EventBus 源码分析

这篇文章主要是根据我们平时的使用,一步一步的分析EventBus源码流程,因此分为三步:

1、注册订阅者
2、事件发布
3、反注册订阅者

1、register 注册订阅者

在使用eventBus的时候,第一个步骤就是注册订阅者

  EventBus.getDefault().register(this);

getDefault方法是一个单例模式的初始化方法,主要就是获取一个实例,源码如下:

   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;
    }

可以很清楚的看到,这里只是一个DoubleCheck的单例模式,接下来直接看一下构造方法:

  /**
     * 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;
    }

构造方法里面是通过一个Builder模式来对EventBus各项配置进行初始化
在getDefault获取到实例之后,就会调用register方法进行注册,这时候进入register方法看一下注册过程:

   public void register(Object subscriber) {
        Class<?> subscriberClass = subscriber.getClass();
        //获取到订阅者中所有的订阅方法,并储存到list集合中
        List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
        synchronized (this) {
            for (SubscriberMethod subscriberMethod : subscriberMethods) {
                subscribe(subscriber, subscriberMethod);
            }
        }
    }

从源码里面看,注册过程只有两步:

a、根据注册时传入的订阅者对象,找到所有的订阅方法;
b、订阅所有的订阅方法

a1)、
首先看一下如何查找到订阅者中所有的订阅方法,即findSubscriberMethods()方法,subscriberMethodFinder是在EventBus构造方法中进行的初始化,进入findSubscriberMethods方法中进行查看:

        List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
        //METHOD_CACHE 一个map集合,以订阅者为key,以订阅方法的list集合为value进行缓存
        List<SubscriberMethod> subscriberMethods =  METHOD_CACHE.get(subscriberClass);
        //如果当前的对象已经被缓存,则直接获取返回
        if (subscriberMethods != null) {
            return subscriberMethods;
        }
     if (ignoreGeneratedIndex) {
           // 如果忽略索引,就根据反射来获取
            subscriberMethods = findUsingReflection(subscriberClass);
        } else {
            //否则使用索引
            subscriberMethods = findUsingInfo(subscriberClass);
        }
      //如果该订阅者中没有订阅方法,此处会抛出一个异常,提醒你该订阅者和他的父类里面都没有订阅方法(public 修饰并且@Subscribe 进行注解)
        if (subscriberMethods.isEmpty()) {
            throw new EventBusException("Subscriber " + subscriberClass
                    + " and its super classes have no public methods with the @Subscribe annotation");
        } else {
          //如果有订阅方法,则把该订阅者和订阅方法进行缓存,并返回订阅方法的list集合
            METHOD_CACHE.put(subscriberClass, subscriberMethods);
            return subscriberMethods;
        }
    }

a1.1)、
findSubscriberMethods 里面的逻辑已经很清晰了,现在看一下 findUsingInfo()和findUsingReflection(),一般使用的时候,并没有自定义进行配置,所以一般都是使用索引进行查找

a1.1.1)
这里就先分析一下findUsingInfo()方法,反射的稍后再看:

//忽略索引时,查找方法
 private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
        // 创建并初始化FindState对象 FindState是订阅者的一个辅助类,用于获取到订阅方法
        FindState findState = prepareFindState();
        //关联订阅者
        findState.initForSubscriber(subscriberClass);
       //此处通过循环,获取到订阅者父类中的订阅方法
        while (findState.clazz != null) {
            //获取到订阅者的信息
            findState.subscriberInfo = getSubscriberInfo(findState);
            if (findState.subscriberInfo != null) {
                //如果可以找到订阅者信息,将订阅方法缓存到 subscriberMethods 中
                SubscriberMethod[] array = findState.subscriberInfo.getSubscriberMethods();
                for (SubscriberMethod subscriberMethod : array) {
                    if (findState.checkAdd(subscriberMethod.method, subscriberMethod.eventType)) {
                        findState.subscriberMethods.add(subscriberMethod);
                    }
                }
            } else {
                //如果找不到订阅者的信息,则通过反射方法进行获取
                findUsingReflectionInSingleClass(findState);
            }
            findState.moveToSuperclass();
        }
        return getMethodsAndRelease(findState);
    }

FindState类是一个辅助类,用来辅助订阅者获取订阅方法,代码比较简单,篇幅原因此处就不在进行源码解析,只要清楚这个类主要用途即可
prepareFindState() 方法则是用来创建并初始化FindState的方法,同时坐着也通过一个缓存池进行了优化
从上面源码中可以看到,在可以直接查找到订阅信息的情况比较简单,直接将查找到的信息进行缓存即可,下面分析一下找不到订阅信息的时候,通过反射方法进行查找:

//通过反射方式,获取到订阅者中所有的订阅方法
    private void findUsingReflectionInSingleClass(FindState findState) {
        // 1 获取订阅者内所有方法
        Method[] methods;
        try {
            // This is faster than getMethods, especially when subscribers are fat classes like Activities
            methods = findState.clazz.getDeclaredMethods();
        } catch (Throwable th) {
            // Workaround for java.lang.NoClassDefFoundError, see https://github.com/greenrobot/EventBus/issues/149
            methods = findState.clazz.getMethods();
            findState.skipSuperClasses = true;
        }
        // 2 遍历所有方法,找到所有的订阅方法,并将所有的订阅方法进行缓存到 findState 中
        for (Method method : methods) {
            int modifiers = method.getModifiers();
            //如果是非静态、非抽象、public 修饰的
            if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
                Class<?>[] parameterTypes = method.getParameterTypes();
                //只有一个参数
                if (parameterTypes.length == 1) {
                    Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);
                    // Subscribe 进行注解
                    if (subscribeAnnotation != null) {
                        Class<?> eventType = parameterTypes[0];
                        if (findState.checkAdd(method, eventType)) {
                            ThreadMode threadMode = subscribeAnnotation.threadMode();
                            //符合条件的进行缓存
                            findState.subscriberMethods.add(new SubscriberMethod(method, eventType, threadMode,
                                    subscribeAnnotation.priority(), subscribeAnnotation.sticky()));
                        }
                    }
                } else if (strictMethodVerification && method.isAnnotationPresent(Subscribe.class)) {
                    String methodName = method.getDeclaringClass().getName() + "." + method.getName();
                    throw new EventBusException("@Subscribe method " + methodName +
                            "must have exactly 1 parameter but has " + parameterTypes.length);
                }
            } else if (strictMethodVerification && method.isAnnotationPresent(Subscribe.class)) {
                String methodName = method.getDeclaringClass().getName() + "." + method.getName();
                throw new EventBusException(methodName +
                        " is a illegal @Subscribe method: must be public, non-static, and non-abstract");
            }
        }
    }

认真看一下每一个步骤,可以发现其实就是通过反射查找到符合条件的订阅方法,然后将订阅方法缓存到findState中,同时对注解的订阅方法进行校验,给出提示,在查找到所有的订阅方法之后,代码执行到了,a1.1.1) findUsingReflectionInSingleClass 执行完毕,然后 findUsingInfo()在循环查找并缓存订阅者中所有的订阅方法,至此, 在a1.1) findUsingInfo 方法也执行完毕,成功找到了所有的订阅方法,a1)、中忽略索引的情况已经执行完毕
查找完成之后,我们在回过头看一下a1)、findSubscriberMethods 方法中不忽略 索引的方法 findUsingReflection()

 FindState findState = prepareFindState();
        findState.initForSubscriber(subscriberClass);
        while (findState.clazz != null) {
            findUsingReflectionInSingleClass(findState);
            findState.moveToSuperclass();
        }
        return getMethodsAndRelease(findState);

这段代码就非常简单了,上面已经分析过了,直接使用反射查找到所有的方法,并进行缓存;
在查找到所有的订阅方法之后,将所有的方法进行缓存到 METHOD_CACHE ,至此,注册第一步就完成了;
无论使用哪一种方法,到这里都已经找到所有的订阅方法了,在再次返回到 register()方法中:

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

这段代码通过循环,将所有的订阅方法和订阅者关联起来,看一下subscribe 方法

  private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
        Class<?> eventType = subscriberMethod.eventType;
        //创建订阅者和订阅方法的封装对象
        Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
        //由于所有的订阅方法都只有一个参数,并且subscriptionsByEventType 是以 订阅方法的参数的class为key,Subscription集合为value的
//      //的map集合,所以,根据eventType 可以找到所有的含有此参数的订阅方法所在的订阅者
        // 根据订阅方法的参数类型,查找到所有包含有该Event 的订阅者,Event 即为订阅方法的参数
        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);
            }
        }
        //根据优先级,将当前的订阅者插入订阅集合
        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;
            }
        }

        //根据订阅者,查找订阅者里面所有的Event 类型,并储存
        List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
        if (subscribedEvents == null) {
            subscribedEvents = new ArrayList<>();
            typesBySubscriber.put(subscriber, subscribedEvents);
        }
        subscribedEvents.add(eventType);
        //对于粘性事件,则立刻执行
        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);
            }
        }
    }

至此,订阅者和订阅事件进行了关联,并且进行了缓存,register()完成

2、事件发布

事件发布一般通过以下两种方法进行发布,postSticky 为发送粘性事件

 EventBus.getDefault().post(new Event("我是测试event数据"));
 EventBus.getDefault().postSticky(new Event("我是测试event数据"));

首先先分析一下post 事件

  public void post(Object event) {
        //获取到ThreadLocal中储存的数据,并将当前post的数据添加到队列中
        PostingThreadState postingState = currentPostingThreadState.get();
        List<Object> eventQueue = postingState.eventQueue;
        eventQueue.add(event);
        //如果当前没有在post 事件
        if (!postingState.isPosting) {
            postingState.isMainThread = isMainThread();
            postingState.isPosting = true;
            if (postingState.canceled) {
                throw new EventBusException("Internal error. Abort state was not reset");
            }
            //通过循环取出队列中所有数据,并进行订阅
            try {
                while (!eventQueue.isEmpty()) {
                    postSingleEvent(eventQueue.remove(0), postingState);
                }
            } finally {
                postingState.isPosting = false;
                postingState.isMainThread = false;
            }
        }
    }

 private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
        Class<?> eventClass = event.getClass();
        boolean subscriptionFound = false;
        //此处仅判断是否允许继承,如果允许就会执行父类中的订阅方法,不作为重点关注内容
        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));
            }
        }
    }

    private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass) {
        CopyOnWriteArrayList<Subscription> subscriptions;
        //subscriptionsByEventType,key 为订阅方法的参数类型,value为订阅者和订阅方法封装的list集合
        //根据register()的源码,已经知道在注册的时候,会将订阅者和订阅方法缓存到subscriptionsByEventType里面
        synchronized (this) {
            subscriptions = subscriptionsByEventType.get(eventClass);
        }
        //如果post的Event有订阅者接收,循环进行订阅
        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;
    }

  // 订阅方法的执行方法,会根据threadMode将订阅方法在不同的线程进行执行
    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 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);
        }
    }

post方法本身并不复杂,重点关注一下postToSubscription()方法,在此方法中,根据不同的线程进行处理,执行订阅方法,到这里,事件发布就完成了,订阅方法也已经被执行了,完成了事件的分发

3、反注册订阅者

反注册就非常简单了,仅仅是将缓存的订阅者及其订阅方法移除即可

    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--;
                }
            }
        }
    }

从代码中可以看出,反注册的时候将注册时候缓存在 typesBySubscriber 和subscriptionsByEventType 中的数据移除,完成了反注册

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