EventBus 源码

构造方法

当要使用 EventBus 时，首先会调用 EventBus.getDefault() 来获取 EventBus 实例：

public static EventBus getDefault() {
    // 双重检查模式（DCL）的单例模式
    if (defaultInstance == null) {
        synchronized (EventBus.class) {
            if (defaultInstance == null) {
                defaultInstance = new EventBus();
            }
        }
    }
    return defaultInstance;
}

接下来查看构造方法：

// class：EventBus
// EventBusBuilder 内部又调用到了 EventBus 的另一个构造方法
private static final EventBusBuilder DEFAULT_BUILDER = new EventBusBuilder();

public EventBus() {
    // DEFAULT_BUILDER 是默认的 EventBusBuilder，用来构造 EventBus。
    this(DEFAULT_BUILDER);
}

// class：EventBus
// 可通过构造一个 EventBusBuilder 来对 EventBus 进行配置。建造者模式。
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;
}

订阅者注册

获取 EventBus 后，便可以将订阅者注册到 EventBus 中：

public void register(Object subscriber) {
    Class<?> subscriberClass = subscriber.getClass();
  // findSubscriberMethods 方法找出一个 SubscriberMethod 集合，也就是传进来的订阅者的所有订阅方法。
  // SubscriberMethod 类中主要用来保存订阅方法的 Method 对象、线程模式、事件类型、优先级、是否是粘性事件等属性。
    List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
    synchronized (this) {
      // 遍历集合，完成订阅者的注册操作。
        for (SubscriberMethod subscriberMethod : subscriberMethods) {
            subscribe(subscriber, subscriberMethod);
        }
    }
}

查找订阅者的订阅方法

List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
  // 从缓存中查找是否有订阅方法的集合，如果找到了就立马返回。否则根据 ignoreGeneratedIndex 的值来选择采用何种方法来查找订阅方法的集合。
    List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
  
    if (subscriberMethods != null) {
        return subscriberMethods;
    }
  // ignoreGeneratedIndex 属性表示是否忽略注解器生成的 MyEventBusIndex。
  // 默认值 false，可通过 EventBusBuilder 来设置它的值。
    if (ignoreGeneratedIndex) {
        subscriberMethods = findUsingReflection(subscriberClass);
    } else {
        subscriberMethods = findUsingInfo(subscriberClass);
    }
    if (subscriberMethods.isEmpty()) {
        throw new EventBusException("Subscriber " + subscriberClass
                + " and its super classes have no public methods with the @Subscribe annotation");
    } else {
      // 在找到订阅方法的集合后,放入缓存,以免下次继续查找。
        METHOD_CACHE.put(subscriberClass, subscriberMethods);
        return subscriberMethods;
    }
}

当通过 EventBus 单例模式来获取默认的 EventBus 对象，也就是 ignoreGeneratedIndex 为 false 的情况，会调用 findUsingInfo 方法：

private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
    FindState findState = prepareFindState();
    findState.initForSubscriber(subscriberClass);
    while (findState.clazz != null) {
      // 获取订阅者信息。在开始查找订阅方法时并没有忽略注解器生成的索引 MyEventBusIndex。如果通过 EventBusBuilder 配置了 MyEventBusIndex，便会获取 subscriberInfo。
        findState.subscriberInfo = getSubscriberInfo(findState);
        if (findState.subscriberInfo != null) {
          // 得到订阅方法相关的信息。
            SubscriberMethod[] array = findState.subscriberInfo.getSubscriberMethods();
            for (SubscriberMethod subscriberMethod : array) {
                if (findState.checkAdd(subscriberMethod.method, subscriberMethod.eventType)) {
                    findState.subscriberMethods.add(subscriberMethod);
                }
            }
        } else {
          // 如果没有配置 MyEventBusIndex，便会执行此方法，将订阅方法保存到 findState 中。
            findUsingReflectionInSingleClass(findState);
        }
        findState.moveToSuperclass();
    }
  // 对 findState 做回收处理并返回订阅方法的 List 集合。
    return getMethodsAndRelease(findState);
}

默认情况下是没有配置 MyEventBusIndex 的，因此查看 findUsingReflectionInSingleClass 方法：

private void findUsingReflectionInSingleClass(FindState findState) {
    Method[] methods;
    try {
        // This is faster than getMethods, especially when subscribers are fat classes like Activities
      // 通过反射来获取订阅者中所有方法，并根据方法的类型、参数和注解来找到订阅方法。找到订阅方法后将订阅方法的相关信息保存到 findState 中。
        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;
    }
    for (Method method : methods) {
        int modifiers = method.getModifiers();
        if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
            Class<?>[] parameterTypes = method.getParameterTypes();
            if (parameterTypes.length == 1) {
                Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);
                if (subscribeAnnotation != null) {
                    Class<?> eventType = parameterTypes[0];
                    if (findState.checkAdd(method, eventType)) {
                        ThreadMode threadMode = subscribeAnnotation.threadMode();
                        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");
        }
    }
}

订阅者的注册过程

在查找完订阅者的订阅方法后便开始对所有的订阅方法进行注册，在 register() 中调用了 subscribe() 来对订阅方法进行注册：

// 主要做了两件事：一是将 Subscriptions 根据 eventType 封装到 subscriptionsByEventType 中，将 subscribedEvents 根据 subscriber 封装到 typesBySubscriber 中。二是对粘性事件的处理。
private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
    Class<?> eventType = subscriberMethod.eventType;
  // 根据订阅者和订阅方法创建一个订阅对象 Subscription。
    Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
  // 根据事件类型获取订阅集合对象。
    CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
    if (subscriptions == null) {
      // 如果为 null 则重新创建，并将 Subscriptions 根据 eventType 保存在 Map 集合。
        subscriptions = new CopyOnWriteArrayList<>();
        subscriptionsByEventType.put(eventType, subscriptions);
    } else {
      // 判断订阅者是否已经被注册
        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) {
          // 按照订阅方法的优先级插入到订阅对象集合中，完成订阅方法的注册。
            subscriptions.add(i, newSubscription);
            break;
        }
    }
  // 通过 subscriber 获取事件类型集合。
    List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
    if (subscribedEvents == null) {
      // 如果为 null 则重新创建，并将 eventType 添加到 subscribedEvents 中，并根据 subscriber 将 subscribedEvents 存储在 typesBySubscriber（Map 集合）。
        subscribedEvents = new ArrayList<>();
        typesBySubscriber.put(subscriber, subscribedEvents);
    }
    subscribedEvents.add(eventType);
    // 如果是粘性事件，则从 stickyEvents 事件保存队列中取出该事件类型的事件发送给当前订阅者。
    if (subscriberMethod.sticky) {
        if (eventInheritance) {
          // 粘性事件处理
            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);
        }
    }
}

事件的发送

在获取 EventBus 对象后，可通过 post() 进行对事件的提交：

// class：EventBus
public void post(Object event) {
  // PostingThreadState 保存着事件队列和线程状态信息。
    PostingThreadState postingState = currentPostingThreadState.get();
  // 获取事件队列，并将当前事件插入事件队列。
    List<Object> eventQueue = postingState.eventQueue;
    eventQueue.add(event);

    if (!postingState.isPosting) {
        postingState.isMainThread = isMainThread();
        postingState.isPosting = true;
        if (postingState.canceled) {
            throw new EventBusException("Internal error. Abort state was not reset");
        }
        try {
          // 处理队列中的所有方法
            while (!eventQueue.isEmpty()) {
                postSingleEvent(eventQueue.remove(0), postingState);
            }
        } finally {
            postingState.isPosting = false;
            postingState.isMainThread = false;
        }
    }
}

// class：EventBus
private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
    Class<?> eventClass = event.getClass();
    boolean subscriptionFound = false;
  // 表示是否向上查找事件的父类，默认为 true，可在 EventBusBuilder 中进行配置。
    if (eventInheritance) {
      // 通过 lookupAllEventTypes 找到所有父类事件并存在 List 中
        List<Class<?>> eventTypes = lookupAllEventTypes(eventClass);
        int countTypes = eventTypes.size();
        for (int h = 0; h < countTypes; h++) {
            Class<?> clazz = eventTypes.get(h);
          // 然后通过 postSingleEventForEventType 对事件逐一处理。
            subscriptionFound |= postSingleEventForEventType(event, postingState, clazz);
        }
    } else {
        subscriptionFound = postSingleEventForEventType(event, postingState, eventClass);
    }
  // 找不到该事件时的异常处理
    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;
    synchronized (this) {
      // 同步取出该事件对应的 Subscriptions（订阅对象集合）。
        subscriptions = subscriptionsByEventType.get(eventClass);
    }
    if (subscriptions != null && !subscriptions.isEmpty()) {
      // 遍历 Subscriptions，将事件 event 和对应的 Subscription（订阅对象）传递给 postingState 并调用 postToSubscription 方法对事件进行处理。
        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;
}

private void postToSubscription(Subscription subscription, Object event, boolean isMainThread) {
  // 取出订阅方法的 threadMode（线程模式），之后据其来分别处理。
    switch (subscription.subscriberMethod.threadMode) {
        case POSTING:
            invokeSubscriber(subscription, event);
            break;
        case MAIN:
            if (isMainThread) {
              // 若提交事件的线程是主线程，则通过反射直接运行订阅的方法。
                invokeSubscriber(subscription, event);
            } else {
              // 若其不是主线程，则需要 mainThreadPoster 将订阅事件添加到主线程队列中。
              // mainThreadPoster 是 HandlerPoster 类型的，继承自 Handler，通过 Handler 将订阅方法切换到主线程执行。
                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);
    }
}

订阅者取消注册

public synchronized void unregister(Object subscriber) {
  // typesBySubscriber 是一个 map 集合。通过 subscriber 找到 subscribedTypes(事件类型集合)。
    List<Class<?>> subscribedTypes = typesBySubscriber.get(subscriber);
    if (subscribedTypes != null) {
        for (Class<?> eventType : subscribedTypes) {
          // 遍历 subscribedTypes，并调用 unsubscribeByEventType 方法。
            unsubscribeByEventType(subscriber, eventType);
        }
      // 将 subscriber 对应的 eventType 从 typesBySubscriber 中移除。
        typesBySubscriber.remove(subscriber);
    } else {
        logger.log(Level.WARNING, "Subscriber to unregister was not registered before: " + subscriber.getClass());
    }
}

private void unsubscribeByEventType(Object subscriber, Class<?> eventType) {
  // 通过 eventType 来得到对应的 Subscriptions(订阅对象集合)
    List<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
    if (subscriptions != null) {
        int size = subscriptions.size();
        for (int i = 0; i < size; i++) {
          // 并在 for 循环中判断如果 Subscription(订阅对象) 的 subscriber(订阅者) 属性等于传进来的 subscriber，则从 Subscriptions 中移除该 Subscription。
            Subscription subscription = subscriptions.get(i);
            if (subscription.subscriber == subscriber) {
                subscription.active = false;
                subscriptions.remove(i);
                i--;
                size--;
            }
        }
    }
}

链接

参考资料：

GitHub

Android 进阶之光