EventBus 2.4 源码分析

EventBus简介

本篇基于EventBus 2.4撰写。

Android optimized event bus that simplifies communication between Activities, Fragments, Threads, Services, etc. Less code, better quality.

上面是从官方repo拉来的代码，大致是说简化的组件之间的交流通信，减少代码，提高质量。

其实和EventBus最早是在qzone的代码里认识的，空间内部有一个叫eventcenter的东西，曾经有优化过一些，当时看源码实现的时候发现的原来是根据EventBus改的一个实现。大概就是把annotation的实现改成了接口实现，另外去掉了根据Event类型来找订阅者的模式，完全通过Event的TYPE类型常量来判断，register的时候直接指定对哪种TYPE感兴趣，辅助的判断则有事件发送者引用。这种实现见仁见智吧，虽然直接通过接口肯定是能提高性能的。这里要吐槽的是实现修改的时候，直接把很多对外的接口名字改掉了，何必呢。

EventBus的好处是显而易见的，完全解耦了请求链之间的关系，避免了请求者被长持有，又比广播更轻量，比LocalBroadcast则更强大，接口也简单实用。缺点的话，像是各种Event的定义是一个工作量。

源码分析 - 注册(register)

EventBus.java:

private synchronized void register(Object subscriber, boolean sticky, int priority) {
    List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriber.getClass());
    for (SubscriberMethod subscriberMethod : subscriberMethods) {
        subscribe(subscriber, subscriberMethod, sticky, priority);
    }
}

register的时候，大致就是去subscriber里面首先找到那些onEvent方法（目前实现仍然是根据onEvent这个前缀），寻找的时候会去判断后缀，分为post线程、主线程、background线程，以及异步线程，官方repo提到这里之后在3.0可能会换成annotation的实现。

sticky参数是粘性事件概念，postSticky和registerSticky相对应，stickyEvent会记录该EventType对应的最后一次postSticky的事件，这样在registerSticky的时候，会立即检查是否有之前post的事件，从而避免了某些事件去实现自己的缓存。应用场景大概就是某些activity/fragment感兴趣的事件发生在创建前，这样则可以避免必须实现缓存（当然事实上应用场景还是比较少的，因为大部分东西我们还是会在哪里记录一下）

SubscriberMethod.java:

final class SubscriberMethod {
    final Method method;
    final ThreadMode threadMode;
    final Class<?> eventType;
    /** Used for efficient comparison */
    String methodString;
}

SubscriberMethod里面记录了Method引用，线程模式（在findSubscriberMethods里拿到的），eventType，以及用来提高method.equals性能的methodString。

接着再看subscribe方法的实现，在register最后，对找到的所有方法都去执行了一遍subscribe
EventBus.java:

// Must be called in synchronized block
private void subscribe(Object subscriber, SubscriberMethod subscriberMethod, boolean sticky, int priority) {
    Class<?> eventType = subscriberMethod.eventType;

    // CopyOnWriteArrayList就是个ImmutableArrayList, add/set等方法会返回一个新的ArrayList
    // subscriptionsByEventType是一个hashmap，key是事件类型，value则是订阅者数组
    CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);


    Subscription newSubscription = new Subscription(subscriber, subscriberMethod, priority);

    // 该eventType在map里还不存在，新建一下对应的subscription数组，放进去map
    if (subscriptions == null) {
        subscriptions = new CopyOnWriteArrayList<Subscription>();
        subscriptionsByEventType.put(eventType, subscriptions);
    } else {
        // 重复注册的时候抛出异常，这里如果应用如果觉得无伤大雅其实可以直接return
        if (subscriptions.contains(newSubscription)) {
            throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event "
                    + eventType);
        }
    }

    // 根据优先级去添加到对应的位置，高优先级在前面也就会先处理
    int size = subscriptions.size();
    for (int i = 0; i <= size; i++) {
        if (i == size || newSubscription.priority > subscriptions.get(i).priority) {
            subscriptions.add(i, newSubscription);
            break;
        }
    }

    // typesBySubscriber是另一个map，顾名思义是以subscriber为key的一个map，被用在
    // 1) isRegistered(Object subscriber)方法加速判断是否已注册，用空间换时间
    // 2) unregister的时候直接可以拿到subscriber订阅的所有eventType，然后去从map移除，避免需要遍历所有eventType的map
    List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
    if (subscribedEvents == null) {
        subscribedEvents = new ArrayList<Class<?>>();
        typesBySubscriber.put(subscriber, subscribedEvents);
    }
    subscribedEvents.add(eventType);

    // 粘性事件的话，就去立刻找一下是否有之前post过的事件，有则立即post给该subscriber
    if (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);
        }
    }
}

源码分析 - 发送事件(post)

再来看一下对应的post逻辑

EventBus.java:

/** Posts the given event to the event bus. */
public void post(Object event) {
    // 获得当前post线程的状态，实现贴在下面了，currentPostingThreadState是ThreadLocal<PostingThreadState>变量，每个线程get和set的都是单独的一份数据
    PostingThreadState postingState = currentPostingThreadState.get();
    // 往事件队列里面添加该event
    List<Object> eventQueue = postingState.eventQueue;
    eventQueue.add(event);

    // 如果当前不在posting事件
    if (!postingState.isPosting) {
        // 设置是否在主线程
        postingState.isMainThread = Looper.getMainLooper() == Looper.myLooper();
        // 设置当前正在post事件
        postingState.isPosting = true;
        // canceled状态，抛出异常
        if (postingState.canceled) {
            throw new EventBusException("Internal error. Abort state was not reset");
        }
        try {
            // 循环post从eventQueue里面拿出来的event
            while (!eventQueue.isEmpty()) {
                postSingleEvent(eventQueue.remove(0), postingState);
            }
        } finally {
            // 置位回去
            postingState.isPosting = false;
            postingState.isMainThread = false;
        }
    }
}

final static class PostingThreadState {
    final List<Object> eventQueue = new ArrayList<Object>();
    boolean isPosting;
    boolean isMainThread;
    Subscription subscription;
    Object event;
    // 可以通过cancelEventDelivery去取消事件传递
    boolean canceled;
}

// 单个事件的post处理
private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
    Class<?> eventClass = event.getClass();
    boolean subscriptionFound = false;
    // 继承链处理，比如Event本身的父类的subscriber也会收到，getDefault的时候默认为true。
    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);
    }
    // 竟然没找到，太诡异了
    if (!subscriptionFound) {
        if (logNoSubscriberMessages) {
            Log.d(TAG, "No subscribers registered for event " + eventClass);
        }
        if (sendNoSubscriberEvent && eventClass != NoSubscriberEvent.class &&
                eventClass != SubscriberExceptionEvent.class) {
            // 没想到还有这种逻辑吧，没找到订阅者的，则会发送一个NoSubscriberEvent出去
            post(new NoSubscriberEvent(this, event));
        }
    }
}

// 对特定的event去post单个事件
private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass) {
    CopyOnWriteArrayList<Subscription> subscriptions;
    // 找到该事件的所有订阅
    synchronized (this) {
        subscriptions = subscriptionsByEventType.get(eventClass);
    }
    if (subscriptions != null && !subscriptions.isEmpty()) {
        // 遍历所有订阅
        for (Subscription subscription : subscriptions) {
            postingState.event = event;
            postingState.subscription = subscription;
            boolean aborted = false;
            try {
                // 结果实际post还在这个方法内实现
                postToSubscription(subscription, event, postingState.isMainThread);
                aborted = postingState.canceled;
            } finally {
                postingState.event = null;
                postingState.subscription = null;
                postingState.canceled = false;
            }
            // 如果cancel了，则不再继续传递事件
            if (aborted) {
                break;
            }
        }
        return true;
    }
    return false;
}

// 具体的事件分发
private void postToSubscription(Subscription subscription, Object event, boolean isMainThread) {
    // 这里就是EventBus的一个很强大的功能了，根据订阅者的订阅方法监听线程去处理
    // 如果post和监听方法在同一个线程则立即invoke对应方法
    // 否则会去入队列到对应线程handler进行处理
    switch (subscription.subscriberMethod.threadMode) {
        case PostThread:
            invokeSubscriber(subscription, event);
            break;
        case MainThread:
            if (isMainThread) {
                invokeSubscriber(subscription, event);
            } else {
                mainThreadPoster.enqueue(subscription, event);
            }
            break;
        case BackgroundThread:
            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);
    }
}

End

大致就讲了一下register和post这一对比较常用的接口，其他还有一些实现像是EventBusBuilder，SubscriberException，cancelEventDelivery，AsyncExecutor就不在这里进行赘述，之后可能会对AsyncExecutor单独开一篇讲一下，另外也会对otto的实现做一下分析。