整体流程阅读

EventBus在使用的时候基本分为以下几步：
1、注册订阅者

EventBus.getDefault().register(this);

2、订阅者解注册，否者会导致内存泄漏

EventBus.getDefault().unregister(this);

3、在订阅者中编写注解为Subscribe的事件处理函数

@Subscribe(threadMode = ThreadMode.MAIN, sticky = true, priority = 1)
public void onMsgEventReceived(MsgEvent event) {
     Toast.makeText(this, event.getMsg(), Toast.LENGTH_LONG).show();
}

4、事件发送

EventBus.getDefault().post("msg1 - coming！！！");

我们先按使用的流程大体看下源码逻辑，源码版本3.3.1：

1、注册源码逻辑

    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;
    }

EventBus 使用了双重校验锁的单例设计模式，保证用到的对象是唯一的，首次使用对象为空的时候通过下面构造创建一个。

    public EventBus() {
        this(DEFAULT_BUILDER);
    }

DEFAULT_BUILDER是一个final常量，在加载的时候就进行初始化，赋一个EventBusBuilder对象如下面代码所示。

private static final EventBusBuilder DEFAULT_BUILDER = new EventBusBuilder();

EventBusBuilder是EventBus的建造类，里面参数在加载的时候进行了初始化。

public class EventBusBuilder {
    private final static ExecutorService DEFAULT_EXECUTOR_SERVICE = Executors.newCachedThreadPool();

    boolean logSubscriberExceptions = true;
    boolean logNoSubscriberMessages = true;
    boolean sendSubscriberExceptionEvent = true;
    boolean sendNoSubscriberEvent = true;
    boolean throwSubscriberException;
    boolean eventInheritance = true;
    boolean ignoreGeneratedIndex;
    boolean strictMethodVerification;
    ExecutorService executorService = DEFAULT_EXECUTOR_SERVICE;
    List<Class<?>> skipMethodVerificationForClasses;
    List<SubscriberInfoIndex> subscriberInfoIndexes;
    Logger logger;
    MainThreadSupport mainThreadSupport;
    ...
}

如果有需要的话，我们也可以通过配置EventBusBuilder来更改EventBus的属性，在EventBus中有一个静态方法直接返回一直新的EventBusBuilder对象，设置完参数后调用build()来以新的配置来新建一个EventBus对象。

#EventBus
    public static EventBusBuilder builder() {
        return new EventBusBuilder();
    }

#EventBusBuilder
    /** Builds an EventBus based on the current configuration. */
    public EventBus build() {
        return new EventBus(this);
    }

然后通过下面的调用来设置：

EventBus.builder()
        .eventInheritance(false)
        .logSubscriberExceptions(false)
        .build()
        .register(this);

拿到EventBus对象以后，我们可以调用其register方法进行订阅者注册了。

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

首先获取订阅者类subscriber，然后通过findSubscriberMethods方法获取该类中以@Subscribe注解的函数，由于一个类中可能监听多个事件，因此获取的方法可能是多个，所有的方法赋值到一个List列表中，然后遍历这个列表进行注册。

    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);
        }
        subscribedEvents.add(eventType);

        ....
    }

上面就是注册最主要的代码，步骤解析如下：

• 根据注解方法获取监听事件的类型eventType，并对订阅者subscriber和订阅函数subscriberMethod建立一个订阅关系对象newSubscription；

• 根据eventType在subscriptionsByEventType（HashMap）中获取所有该事件类型的订阅关系列表subscriptions；

• 如果订阅关系列表subscriptions为空就新建一个，然后以key为eventType，value为newSubscription添加进去；

• 如果订阅关系列表subscriptions不为空，判断是否存在newSubscription，如果存在，说明之前已经注册过，抛出异常；

• 如果订阅关系列表subscriptions不为空，列表页没有订阅关系newSubscription，我们遍历添加进去，这里通过订阅函数的priority 来决定存放在列表中的位置，从这里也能看出priority越大，存放位置越靠前，和上一篇中分析的：值越大，优先级越高，越优先接收到事件。我们可以猜出是通过遍历这个表来进行事件发送的，在表里的位置越靠前，越先收到事件。

• 然后通过订阅者subscriber在另一个HashMap - typesBySubscriber中获取该订阅者订阅的所有事件，因为一个订阅者可以订阅多个不同的事件，因此获取的是个List列表subscribedEvents；

• 首先判断subscribedEvents列表是不是空的，如果是空说明以前没有订阅过任何事件，新建一个List，然后以key为subscriber，value为subscribedEvents添加到typesBySubscriber；

• 然后在新建的subscribedEvents中添加我们订阅的事件eventType。

这里出现了两个HashMap：subscriptionsByEventType与typesBySubscriber，通过上面的解析可以知道：
subscriptionsByEventType：一个事件可能有多个订阅者，key是事件，value是所有订阅该事件的所有的订阅者；
typesBySubscriber：一个订阅者可能订阅多个事件，key是订阅者，value是订阅者订阅的所有事件；
看命名就知道By后面的是key值，前面的是value值，一个好的命名就是这样吧。

2、解注册源码逻辑
在使用EventBus时，注册完后我习惯接着去写解注册的代码，怕后面会忘，因此按照写代码习惯在讲下解注册的源码实现。

    /** Unregisters the given subscriber from all event classes. */
    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());
        }
    }

如果是我们自己写这部分代码，根据前面注册的过程，我们肯定是要把添加到两个HashMap中的值移除掉。好了，看上面源码，首先在typesBySubscriber中获取该订阅者订阅的事件列表subscribedTypes；

如果为空说明该订阅者没有订阅任何事件，无任何操作；如果不为空，遍历所有事件调用unsubscribeByEventType方法进行解注册，然后在typesBySubscriber中移除这个订阅者subscriber。

    /** Only updates subscriptionsByEventType, not typesBySubscriber! Caller must update typesBySubscriber. */
    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--;
                }
            }
        }
    }

在上一个方法中，已经把typesBySubscriber这个订阅者移除了，那么unsubscribeByEventType函数就是遍历事件所有的订阅者，然后把解注册的订阅者在subscriptionsByEventType中给移除掉。上面的逻辑就是干这个事情。不过一边遍历一边移除是有风险的，这个大家要注意，index需要也跟着进行减少。

总结下：注册和解注册就是往两个HashMap添加和移除数据的过程。

3、事件发送post源码逻辑

    /** Posts the given event to the event bus. */
    public void post(Object event) {
        PostingThreadState postingState = currentPostingThreadState.get();
        List<Object> eventQueue = postingState.eventQueue;
        eventQueue.add(event);
        ····
    }

上面是部分post代码，第一行先解释这个变量currentPostingThreadState及内部类PostingThreadState。

    private final ThreadLocal<PostingThreadState> currentPostingThreadState = new ThreadLocal<PostingThreadState>() {
        @Override
        protected PostingThreadState initialValue() {
            return new PostingThreadState();
        }
    };


    /** For ThreadLocal, much faster to set (and get multiple values). */
    final static class PostingThreadState {
        final List<Object> eventQueue = new ArrayList<>();
        boolean isPosting;
        boolean isMainThread;
        Subscription subscription;
        Object event;
        boolean canceled;
    }

currentPostingThreadState是一个ThreadLocal，相当于是线程的私有财产，里面维护的变量只属于当前线程，线程间不会共享。维护的变量是一个自定义类PostingThreadState，用来保存发送线程的发送状态信息：当前线程是否为主线程，是否在发送事件，发送的事件列表、接收事件的订阅者等。

    /** Posts the given event to the event bus. */
    public void post(Object event) {
        PostingThreadState postingState = currentPostingThreadState.get();
        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 {
                while (!eventQueue.isEmpty()) {
                    postSingleEvent(eventQueue.remove(0), postingState);
                }
            } finally {
                postingState.isPosting = false;
                postingState.isMainThread = false;
            }
        }
    }

现在继续看post逻辑。首先，获取到当前线程的发送状态postingState，然后拿到事件列表，并把需要post的事件加入到列表中。判断是否启动了事件发送流程，如果已经启动了，不在做处理，加入列表中的事件会轮到处理。如果没有启动就启动处理流程，并将isPosting赋值为true。循环从事件列表中获取事件，通过postSingleEvent进行处理。

    private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
        Class<?> eventClass = event.getClass();
        boolean subscriptionFound = false;
        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));
            }
        }
    }

在postSingleEvent中，有一个变量的判断eventInheritance，这个变量表示是否查找发送事件的父类或接口的订阅者，默认是true。例如，发送事件MsgEvent这个对象，MsgEvent继承至Event这个抽象类，发送后，不光所有订阅MsgEvent的订阅者可以收到事件，所有订阅Event的这个事件的订阅者也会收到事件。前面有分析这个字段的值是可以重新配置的，代码如下。

EventBus.builder()
        .eventInheritance(false)   //发送的时候不考虑事件父类
        .logSubscriberExceptions(false)
        .build()
        .register(this);

eventInheritance为true，通过lookupAllEventTypes，向上找到所有父类事件类，然后遍历找到所有事件的订阅者，并发送事件，如果为false，直接将当前事件发送给订阅者。

postSingleEvent函数主要是找到需要post的所有相关事件，然后进一步调用postSingleEventForEventType发送给订阅者，因此函数后面的逻辑是判断postSingleEventForEventType的返回值subscriptionFound，即判断当前事件有没有订阅者进行处理，如果没有处理的，会发送一个NoSubscriberEvent。例如：如果我在上一节的例子中post一个新的对象student，但是没有订阅者和接收函数，如果监听了NoSubscriberEvent，会收到一个NoSubscriberEvent的事件，告知调用者你post的对象没有订阅者。

@Subscribe()
public void onMsgEventReceived(NoSubscriberEvent event) {
    Log.i(TAG, "NoSubscriberEvent : " + event);
}

Student student = new Student(1,"jane");
EventBus.getDefault().post(student);

private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass) {
        CopyOnWriteArrayList<Subscription> subscriptions;
        synchronized (this) {
            subscriptions = subscriptionsByEventType.get(eventClass);
        }
        if (subscriptions != null && !subscriptions.isEmpty()) {
            for (Subscription subscription : subscriptions) {
                postingState.event = event;
                postingState.subscription = subscription;
                boolean aborted;
                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;
    }

从前面分析可知，最后都走到了postSingleEventForEventType这个函数里面，首先在subscriptionsByEventType中找到所有订阅该事件的订阅者subscriptions，subscriptions为空或者个数是0则返回false。有订阅者的话，遍历订阅者，然后通过postToSubscription进行发送，并返回true。

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);
        }
    }

这段代码比较简单就是根据订阅者中注解@Subscribe中threadMode值来分别进行处理，上一节也讲过，POSTING表示在哪个线程发送就在哪个线程接收处理，因此直接调用invokeSubscriber通过反射来调用订阅者中的接收事件的方法。

    void invokeSubscriber(Subscription subscription, Object event) {
        try {
            subscription.subscriberMethod.method.invoke(subscription.subscriber, event);
        } catch (InvocationTargetException e) {
            handleSubscriberException(subscription, event, e.getCause());
        } catch (IllegalAccessException e) {
            throw new IllegalStateException("Unexpected exception", e);
        }
    }

如果threadMode是MAIN，而当前线程是子线程，通过 mainThreadPoster.enqueue(subscription, event)将事件加入主线程队列。

mainThreadSupport = builder.getMainThreadSupport();
mainThreadPoster = mainThreadSupport != null ? mainThreadSupport.createPoster(this) : null;

public interface MainThreadSupport {
    boolean isMainThread();
    Poster createPoster(EventBus eventBus);
}

public class DefaultAndroidMainThreadSupport implements MainThreadSupport {
    public boolean isMainThread() {
        return Looper.getMainLooper() == Looper.myLooper();
    }

    @Override
    public Poster createPoster(EventBus eventBus) {
        return new HandlerPoster(eventBus, Looper.getMainLooper(), 10);
    }
}

mainThreadPoster是由mainThreadSupport创建的，mainThreadSupport是MainThreadSupport实例，而MainThreadSupport是一个接口，实现类为DefaultAndroidMainThreadSupport，因此，最终是调用到的DefaultAndroidMainThreadSupport中的createPoster，新建了一个HandlerPoster。

public class HandlerPoster extends Handler implements Poster {
    private final PendingPostQueue queue;
    private final int maxMillisInsideHandleMessage;
    private final EventBus eventBus;
    private boolean handlerActive;

    public HandlerPoster(EventBus eventBus, Looper looper, int maxMillisInsideHandleMessage) {
        super(looper);
        this.eventBus = eventBus;
        this.maxMillisInsideHandleMessage = maxMillisInsideHandleMessage;
        queue = new PendingPostQueue();
    }

    public void enqueue(Subscription subscription, Object event) {
        PendingPost pendingPost = PendingPost.obtainPendingPost(subscription, event);
        synchronized (this) {
            queue.enqueue(pendingPost);
            if (!handlerActive) {
                handlerActive = true;
                if (!sendMessage(obtainMessage())) {
                    throw new EventBusException("Could not send handler message");
                }
            }
        }
    }

    @Override
    public void handleMessage(Message msg) {
      ...
    }
}

从上面的源码可以看到，HandlerPoster就是一个Handler，当执行mainThreadPoster.enqueue(subscription, event)时，会将订阅者和事件封装成一个PendingPost，然后加入到PendingPostQueue这个队列中，如果handlerActive为true表示当前Handler正常处理事件，将入队列的事件等着被处理即可。如果为false则启动处理，调用sendMessage发送消息。

public class HandlerPoster extends Handler implements Poster {
    private final PendingPostQueue queue;
    private final int maxMillisInsideHandleMessage;
    private final EventBus eventBus;
    private boolean handlerActive;

    public HandlerPoster(EventBus eventBus, Looper looper, int maxMillisInsideHandleMessage) {
        super(looper);
        ...
    }

    public void enqueue(Subscription subscription, Object event) {
       ...
    }

    @Override
    public void handleMessage(Message msg) {
      boolean rescheduled = false;
        try {
            long started = SystemClock.uptimeMillis();
            while (true) {
                PendingPost pendingPost = queue.poll();
                if (pendingPost == null) {
                    synchronized (this) {
                        // Check again, this time in synchronized
                        pendingPost = queue.poll();
                        if (pendingPost == null) {
                            handlerActive = false;
                            return;
                        }
                    }
                }
                eventBus.invokeSubscriber(pendingPost);
                long timeInMethod = SystemClock.uptimeMillis() - started;
                if (timeInMethod >= maxMillisInsideHandleMessage) {
                    if (!sendMessage(obtainMessage())) {
                        throw new EventBusException("Could not send handler message");
                    }
                    rescheduled = true;
                    return;
                }
            }
        } finally {
            handlerActive = rescheduled;
        }
    }
}

在handleMessage()方法将中将PendingPost对象循环出队列，交给invokeSubscriber()方法进一步处理。这样就把线程通过Handler切回了主线程。

backgroundPoster.enqueue()和asyncPoster.enqueue也类似，内部都是先将事件入队列，然后再出队列，但是会通过线程池去进一步处理事件。

4、粘性事件发送postSticky源码逻辑

    public void postSticky(Object event) {
        synchronized (stickyEvents) {
            stickyEvents.put(event.getClass(), event);
        }
        // Should be posted after it is putted, in case the subscriber wants to remove immediately
        post(event);
    }

从上面代码可以看到，先把事件放在了stickyEvents列中中，然后调用了post，也就是上面我们解析过的流程。让我们再来回顾下注册的代码：

// Must be called in synchronized block
    private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
        Class<?> eventType = subscriberMethod.eventType;
        Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
        CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
        ...
        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);
            }
        }
    }

注册源码分析的时候，就分析到subscriberMethod.sticky这个句上面，现在让我们看看下面的逻辑，同样先判断eventInheritance的值，然后将之前放在stickyEvents中的事件拿出来，执行checkPostStickyEventToSubscription。

private void checkPostStickyEventToSubscription(Subscription newSubscription, Object stickyEvent) {
        if (stickyEvent != null) {
            // If the subscriber is trying to abort the event, it will fail (event is not tracked in posting state)
            // --> Strange corner case, which we don't take care of here.
            postToSubscription(newSubscription, stickyEvent, isMainThread());
        }
    }

checkPostStickyEventToSubscription中的代码比较水，又调用了postToSubscription，这个方法上面贴出来了，这里方面看再贴一遍，熟悉的味道：

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);
        }
    }

所以，在注册的时候，如果你订阅者的接收方法加了sticky注解，那么在注册的时候就会看下订阅的事件之前有没有通过postSticky发送过，如果有就会立马收到这个事件。

以上是我们平时使用过程的源码解析，码字不易，喜欢就点赞收藏啊。

参考文章：
EventBus 原理解析