Android EventBus

EventBus源码解析

Hello EventBus

Posted by XYH on November 28, 2018

本文记录EventBus源码解析内容，版本为3.1.1，从几个核心方法依次入手，register、post、postSticky、unregister。

EventBus

EventBus是Android和Java上的一个事件发布、订阅框架。

核心功能：

  • 简化了组件之间的通信。
    • 将事件的发送者跟接受者隔离。
    • 同时适用于Activity、Fragment及后台线程。
    • 避免了错综复杂的生命周期依赖。
  • 使组价你通信代码变得更为简单。
  • 通信速度快
  • jar包小（~50k）
  • 超过100000000+的应用使用。
  • 可以定义接受者的优先级等其他辅助功能。

流程图如下： EventBus流程图

类关系图如下： EventBus类关系图

register

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

该函数会根据订阅者的类名去subscriberMethodFinder查找当前订阅者的所有响应方法。具体是获取有注解@Subscribe的方法，同时方法必须是public不能为static、abstract。接着循环每个响应方法，依次执行subscribe。

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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);
        }
        //暂存
        subscribedEvents.add(eventType);
        //处理sticky事件，如果之前有发布对应的sticky事件则在当前subscriber注册之后去触发对应的方法。
        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);
            }
        }
    }

subscribe通过subscriptionsByEventType得到该事件类型所有订阅者信息队列，根据优先级将当前订阅者信息插入到订阅者队列subscriptionsByEventType中。 在typesBySubscriber中得到当前订阅者订阅的所有事件队列，将此事件保存到队列typesBySubscriber中，用于后续取消订阅； 检查这个事件是否是 Sticky 事件，如果是则从stickyEvents事件保存队列中取出该事件类型最后一个事件发送给当前订阅者。

post

post流程

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public void post(Object event) {
        //从ThreadLocal中取出当前的PostingThreadState
        PostingThreadState postingState = currentPostingThreadState.get();
        //获取事件队列
        List<Object> eventQueue = postingState.eventQueue;
        //添加事件到事件队列中
        eventQueue.add(event);

        if (!postingState.isPosting) {
            //判断当前是否是主线程
            postingState.isMainThread = isMainThread();
            postingState.isPosting = true;
            if (postingState.canceled) {
                throw new EventBusException("Internal error. Abort state was not reset");
            }
            try {
                while (!eventQueue.isEmpty()) {
                    //具体调用方法
                    postSingleEvent(eventQueue.remove(0), postingState);
                }
            } finally {
                postingState.isPosting = false;
                postingState.isMainThread = false;
            }
        }
    }

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private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
        Class<?> eventClass = event.getClass();
        boolean subscriptionFound = false;
        //EventBus提供的DEFAULT_BUILDER中，这个参数为true，是否从父类查找对应的@subscribe方法。
        if (eventInheritance) {
            List<Class<?>> eventTypes = lookupAllEventTypes(eventClass);
            int countTypes = eventTypes.size();
            for (int h = 0; h < countTypes; h++) {
                Class<?> clazz = eventTypes.get(h);
                //继续调用
                subscriptionFound |= postSingleEventForEventType(event, postingState, clazz);
            }
        } else {
            subscriptionFound = postSingleEventForEventType(event, postingState, eventClass);
        }
        //如果没有@subscribe方法
        if (!subscriptionFound) {
            if (logNoSubscriberMessages) {
                logger.log(Level.FINE, "No subscribers registered for event " + eventClass);
            }
            //sendNoSubscriberEvent表示是否继续发送没有接收者的Event，默认为true。
            if (sendNoSubscriberEvent && eventClass != NoSubscriberEvent.class &&
                    eventClass != SubscriberExceptionEvent.class) {
                post(new NoSubscriberEvent(this, event));
            }
        }
    }

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private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass) {
        CopyOnWriteArrayList<Subscription> subscriptions;
        synchronized (this) {
            //这个在调用register会先put对应的eventClass。
            subscriptions = subscriptionsByEventType.get(eventClass);
        }
        if (subscriptions != null && !subscriptions.isEmpty()) {
            for (Subscription subscription : subscriptions) {
                postingState.event = event;
                postingState.subscription = subscription;
                boolean aborted = false;
                try {
                    //这里继续调用
                    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;
    }

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private void postToSubscription(Subscription subscription, Object event, boolean isMainThread) {
        //该方法都会根据反射去调用对应的方法。
        switch (subscription.subscriberMethod.threadMode) {
            //默认线程
            case POSTING:
                invokeSubscriber(subscription, event);
                break;
            //主线程
            case MAIN:
                if (isMainThread) {
                    invokeSubscriber(subscription, event);
                } else {
                    mainThreadPoster.enqueue(subscription, event);
                }
                break;
            //主线程有序调用
            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;
            //后台线程
            case BACKGROUND:
                if (isMainThread) {
                    backgroundPoster.enqueue(subscription, event);
                } else {
                    invokeSubscriber(subscription, event);
                }
                break;
            //异步
            case ASYNC:
                asyncPoster.enqueue(subscription, event);
                break;
            default:
                throw new IllegalStateException("Unknown thread mode: " + subscription.subscriberMethod.threadMode);
        }
    }

以上代码就是EventBus post的大致流程。

unregister

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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());
        }
    }
    
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--;
                }
            }
        }
    }

该方法总结一下就是清除之前register的遗留信息。

Sticky事件

一般的事件post之后，在post之后注册的接收者无法接收到该事件，鉴于这种情况，有了Sticky事件，Sticky事件post之后，在之后注册的接收者依然能够接收到该事件。

在源码中的实现如下：

EventBus#postSticky(object)

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public void postSticky(Object event) {
        synchronized (stickyEvents) {
            //这里会暂存sticky事件
            stickyEvents.put(event.getClass(), event);
        }
        //调用正常的post流程
        post(event);
    }

再回到EventBus#register()方法中调用的subscribe()

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 private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
        xxx
        //处理sticky事件，如果之前有发布对应的sticky事件则在当前subscriber注册之后去触发对应的方法。
        if (subscriberMethod.sticky) {
            if (eventInheritance) {
                //获取暂存的对应sticky事件
                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();
                        //触发对应的接收者方法       
                        checkPostStickyEventToSubscription(newSubscription, stickyEvent);
                    }
                }
            } else {
                Object stickyEvent = stickyEvents.get(eventType);
                checkPostStickyEventToSubscription(newSubscription, stickyEvent);
            }
        }
    }

如果想移除sticky事件的话，只需要调用

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 EventBus.getDefault().removeStickyEvent(stickyEvent)

对应EventBus中的源码：

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public <T> T removeStickyEvent(Class<T> eventType) {
        synchronized (stickyEvents) {
            return eventType.cast(stickyEvents.remove(eventType));
        }
    }
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