<Class<?>> eventTypes = eventTypesCache.get(eventClass);
            if (eventTypes == null) {
                eventTypes = new ArrayList<>();
                Class<?> clazz = eventClass;
                while (clazz != null) {
                    eventTypes.add(clazz);
                    addInterfaces(eventTypes, clazz.getInterfaces());
                    clazz = clazz.getSuperclass();
                }
                eventTypesCache.put(eventClass, eventTypes);
            }
            return eventTypes;
        }
    }

    /** Recurses through super interfaces. */
    static void addInterfaces(List<Class<?>> eventTypes, Class<?>[] interfaces) {
        for (Class<?> interfaceClass : interfaces) {
            if (!eventTypes.contains(interfaceClass)) {
                eventTypes.add(interfaceClass);
                addInterfaces(eventTypes, interfaceClass.getInterfaces());
            }
        }
    }

    /**
     * Invokes the subscriber if the subscriptions is still active. Skipping subscriptions prevents race conditions
     * between {@link #unregister(Object)} and event delivery. Otherwise the event might be delivered after the
     * subscriber unregistered. This is particularly important for main thread delivery and registrations bound to the
     * live cycle of an Activity or Fragment.
     */
    void invokeSubscriber(PendingPost pendingPost) {
        Object event = pendingPost.event;
        Subscription subscription = pendingPost.subscription;
        PendingPost.releasePendingPost(pendingPost);
        if (subscription.active) {
            invokeSubscriber(subscription, event);
        }
    }

    void invokeSubscriber(Subscription subscription, Object event) {
        try {
            subscription.subscriberMethod.method.invoke(subscription.subscriber, event); // 通过反射调用订阅者的方法
        } catch (InvocationTargetException e) {
            handleSubscriberException(subscription, event, e.getCause());
        } catch (IllegalAccessException e) {
            throw new IllegalStateException("Unexpected exception", e);
        }
    }

    private void handleSubscriberException(Subscription subscription, Object event, Throwable cause) {
        if (event instanceof SubscriberExceptionEvent) {
            if (logSubscriberExceptions) {
                // Don't send another SubscriberExceptionEvent to avoid infinite event recursion, just log
                logger.log(Level.SEVERE, "SubscriberExceptionEvent subscriber " + subscription.subscriber.getClass()
                        + " threw an exception", cause);
                SubscriberExceptionEvent exEvent = (SubscriberExceptionEvent) event;
                logger.log(Level.SEVERE, "Initial event " + exEvent.causingEvent + " caused exception in "
                        + exEvent.causingSubscriber, exEvent.throwable);
            }
        } else {
            if (throwSubscriberException) {
                throw new EventBusException("Invoking subscriber failed", cause);
            }
            if (logSubscriberExceptions) {
                logger.log(Level.SEVERE, "Could not dispatch event: " + event.getClass() + " to subscribing class "
                        + subscription.subscriber.getClass(), cause);
            }
            if (sendSubscriberExceptionEvent) {
                SubscriberExceptionEvent exEvent = new SubscriberExceptionEvent(this, cause, event,
                        subscription.subscriber);
                post(exEvent);
            }
        }
    }

    /** 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;
    }

    ExecutorService getExecutorService() {
        return executorService;
    }

    /**
     * For internal use only.
     */
    public Logger getLogger() {
        return logger;
    }

    // Just an idea: we could provide a callback to post() to be notified, an alternative would be events, of course...
    /* public */interface PostCallback {
        void onPostCompleted(List<SubscriberExceptionEvent> exceptionEvents);
    }

    @Override
    public String toString() {
        return "EventBus[indexCount=" + indexCount + ", eventInheritance=" + eventInheritance + "]";
    }

在上述代码加了注释之后, 读者应该能较好地理解EventBus主类的逻辑了. 其他的例如Poster, EventBusBuilder, SubscriberMethod, SubscriberMethodFinder等都是一些具体的实现方法了, 这里不再赘述, 有兴趣的朋友可以深入看一下.
以上代码需要注意的是, 事件和订阅者都是注册到具体的EventBus对象上的, 也就是说注册到不同EventBus对象上的事件和订阅者互相不可见. 作者在注释中说, 如果要让EventBus发挥集中式事件总线的作用, 应该用EventBus.getDefault()的方式获取EventBus对象.

注解处理器

EventBus项目在3.0版本加入了EventBusAnnotationProcessor, 可以在编译期处理注解, 达到性能优化的目的.

一点疑问

根据EventBus官方库的文档, 它比otto库快很多, 但是我不太明白它为什么比otto快.
可以想到的点是, 它将SubscriberMethod进行了缓存, 从而不必每次都反射调用. 它使用了CopyOnWriteArrayList, 从而不必进行同步操作.

参考资料

• EventBus框架库代码走读 工匠若水
• EventBus源码
• EventBus Wiki