EventBus的原理

小知识

观察者模式 :定义对象间一种一对多的依赖关系,使得每当一个对象改变状态,则所有依赖它的对象都会得到通知并自动更新。三个角色:
1.Subject:就是“被观察”的角色,它将所有观察者对象的引用保存在一个集合中。
2.Observer:是抽象的“观察”角色,它定义了一个更新接口,使得在被观察者状态发生改变时通知自己。
3.ConcreteObserver:具体的观察者。

承接上一篇基本用法的例子来理解这三个角色,其实就是Subject(被观察者)是考试中班里的那个优生,订阅的方法就是提前和人家打好招呼,写完了答案发过来。ConcreteObserve(观察者)就是提前和大神说好的我们这些考试等答案的人。大神做完一条短信发给众多同学,这就是一个发布过程。
这个设计模式也是为了实现上一篇的小知识 解耦。
书归正传。。。

原理图

这是EventBus的官方原理图,抄一下


image.png

这就是个流程图 流程图也可以大概看出 这里使用的是观察者模式。
Publisher就是那个大神,我们就是那些Subscriber。这些Event就是答案。

看下源码

注解 @Subscribe

package org.greenrobot.eventbus;


import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.METHOD})
public @interface Subscribe {
    ThreadMode threadMode() default ThreadMode.POSTING;

    /**
     * If true, delivers the most recent sticky event (posted with
     * {@link EventBus#postSticky(Object)}) to this subscriber (if event available).
     */
    boolean sticky() default false;

    /** Subscriber priority to influence the order of event delivery.
     * Within the same delivery thread ({@link ThreadMode}), higher priority subscribers will receive events before
     * others with a lower priority. The default priority is 0. Note: the priority does *NOT* affect the order of
     * delivery among subscribers with different {@link ThreadMode}s! */
    int priority() default 0;
}

ThreadMode threadMode() default ThreadMode.POSTING;

很直接 设置默认的模式为POSTING,这个上篇的例子讲过,就是接收答案的那些人和大神在一个教室(接收者和发布者在一个线程中)。

boolean sticky() default false;

设置粘性事件开关默认为false,就是我默认还没有注册的情况下不先接收消息。

int priority() default 0;

设置默认优先级为0,这个数越大优先级越高,优先级高的在同一模式下会优先收到消息。

注册 EventBus.getDefault().register(this);

这里先看下EventBus.getDefault()

  /** Convenience singleton for apps using a process-wide EventBus instance. */
    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;
    }

我这少儿英语水平看最上面的英文也猜的出来,这是一个线程安全的单例模式。
其实没什么特别的,跟我们平时写一个单例一样。只是里面的

instance = EventBus.defaultInstance = new EventBus();

需要看下:

   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;
    }

这个也眼熟,其实就是初始化一些设置参数,这里使用的是构建者模式 。就和大神问你怎么传答案一样。是短信还是小抄,只要选择题还是只要最后几道大题,要能及格还是考高一点。一切满足客户需求。你不设置就是一个默认套餐,你要做一些私人定制 那就自定义:

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

都商量好了那就定下来了

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

Class subscriberClass = subscriber.getClass();

这个好理解 拿到注册我的那个类。相当于找到一份学生名单

List subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);

这个里面 SubscriberMethod 这个家伙里装的就是那些私人定制(是否黏性事件啦,线程模式啦,优先级啦),没有私人定制就是默认值。

然后 findSubscriberMethods 方法就是在注册的那个类里寻找
Subscribe注解、有public修饰符、一个参数的方法。相当于在这份学生名单中找到需要答案的那部分学生。

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

最后遍历这些订阅了的方法 完成注册。相当于确认这些人,准备好写完答案发给他们。

我们看下findSubscriberMethods方法 看看具体是怎么找的

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

        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 {
            METHOD_CACHE.put(subscriberClass, subscriberMethods);
            return subscriberMethods;
        }
    }

List subscriberMethods = METHOD_CACHE.get(subscriberClass);

这里我们看下 METHOD_CACHE

private static final Map, List> METHOD_CACHE = new ConcurrentHashMap<>();

就是一个存储订阅者和其中订阅方法的类,这样做应该是为了重复查找时的效率。这里就相当于将找到班级里对应需要答案的人,记下来.

我们先看

if (ignoreGeneratedIndex) {subscriberMethods =
findUsingReflection(subscriberClass);
} else {
subscriberMethods = findUsingInfo(subscriberClass);
}

ignoreGeneratedIndex在这里时一个注解管理器 这个下次说

findUsingInfo
findUsingInfo()方法会在当前要注册的类以及其父类中查找订阅事件的方法,这里出现了一个FindState类,它是SubscriberMethodFinder的内部类,用来辅助查找订阅事件的方法,具体的查找过程在findUsingReflectionInSingleClass()方法里,它主要通过反射来查找订阅事件的方法。

private void findUsingReflectionInSingleClass(FindState findState) {
        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;
        }
        // 循环遍历当前类的方法,筛选出符合条件的
        for (Method method : methods) {
            // 获得方法的修饰符
            int modifiers = method.getModifiers();
            // 如果是public类型,但非abstract、static等
            if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
                // 获得当前方法所有参数的类型
                Class[] parameterTypes = method.getParameterTypes();
                // 如果当前方法只有一个参数
                if (parameterTypes.length == 1) {
                    Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);
                    // 如果当前方法使用了Subscribe注解
                    if (subscribeAnnotation != null) {
                        // 得到该参数的类型
                        Class eventType = parameterTypes[0];
                        // checkAdd()方法用来判断FindState的anyMethodByEventType map是否已经添加过以当前eventType为key的键值对,没添加过则返回true
                        if (findState.checkAdd(method, eventType)) {
                             // 得到Subscribe注解的threadMode属性值,即线程模式
                            ThreadMode threadMode = subscribeAnnotation.threadMode();
                            // 创建一个SubscriberMethod对象,并添加到subscriberMethods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");
            }
        }
    }

再来看

if (subscriberMethods.isEmpty()) {
throw new EventBusException("Subscriber " + subscriberClass
+ " and its super classes have no public methods with the @Subscribe annotation");
} else {
METHOD_CACHE.put(subscriberClass, subscriberMethods);
return subscriberMethods;
}

再接着就是判断,没人订阅的时候就抛出一个异常,有人订阅的时候就把这个人从名单中记到自己小本本上(METHOD_CACHE缓存中)

注册方法

private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
        // 得到当前订阅了事件的方法的参数类型
        Class eventType = subscriberMethod.eventType;
        // Subscription类保存了要注册的类对象以及当前的subscriberMethod
        Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
        // subscriptionsByEventType是一个HashMap,保存了以eventType为key,Subscription对象集合为value的键值对
        // 先查找subscriptionsByEventType是否存在以当前eventType为key的值
        CopyOnWriteArrayList subscriptions = subscriptionsByEventType.get(eventType);
        // 如果不存在,则创建一个subscriptions,并保存到subscriptionsByEventType
        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);
            }
        }
        // 添加上边创建的newSubscription对象到subscriptions中
        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;
            }
        }
        // typesBySubscribere也是一个HashMap,保存了以当前要注册类的对象为key,注册类中订阅事件的方法的参数类型的集合为value的键值对
        // 查找是否存在对应的参数类型集合
        List> subscribedEvents = typesBySubscriber.get(subscriber);
        // 不存在则创建一个subscribedEvents,并保存到typesBySubscriber
        if (subscribedEvents == null) {
            subscribedEvents = new ArrayList<>();
            typesBySubscriber.put(subscriber, subscribedEvents);
        }
        // 保存当前订阅了事件的方法的参数类型
        subscribedEvents.add(eventType);
        // 粘性事件相关的,后边具体分析
        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).
                Set, Object>> entries = stickyEvents.entrySet();
                for (Map.Entry, 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()方法主要是得到了subscriptionsByEventType、typesBySubscriber两个 HashMap。其中,发送事件的时候要用到subscriptionsByEventType来完成事件的处理。当取消 EventBus 注册的时候要用到typesBySubscriber、subscriptionsByEventType,完成相关资源的释放。

网上这样说的居多,其实大白话就是 :
订阅的过程是把之后要做的需要的事准备好,比如发送需要的 按照优先级排列顺序,这样发送的时候按照优先级好发送 把粘性事件的处理下 把取消注册的放到typesBySubscriber中,方便之后取消注册时处理typesBySubscriber

取消注册

EventBus.getDefault().unregister(this);

   /** Unregisters the given subscriber from all event classes. */
    public synchronized void unregister(Object subscriber) {
        List> 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());
        }
    }

List> subscribedTypes = typesBySubscriber.get(subscriber);

得到当前注册类对象 对应的 订阅事件方法的参数类型 的集合;
typesBySubscriber这个上面注册方法里刚提到说过,初始化了然后再解除订阅的时候要用,这里用到了

for (Class eventType : subscribedTypes) {
unsubscribeByEventType(subscriber, eventType);
}

遍历参数类型集合,释放之前缓存的当前类中的Subscription

typesBySubscriber.remove(subscriber);

然后删除这个key对应的键值对儿

logger.log(Level.WARNING, "Subscriber to unregister was not registered before: " + subscriber.getClass());

字面意思也看得出来 这个报错是之前没注册无法删除

unsubscribeByEventType(subscriber, eventType);

private void unsubscribeByEventType(Object subscriber, Class eventType) {
        // 得到当前参数类型对应的Subscription集合
        List subscriptions = subscriptionsByEventType.get(eventType);
        if (subscriptions != null) {
            int size = subscriptions.size();
            // 遍历Subscription集合
            for (int i = 0; i < size; i++) {
                Subscription subscription = subscriptions.get(i);
                // 如果当前subscription对象对应的注册类对象 和 要取消注册的注册类对象相同,则删除当前subscription对象
                if (subscription.subscriber == subscriber) {
                    subscription.active = false;
                    subscriptions.remove(i);
                    i--;
                    size--;
                }
            }
        }
    }

删除的目的就是
释放typesBySubscriber、subscriptionsByEventType中缓存的资源。

发送事件

EventBus.getDefault().post("Hello AZ!")

post发放发送时间 源码

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

PostingThreadState postingState = currentPostingThreadState.get();
List eventQueue = postingState.eventQueue;
eventQueue.add(event);

这里的PostingThreadState类保存了事件队列和线程模式等信息 你是以哪种线程模式处理 并且新建了一个队列 把事件添加了进去

 if (!postingState.isPosting) {
            // 是否为主线程
            postingState.isMainThread = isMainThread();
            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()) {
                    // 发送单个事件
                    // eventQueue.remove(0),从事件队列移除事件
                    postSingleEvent(eventQueue.remove(0), postingState);
                }
            } finally {
                postingState.isPosting = false;
                postingState.isMainThread = false;
            }
        }

这里就是就是判断线程,然后遍历事件 把事件通过postSingleEvent方法发送出去

其实post发放是把消息添加到了list中 然后遍历之后通过
postSingleEvent方法处理。

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

其实这里也没处理 而是判断了下这个类的父类有没有接收方法,决定是否向上传递事件 然后把事件交给了postSingleEventForEventType去处理

    private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class eventClass) {
        CopyOnWriteArrayList 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;
    }

CopyOnWriteArrayList subscriptions;
synchronized (this) {
subscriptions = subscriptionsByEventType.get(eventClass);
}

这里是通过事件类型这个key去map里找对应的集合

postingState.event = event;
postingState.subscription = subscription;

保存下来subscription 和event 之后 交给postToSubscription做最终处理

    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);
        }
    }

switch (subscription.subscriberMethod.threadMode)

这里首先判断了一下线程模式是哪种 然后

case POSTING:
invokeSubscriber(subscription, event);
break;

如果是POSTING模式 则发送的在哪个线程 就在那个线程下处理

case MAIN: if (isMainThread) {
invokeSubscriber(subscription, event);
} else {
mainThreadPoster.enqueue(subscription, event);
}
break;

如果是MAIN模式 如果在主线程就在主线程处理 如果发送者在子线程 就将事件放入消息队列 通过线程切换到主线程处理

case BACKGROUND:

            if (isMainThread) {
                backgroundPoster.enqueue(subscription, event);
            } else {
                // 如果在子线程发送事件,则直接在发送事件的线程通过反射处理事件
                invokeSubscriber(subscription, event);
            }
            break;

如果是BACKGROUND模式 如果发送者在主线程 则入消息队列 在子线程处理 如果发送者在子线程 则直接处理

case ASYNC:
asyncPoster.enqueue(subscription, event);
break;

如果是ASYNC模式 则将消息如消息队列 通过线程池处理

这里的postToSubscription()方法就是根据发送者的线程和当线程模式来处理事件,处理方式有两种,一种是invokeSubscriber()方法 ,一种是进线程池处理

进线程池处理就是把事件包装成了BackgroundPoster对象 然后BackgroundPoster类内就是一个线程池实现 然后去处理

final class BackgroundPoster implements Runnable, Poster {

    private final PendingPostQueue queue;
    private final EventBus eventBus;

    private volatile boolean executorRunning;

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

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

    @Override
    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;
                            }
                        }
                    }
                    eventBus.invokeSubscriber(pendingPost);
                }
            } catch (InterruptedException e) {
                eventBus.getLogger().log(Level.WARNING, Thread.currentThread().getName() + " was interruppted", e);
            }
        } finally {
            executorRunning = false;
        }
    }

}

他继承了Runnable接口 然后run里实现处理方法

至于invokeSubscriber()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);
        }
    }

没啥说的 就是用反射来执行订阅事件的方法,消息就传到了方法中。

粘性事件

EventBus.getDefault().postSticky("Hello AZ!");

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);
    }

这个方法将事件类型和对应事件保存到stickyEvents中,方便后续使用;然后执行post(event)继续发送事件,这个post()方法就是之前发送的post()方法

private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
        ......
        ......
        ......
        // 如果当前订阅事件的方法的Subscribe注解的sticky属性为true,即该方法可接受粘性事件
        if (subscriberMethod.sticky) {
            // 默认为true,表示是否向上查找事件的父类
            if (eventInheritance) {
                // stickyEvents就是发送粘性事件时,保存了事件类型和对应事件
                Set, Object>> entries = stickyEvents.entrySet();
                for (Map.Entry, Object> entry : entries) {
                    Class candidateEventType = entry.getKey();
                    // 如果candidateEventType是eventType的子类或
                    if (eventType.isAssignableFrom(candidateEventType)) {
                        // 获得对应的事件
                        Object stickyEvent = entry.getValue();
                        // 处理粘性事件
                        checkPostStickyEventToSubscription(newSubscription, stickyEvent);
                    }
                }
            } else {
                Object stickyEvent = stickyEvents.get(eventType);
                checkPostStickyEventToSubscription(newSubscription, stickyEvent);
            }
        }
    }

之前的post()方法中 有执行粘性事件的
可以看到,处理粘性事件就是在 EventBus 注册时,遍历stickyEvents,如果当前要注册的事件订阅方法是粘性的,并且该方法接收的事件类型和stickyEvents中某个事件类型相同或者是其父类,则取出stickyEvents中对应事件类型的具体事件,交给checkPostStickyEventToSubscription()处理

    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());
        }
    }

这个方法中 只是判断了一下粘性事件不为空的话,就调用postToSubscription()方法 postToSubscription()方法就是上面讲过的postToSubscription()方法 最终会判断线程模式将消息发送到接受方法里
这种方式实现了在注册之前 发送了粘性事件
最后 附上原理图

EventBus.png

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