一、构建

1、EventBus实例创建采用了DCL单例：

static volatile EventBus defaultInstance;

public static EventBus getDefault() {
        if (defaultInstance == null) {
            synchronized (EventBus.class) {
                if (defaultInstance == null) {
                    defaultInstance = new EventBus();
                }
            }
        }
        return defaultInstance;
 }

2、其构造器相关代码如下:

private static final EventBusBuilder DEFAULT_BUILDER = new EventBusBuilder();
// 基于事件类型, 订阅列表构成的集合
private final Map<Class<?>, CopyOnWriteArrayList<Subscription>> subscriptionsByEventType;
private final Map<Object, List<Class<?>>> typesBySubscriber;

public EventBus() {
        this(DEFAULT_BUILDER);
 }

EventBus(EventBusBuilder builder) {
        subscriptionsByEventType = new HashMap<>();
        typesBySubscriber = new HashMap<>();
        stickyEvents = new ConcurrentHashMap<>();
        // 线程发送器
        mainThreadPoster = new HandlerPoster(this, Looper.getMainLooper(), 10);
        backgroundPoster = new BackgroundPoster(this);
        asyncPoster = new AsyncPoster(this);

        // 设置build选项
        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;
 }

也就是说，采用的是默认的建造者EventBusBuilder对象来构建EventBus对象。
其中初始化的subscriptionsByEventType 、typesBySubscriber 是两个很重要的map集合。

二、注册

1、注册订阅者对象

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

可以看出，subscriberMethodFinder根据订阅者Class实例来获得subscriberMethod集合。SubscriberMethod是对注册方法以及相关属性的封装：

public class SubscriberMethod {
    final Method method;
    final ThreadMode threadMode;
    final Class<?> eventType;
    final int priority;
    final boolean sticky;
    /** Used for efficient comparison */
    String methodString;

    ...

}

到此，我们可以联想到Method对象之后应该会执行invoke方法，这也是EventBus传递消息的根本。ThreadMode是线程模式，我们都知道EventBus可以通过注解来指定执行线程。

2、查找订阅者方法

// 方法缓存集合，线程安全
private static final Map<Class<?>, List<SubscriberMethod>> METHOD_CACHE = new ConcurrentHashMap<>();

List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
        List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
        if (subscriberMethods != null) {
            return subscriberMethods;
        }

        // 默认为false
        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 {
            // 添加进缓存
            METHOD_CACHE.put(subscriberClass, subscriberMethods);
            return subscriberMethods;
        }
 }

这是一个获得数据并添加到缓存的过程，ignoreGeneratedIndex是在EventBusBuilder中设置的，默认为false，也就是说返回的结果是由findUsingInfo()方法得到的。

private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
        FindState findState = prepareFindState();
        findState.initForSubscriber(subscriberClass);
        while (findState.clazz != null) {
            findState.subscriberInfo = getSubscriberInfo(findState);
            if (findState.subscriberInfo != null) {
                SubscriberMethod[] array = findState.subscriberInfo.getSubscriberMethods();
                for (SubscriberMethod subscriberMethod : array) {
                    if (findState.checkAdd(subscriberMethod.method, subscriberMethod.eventType)) {
                        findState.subscriberMethods.add(subscriberMethod);
                    }
                }
            } else {
                findUsingReflectionInSingleClass(findState);
            }
            findState.moveToSuperclass();
        }
        return getMethodsAndRelease(findState);
 }

findState对象是对查找状态的封装。调用initForSubscriber方法后，findState.clazz为subscriberClass，findState.subscriberInfo为null，故而调用findUsingReflectionInSingleClass方法，这个方法是查找功能的具体实现。

private void findUsingReflectionInSingleClass(FindState findState) {
        Method[] methods;
        try {
            // This is faster than getMethods, especially when subscribers are fat classes like Activities
            // 获得所有声明的方法
            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) {
            // 修饰符
            int modifiers = method.getModifiers();
            // 必须是public，且不是Modifier.ABSTRACT | Modifier.STATIC | BRIDGE | SYNTHETIC;
            if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
                // 参数类型
                Class<?>[] parameterTypes = method.getParameterTypes();
                if (parameterTypes.length == 1) {
                    // 获得注解
                    Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);
                    if (subscribeAnnotation != null) {
                        Class<?> eventType = parameterTypes[0];
                        if (findState.checkAdd(method, eventType)) {
                            ThreadMode threadMode = subscribeAnnotation.threadMode();
                            // 添加到集合
                            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");
            }
        }
}

由上可以看到方法查找的逻辑，最终把满足要求的方法以及相关参数封装起来，添加到 findState.subscriberMethods集合中。

接下来回到findUsingInfo方法中，又调用了moveToSuperclass方法。

 void moveToSuperclass() {
            if (skipSuperClasses) {
                clazz = null;
            } else {
                clazz = clazz.getSuperclass();
                String clazzName = clazz.getName();
                /** Skip system classes, this just degrades performance. */
                if (clazzName.startsWith("java.") || clazzName.startsWith("javax.") || clazzName.startsWith("android.")) {
                    clazz = null;
                }
            }
}

这是一个把遍历目标转移到父类的方法，skipSuperClasses默认为false。该方法不停的循环，一直到系统类，将clazz对象设为null，从而结束循环。

最终返回订阅方法集合。

 private List<SubscriberMethod> getMethodsAndRelease(FindState findState) {
        List<SubscriberMethod> subscriberMethods = new ArrayList<>(findState.subscriberMethods);
        // 回收findState
        findState.recycle();
        synchronized (FIND_STATE_POOL) {
            for (int i = 0; i < POOL_SIZE; i++) {
                if (FIND_STATE_POOL[i] == null) {
                    FIND_STATE_POOL[i] = findState;
                    break;
                }
            }
        }
        return subscriberMethods;
}

3、订阅关系确立
之前查找到的订阅方法集合仅仅是把SubscriberMethod 对象累积起来，并不能根据事件类型，快速得到订阅对象（post方法发送的是事件对象），故而需要遍历订阅方法对象，建立订阅关系。

private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
        Class<?> eventType = subscriberMethod.eventType;
        // 封装订阅对象、订阅方法为订阅关系
        Subscription newSubscription = new Subscription(subscriber, subscriberMethod);

        CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
        if (subscriptions == null) {
            subscriptions = new CopyOnWriteArrayList<>();
            // 建立事件类型-订阅关系映射
            subscriptionsByEventType.put(eventType, subscriptions);
        } else {
            // 重复注册抛出异常
            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) {
                // 添加订阅关系到集合中
                subscriptions.add(i, newSubscription);
                break;
            }
        }
        
        // 根据订阅对象获得事件类型
        List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
        if (subscribedEvents == null) {
            subscribedEvents = new ArrayList<>();
            typesBySubscriber.put(subscriber, subscribedEvents);
        }
        subscribedEvents.add(eventType);

        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);
            }
        }
 }

由上述方法可得到两个集合subscriptionsByEventType、typesBySubscriber，建立了事件类型与订阅对象之间的双重映射。前者用于发送事件处理，后者用于取消注册。

三、发送

1、发送事件对象

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

        // 未发送
        if (!postingState.isPosting) {
            // 判断当前线程
            postingState.isMainThread = Looper.getMainLooper() == Looper.myLooper();
            postingState.isPosting = true;
            if (postingState.canceled) {
                throw new EventBusException("Internal error. Abort state was not reset");
            }

            try {
                // 遍历事件队列
                while (!eventQueue.isEmpty()) {
                    postSingleEvent(eventQueue.remove(0), postingState);
                }
            } finally {
                // 结束后reset
                postingState.isPosting = false;
                postingState.isMainThread = false;
            }
        }
}

currentPostingThreadState是一个ThreadLocal对象，用于存储当前线程的PostingThreadState对象。

 final static class PostingThreadState {
        // 事件队列
        final List<Object> eventQueue = new ArrayList<Object>();
        // 是否发送中
        boolean isPosting;
        // 是否主线程
        boolean isMainThread;
        // 订阅关系
        Subscription subscription;
        Object event;
        boolean canceled;
 }

回到post方法，得到事件队列后，将事件添加到队列中。在事件队列不为空的情况下，遍历调用postSingleEvent方法。

2、单一事件处理

  private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
        Class<?> eventClass = event.getClass();
        boolean subscriptionFound = false;
        // 事件继承,默认为true
        if (eventInheritance) {
            // 事件对象的所有父类及实现接口
            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) {
                post(new NoSubscriberEvent(this, event));
            }
        }
}

可以看出，发送单一事件会继续遍历事件的父类与父接口，也就是处理事件继承。

private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass) {
        CopyOnWriteArrayList<Subscription> subscriptions;
        synchronized (this) {
            // 从集合中根据事件类型取出订阅关系
            subscriptions = subscriptionsByEventType.get(eventClass);
        }

        if (subscriptions != null && !subscriptions.isEmpty()) {
            // 遍历订阅关系（含订阅对象与订阅方法）
            for (Subscription subscription : subscriptions) {
                // 发送状态读取订阅关系
                postingState.event = event;
                postingState.subscription = subscription;
                boolean aborted = false;

                try {
                    // 根据不同模式进行处理
                    postToSubscription(subscription, event, postingState.isMainThread);
                    aborted = postingState.canceled;
                } finally {
                    // 归位清零
                    postingState.event = null;
                    postingState.subscription = null;
                    postingState.canceled = false;
                }
                if (aborted) {
                    break;
                }
            }
            return true;
        }
        return false;
}

3、不同模式下处理事件

private void postToSubscription(Subscription subscription, Object event, boolean isMainThread) {
        switch (subscription.subscriberMethod.threadMode) {
            // 发送事件的线程
            case POSTING:
                invokeSubscriber(subscription, event);
                break;
            // 主线程
            case MAIN:
                // 当前为主线程
                if (isMainThread) {
                    invokeSubscriber(subscription, event);
                } else {
                    mainThreadPoster.enqueue(subscription, event);
                }
                break;
            // 分线程
            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);
        }
 }

可以看出，针对当前线程和目标线程，分别做了处理。最终调用invokeSubscriber方法。

void invokeSubscriber(Subscription subscription, Object event) {
        try {
            // 反射调用订阅方法
            subscription.subscriberMethod.method.invoke(subscription.subscriber, event);
        } catch (InvocationTargetException e) {
            handleSubscriberException(subscription, event, e.getCause());
        } catch (IllegalAccessException e) {
            throw new IllegalStateException("Unexpected exception", e);
        }
}

四、线程变换

1、Main发送器
主要是在HandlerPoster类中进行处理

void enqueue(Subscription subscription, Object event) {
        // 将订阅关系与事件封装为PendingPost对象
        PendingPost pendingPost = PendingPost.obtainPendingPost(subscription, event);
        synchronized (this) {
            // 进入PendingPost队列
            queue.enqueue(pendingPost);
            if (!handlerActive) {
                handlerActive = true;
                // 发送消息开始执行
                if (!sendMessage(obtainMessage())) {
                    throw new EventBusException("Could not send handler message");
                }
            }
        }
}

// 消息处理
public void handleMessage(Message msg) {
        boolean rescheduled = false;
        try {
            long started = SystemClock.uptimeMillis();
            while (true) {
                // 出队
                PendingPost pendingPost = queue.poll();
                if (pendingPost == null) {
                    synchronized (this) {
                        // Check again, this time in synchronized
                        pendingPost = queue.poll();
                        if (pendingPost == null) {
                            // 队列为空
                            handlerActive = false;
                            return;
                        }
                    }
                }
                // 反射调用订阅方法
                eventBus.invokeSubscriber(pendingPost);
                long timeInMethod = SystemClock.uptimeMillis() - started;

                // 重新调度
                if (timeInMethod >= maxMillisInsideHandleMessage) {
                    if (!sendMessage(obtainMessage())) {
                        throw new EventBusException("Could not send handler message");
                    }
                    rescheduled = true;
                    return;
                }
            }
        } finally {
            handlerActive = rescheduled;
        }
}

2、Background发送器
使用线程池处理事件

private final static ExecutorService DEFAULT_EXECUTOR_SERVICE = Executors.newCachedThreadPool();

主要是在BackgroundPoster类中处理

public void enqueue(Subscription subscription, Object event) {
        // 将订阅关系与事件封装为PendingPost对象
        PendingPost pendingPost = PendingPost.obtainPendingPost(subscription, event);
        synchronized (this) {
            // 进入PendingPost队列
            queue.enqueue(pendingPost);
            if (!executorRunning) {
                executorRunning = true;
                // 线程池处理
                eventBus.getExecutorService().execute(this);
            }
        }
    }

public void run() {
        try {
            try {
                while (true) {
                    PendingPost pendingPost = queue.poll(1000);
                    if (pendingPost == null) {
                        synchronized (this) {
                            // Check again, this time in synchronized
                            pendingPost = queue.poll();
                            if (pendingPost == null) {
                                executorRunning = false;
                                return;
                            }
                        }
                    }
                    // 反射调用订阅方法
                    eventBus.invokeSubscriber(pendingPost);
                }
            } catch (InterruptedException e) {
                Log.w("Event", Thread.currentThread().getName() + " was interruppted", e);
            }
        } finally {
            executorRunning = false;
        }
}

3、Async发送器
在新建的子线程进行处理事件

private final PendingPostQueue queue;
    private final EventBus eventBus;

    AsyncPoster(EventBus eventBus) {
        this.eventBus = eventBus;
        queue = new PendingPostQueue();
    }

    public void enqueue(Subscription subscription, Object event) {
        PendingPost pendingPost = PendingPost.obtainPendingPost(subscription, event);
        queue.enqueue(pendingPost);
        eventBus.getExecutorService().execute(this);
    }

    @Override
    public void run() {
        PendingPost pendingPost = queue.poll();
        if(pendingPost == null) {
            throw new IllegalStateException("No pending post available");
        }
        eventBus.invokeSubscriber(pendingPost);
 }