前面一篇文章讲解了EventBus的使用，但是作为开发人员，不能只停留在仅仅会用的层面上，我们还需要弄清楚它的内部实现原理。所以本篇博文将分析EventBus的源码，看看究竟它是如何实现“发布/订阅”功能的。

事件注册

根据前一讲EventBus使用详解我们已经知道EventBus使用首先是需要注册的，注册事件的代码如下：

EventBus.getDefault().register(this);

EventBus对外提供了一个register方法来进行事件注册，该方法接收一个Object类型的参数，下面看下register方法的源码:

public void register(Object subscriber) {
    Class<?> subscriberClass = subscriber.getClass();
    // 判断该类是否是匿名内部类
    boolean forceReflection = subscriberClass.isAnonymousClass();
    List<SubscriberMethod> subscriberMethods =
            subscriberMethodFinder.findSubscriberMethods(subscriberClass, forceReflection);
    for (SubscriberMethod subscriberMethod : subscriberMethods) {
        subscribe(subscriber, subscriberMethod);
    }
}

该方法首先获取获取传进来参数的Class对象，然后判断该类是否是匿名内部类。然后根据这两个参数通过subscriberMethodFinder.findSubscriberMethods方法获取所有的事件处理方法。

List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass, boolean forceReflection) {
    String key = subscriberClass.getName();
    List<SubscriberMethod> subscriberMethods;
    synchronized (METHOD_CACHE) {
        subscriberMethods = METHOD_CACHE.get(key);
    }
    if (subscriberMethods != null) {
        //缓存命中，直接返回
        return subscriberMethods;
    }
    if (INDEX != null && !forceReflection) {
        // 如果INDEX不为空，并且subscriberClass为非匿名内部类，
        // 则通过findSubscriberMethodsWithIndex方法查找事件处理函数
        subscriberMethods = findSubscriberMethodsWithIndex(subscriberClass);
        if (subscriberMethods.isEmpty()) {
            //如果结果为空，则使用findSubscriberMethodsWithReflection方法再查找一次
            subscriberMethods = findSubscriberMethodsWithReflection(subscriberClass);
        }
    } else {
        //INDEX为空或者subscriberClass未匿名内部类，使用findSubscriberMethodsWithReflection方法查找
        subscriberMethods = findSubscriberMethodsWithReflection(subscriberClass);
    }
    if (subscriberMethods.isEmpty()) {
        throw new EventBusException("Subscriber " + subscriberClass
                + " and its super classes have no public methods with the @Subscribe annotation");
    } else {
        //存入缓存并返回
        synchronized (METHOD_CACHE) {
            METHOD_CACHE.put(key, subscriberMethods);
        }
        return subscriberMethods;
    }
}

通过名字我们就知道这个方法是获取subscriberClass类中所有的事件处理方法（即使用了@Subscribe的方法）。该方法首先会从缓存METHOD_CACHE中去获取事件处理方法，如果缓存中不存在，则需要通过findSubscriberMethodsWithIndex或者findSubscriberMethodsWithReflection方法获取所有事件处理方法，获取到之后先存入缓存再返回。

这个方法里面有个INDEX对象，我们看看它是个什么鬼：

/** Optional generated index without entries from subscribers super classes */
private static final SubscriberIndex INDEX;

static {
    SubscriberIndex newIndex = null;
    try {
        Class<?> clazz = Class.forName("de.greenrobot.event.GeneratedSubscriberIndex");
        newIndex = (SubscriberIndex) clazz.newInstance();
    } catch (ClassNotFoundException e) {
        Log.d(EventBus.TAG, "No subscriber index available, reverting to dynamic look-up");
        // Fine
    } catch (Exception e) {
        Log.w(EventBus.TAG, "Could not init subscriber index, reverting to dynamic look-up", e);
    }
    INDEX = newIndex;
}

由上面代码可以看出EventBus会试图加载一个de.greenrobot.event.GeneratedSubscriberIndex类并创建对象赋值给INDEX，但是EventBus3.0 beta并没有为我们提供该类（可能后续版本会提供）。所以INDEX为null。

我们再返回findSubscriberMethods方法，我们知道INDEX已经为null了，所以必然会调用findSubscriberMethodsWithReflection方法查找所有事件处理函数：

private List<SubscriberMethod> findSubscriberMethodsWithReflection(Class<?> subscriberClass) {
    List<SubscriberMethod> subscriberMethods = new ArrayList<SubscriberMethod>();
    Class<?> clazz = subscriberClass;
    HashSet<String> eventTypesFound = new HashSet<String>();
    StringBuilder methodKeyBuilder = new StringBuilder();
    while (clazz != null) {
        String name = clazz.getName();
        // 如果查找的类是java、javax或者android包下面的类，则过滤掉
        if (name.startsWith("java.") || name.startsWith("javax.") || name.startsWith("android.")) {
            // Skip system classes, this just degrades performance
            break;
        }

        // Starting with EventBus 2.2 we enforced methods to be public (might change with annotations again)
        // 通过反射查找所有该类中所有方法
        Method[] methods = clazz.getDeclaredMethods();
        for (Method method : methods) {
            int modifiers = method.getModifiers();
            // 事件处理方法必须为public，这里过滤掉所有非public方法
            if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
                Class<?>[] parameterTypes = method.getParameterTypes();
                // 事件处理方法必须只有一个参数
                if (parameterTypes.length == 1) {
                    Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);
                    if (subscribeAnnotation != null) {
                        String methodName = method.getName();
                        Class<?> eventType = parameterTypes[0];
                        methodKeyBuilder.setLength(0);
                        methodKeyBuilder.append(methodName);
                        methodKeyBuilder.append('>').append(eventType.getName());

                        String methodKey = methodKeyBuilder.toString();
                        if (eventTypesFound.add(methodKey)) {
                            // Only add if not already found in a sub class
                            // 只有在子类中没有找到，才会添加到subscriberMethods
                            ThreadMode threadMode = subscribeAnnotation.threadMode();
                            subscriberMethods.add(new SubscriberMethod(method, eventType, threadMode,
                                    subscribeAnnotation.priority(), subscribeAnnotation.sticky()));
                        }
                    }
                } else if (strictMethodVerification) {
                    // 如果某个方法加了@Subscribe注解，并且不是1个参数，则抛出EventBusException异常
                    if (method.isAnnotationPresent(Subscribe.class)) {
                        String methodName = name + "." + method.getName();
                        throw new EventBusException("@Subscribe method " + methodName +
                                "must have exactly 1 parameter but has " + parameterTypes.length);
                    }
                }
            } else if (strictMethodVerification) {
                // 如果某个方法加了@Subscribe注解，并且不是public修饰，则抛出EventBusException异常
                if (method.isAnnotationPresent(Subscribe.class)) {
                    String methodName = name + "." + method.getName();
                    throw new EventBusException(methodName +
                            " is a illegal @Subscribe method: must be public, non-static, and non-abstract");
                }

            }
        }
        // 会继续查找父类的方法
        clazz = clazz.getSuperclass();
    }
    return subscriberMethods;
}

该方法主要作用就是找出subscriberClass类以及subscriberClass的父类中所有的事件处理方法（添加了@Subscribe注解，访问修饰符为public并且只有一个参数）。值得注意的是：如果子类与父类中同时存在了相同事件处理函数，则父类中的不会被添加到subscriberMethods。

好了，查找事件处理函数的过程已经完了，我们继续回到register方法中：

for (SubscriberMethod subscriberMethod : subscriberMethods) {
    subscribe(subscriber, subscriberMethod);
}

找到事件处理函数后，会遍历找到的所有事件处理函数并调用subscribe方法将所有事件处理函数注册到EventBus中。

private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
    Class<?> eventType = subscriberMethod.eventType;
    // 获取订阅了某种类型数据的 Subscription 。 使用了 CopyOnWriteArrayList ，这个是线程安全的，
    // CopyOnWriteArrayList 会在更新的时候，重新生成一份 copy，其他线程使用的是 
    // copy，不存在什么线程安全性的问题。
    CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
    Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
    if (subscriptions == null) {
        subscriptions = new CopyOnWriteArrayList<Subscription>();
        subscriptionsByEventType.put(eventType, subscriptions);
    } else {
        //如果已经被注册过了，则抛出EventBusException异常
        if (subscriptions.contains(newSubscription)) {
            throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event "
                    + eventType);
        }
    }

    // Starting with EventBus 2.2 we enforced methods to be public (might change with annotations again)
    // subscriberMethod.method.setAccessible(true);

    // Got to synchronize to avoid shifted positions when adding/removing concurrently
    // 根据优先级将newSubscription查到合适位置
    synchronized (subscriptions) {
        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;
            }
        }
    }

    //将处理事件类型添加到typesBySubscriber
    List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
    if (subscribedEvents == null) {
        subscribedEvents = new ArrayList<Class<?>>();
        typesBySubscriber.put(subscriber, subscribedEvents);
    }
    subscribedEvents.add(eventType);

    // 如果该事件处理方法为粘性事件，即设置了“sticky = true”，则需要调用checkPostStickyEventToSubscription
    // 判断是否有粘性事件需要处理，如果需要处理则触发一次事件处理函数
    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);
        }
    }
}

如果事件处理函数设置了“sticky = true”，则会调用checkPostStickyEventToSubscription处理粘性事件。

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, Looper.getMainLooper() == Looper.myLooper());
    }
}

如果存在粘性事件，则立即调用postToSubscription触发该事件的事件处理函数。postToSubscription函数后面讲post时会讲到。

至此，整个register过程就介绍完了。
总结一下，整个过程分为3步：

1. 查找注册的类中所有的事件处理函数（添加了@Subscribe注解且访问修饰符为public的方法）
2. 将所有事件处理函数注册到EventBus
3. 如果有事件处理函数设置了“sticky = true”，则立即处理该事件

post事件

register过程讲完后，我们知道了EventBus如何找到我们定义好的事件处理函数。有了这些事件处理函数，当post相应事件的时候，EventBus就会触发订阅该事件的处理函数。具体post过程是怎样的呢？我们看看代码：

public void post(Object event) {
    PostingThreadState postingState = currentPostingThreadState.get();
    List<Object> eventQueue = postingState.eventQueue;
    eventQueue.add(event);

    if (!postingState.isPosting) {
        // 标识post的线程是否是主线程
        postingState.isMainThread = Looper.getMainLooper() == Looper.myLooper();
        postingState.isPosting = true;
        if (postingState.canceled) {
            throw new EventBusException("Internal error. Abort state was not reset");
        }
        try {
            // 循环处理eventQueue中的每一个event对象
            while (!eventQueue.isEmpty()) {
                postSingleEvent(eventQueue.remove(0), postingState);
            }
        } finally {
            // 处理完之后重置postingState的一些标识信息
            postingState.isPosting = false;
            postingState.isMainThread = false;
        }
    }
}

currentPostingThreadState是一个ThreadLocal类型，里面存储了PostingThreadState；

private final ThreadLocal<PostingThreadState> currentPostingThreadState = new ThreadLocal<PostingThreadState>() {
    @Override
    protected 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<Object>();
    boolean isPosting;
    boolean isMainThread;
    Subscription subscription;
    Object event;
    boolean canceled;
}

PostingThreadState包含了一个事件队列eventQueue和一些标志信息。eventQueue存放所有待post的事件对象。

我们再回到post方法，首先会将event对象添加到事件队列eventQueue中。然后判断是否有事件正在post，如果没有则会遍历eventQueue中每一个event对象，并且调用postSingleEvent方法post该事件。

private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
    Class<?> eventClass = event.getClass();
    boolean subscriptionFound = false;
    if (eventInheritance) {
        // 如果允许事件继承，则会调用lookupAllEventTypes查找所有的父类和接口类
        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) {
            // 如果post的事件没有被注册，则post一个NoSubscriberEvent事件
            post(new NoSubscriberEvent(this, event));
        }
    }
}

如果允许事件继承，则会调用lookupAllEventTypes查找所有的父类和接口类。

private List<Class<?>> lookupAllEventTypes(Class<?> eventClass) {
    synchronized (eventTypesCache) {
        List<Class<?>> eventTypes = eventTypesCache.get(eventClass);
        if (eventTypes == null) {
            eventTypes = new ArrayList<Class<?>>();
            Class<?> clazz = eventClass;
            while (clazz != null) {
                eventTypes.add(clazz);
                addInterfaces(eventTypes, clazz.getInterfaces());
                clazz = clazz.getSuperclass();
            }
            eventTypesCache.put(eventClass, eventTypes);
        }
        return eventTypes;
    }
}

这个方法很简单，就是查找eventClass类的所有父类和接口，并将其保存到eventTypesCache中，方便下次使用。
我们再回到postSingleEvent方法。不管允不允许事件继承，都会执行postSingleEventForEventType方法post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);
                aborted = postingState.canceled;
            } finally {
                postingState.event = null;
                postingState.subscription = null;
                postingState.canceled = false;
            }
            if (aborted) {
                break;
            }
        }
        return true;
    }
    return false;
}

在postSingleEventForEventType方法中，会已eventClass为key从subscriptionsByEventType对象中获取Subscription列表。在上面讲register的时候我们已经看到EventBus在register的时候会将Subscription列表存储在subscriptionsByEventType中。接下来会遍历subscriptions列表然后调用postToSubscription方法进行下一步处理。

private void postToSubscription(Subscription subscription, Object event, boolean isMainThread) {
    switch (subscription.subscriberMethod.threadMode) {
        case PostThread:
            // 如果该事件处理函数没有指定线程模型或者线程模型为PostThread
            // 则调用invokeSubscriber在post的线程中执行事件处理函数
            invokeSubscriber(subscription, event);
            break;
        case MainThread:
            // 如果该事件处理函数指定的线程模型为MainThread
            // 并且当前post的线程为主线程，则调用invokeSubscriber在当前线程（主线程）中执行事件处理函数
            // 如果post的线程不是主线程，将使用mainThreadPoster.enqueue该事件处理函数添加到主线程的消息队列中
            if (isMainThread) {
                invokeSubscriber(subscription, event);
            } else {
                mainThreadPoster.enqueue(subscription, event);
            }
            break;
        case BackgroundThread:
            // 如果该事件处理函数指定的线程模型为BackgroundThread
            // 并且当前post的线程为主线程，则调用backgroundPoster.enqueue
            // 如果post的线程不是主线程，则调用invokeSubscriber在当前线程（非主线程）中执行事件处理函数
            if (isMainThread) {
                backgroundPoster.enqueue(subscription, event);
            } else {
                invokeSubscriber(subscription, event);
            }
            break;
        case Async:
            //添加到异步线程队列中
            asyncPoster.enqueue(subscription, event);
            break;
        default:
            throw new IllegalStateException("Unknown thread mode: " + subscription.subscriberMethod.threadMode);
    }
}

该方法主要是根据register注册的事件处理函数的线程模型在指定的线程中触发事件处理函数。在上一讲EventBus使用详解中已经讲过EventBus的线程模型相关概念了，不明白的可以回去看看。
mainThreadPoster、backgroundPoster和asyncPoster分别是HandlerPoster、BackgroundPoster和AsyncPoster的对象，其中HandlerPoster继承自Handle，BackgroundPoster和AsyncPoster继承自Runnable。
我们主要看看HandlerPoster。

mainThreadPoster = new HandlerPoster(this, Looper.getMainLooper(), 10);

在EventBus的构造函数中，我们看到mainThreadPoster初始化的时候，传入的是Looper.getMainLooper()。所以此Handle是运行在主线程中的。
mainThreadPoster.enqueue方法：

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");
            }
        }
    }
}

enqueue方法最终会调用sendMessage方法，所以该Handle的handleMessage方法会被调用。

@Override
public 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 synchronized
                    pendingPost = 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;
    }
}

在该方法中，最终还是会调用eventBus.invokeSubscriber调用事件处理函数。

BackgroundPoster和AsyncPoster继承自Runnable，并且会在enqueue方法中调用eventBus.getExecutorService().execute(this);具体run方法大家可以自己去看源码，最终都会调用eventBus.invokeSubscriber方法。我们看看eventBus.invokeSubscriber方法的源码：

void invokeSubscriber(PendingPost pendingPost) {
    Object event = pendingPost.event;
    Subscription subscription = pendingPost.subscription;
    PendingPost.releasePendingPost(pendingPost);
    if (subscription.active) {
        invokeSubscriber(subscription, event);
    }
}

该方法会调用invokeSubscriber方法进一步处理：

void invokeSubscriber(Subscription subscription, Object event) {
    try {
        // 通过反射调用事件处理函数
        subscription.subscriberMethod.method.invoke(subscription.subscriber, event);
    } catch (InvocationTargetException e) {
        handleSubscriberException(subscription, event, e.getCause());
    } catch (IllegalAccessException e) {
        throw new IllegalStateException("Unexpected exception", e);
    }
}

该方法最终会通过反射来调用事件处理函数。至此，整个post过程分析完了。
总结一下整个post过程，大致分为3步：

1. 将事件对象添加到事件队列eventQueue中等待处理
2. 遍历eventQueue队列中的事件对象并调用postSingleEvent处理每个事件
3. 找出订阅过该事件的所有事件处理函数，并在相应的线程中执行该事件处理函数

取消事件注册

上面已经分析了EventBus的register和post过程，这两个过程是EventBus的核心。不需要订阅事件时需要取消事件注册：

/** 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) {
            unubscribeByEventType(subscriber, eventType);
        }
        typesBySubscriber.remove(subscriber);
    } else {
        Log.w(TAG, "Subscriber to unregister was not registered before: " + subscriber.getClass());
    }
}

取消事件注册很简单，只是将register过程注册到EventBus的事件处理函数移除掉。

到这里，EventBus源码我们已经分析完了，如有不对的地方还望指点。