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EventBus 源码分析
|Android
字数总计: 4.9k|阅读时长: 21 分钟

摘要

本文分析Android主流的事件框架EventBus，尤其在不同的界面触发函数的调用，避免层层嵌套回调。通过源码分析，了解EventBus如何查找订阅方法，切换线程，发布事件的原理。

前言

我们将根据下面的例子来分析EventBus的源码。

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class MainActivity : AppCompatActivity() {
    private val TAG = "MainActivity"

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        EventBus.getDefault().register(this);
        EventBus.getDefault().post(1)
    }

    @Subscribe(threadMode = ThreadMode.MAIN)
    fun get(event:Int){

    }
  
    override fun onDestroy() {
        super.onDestroy()
        EventBus.getDefault().unregister(this)
    }
}

EventBus.getDefault

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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;
}

通过双重检查单例模式创建EventBus单例。

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public EventBus() {
    this(DEFAULT_BUILDER);
}

EventBus(EventBusBuilder builder) {
    logger = builder.getLogger();
    subscriptionsByEventType = new HashMap<>();//通过事件类型获取所有订阅信息
    typesBySubscriber = new HashMap<>();//通过订阅类获取该订阅类中所有的事件类型
    stickyEvents = new ConcurrentHashMap<>();
    mainThreadSupport = builder.getMainThreadSupport();//支持Android主线程
    mainThreadPoster = mainThreadSupport != null ? mainThreadSupport.createPoster(this) : null;//Handler子类
    backgroundPoster = new BackgroundPoster(this);
    asyncPoster = new AsyncPoster(this);
    indexCount = builder.subscriberInfoIndexes != null ? builder.subscriberInfoIndexes.size() : 0;
  	//用于寻找订阅类中的订阅方法
    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;
}

DEFAULT_BUILDER是EventBusBuilder的实例，所有配置都采用默认值。这里涉及到几个主要对象的创建，例如HandlerPoster对象mainThreadPoster,其实是用来切换到主线程的。

register

接着我们看EventBus如何将一个类与订阅事件绑定。查看EventBus的register函数。

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

register函数通过SubscriberMethodFinder对象findSubscriberMethods函数，获取当前订阅类所有的订阅方法subscriberMethods。然后遍历所有订阅方法SubscriberMethod，然后调用subscribe函数，将当前订阅类和订阅方法进行绑定。

findSubscriberMethods

我们查看SubscriberMethodFinder对象如何通过findSubscriberMethod函数如何寻找当前订阅类中合法的订阅方法，并封装成SubscriberMethod实例。

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List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
  	//先通过缓存获取该订阅类是否解析过（例如Activity销毁后，重新进入）
    List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
    if (subscriberMethods != null) {
        return subscriberMethods;
    }

    if (ignoreGeneratedIndex) {//默认false
        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;
    }
}

这里的METHOD_CACHE是ConcurrentHashMap类型,键值对分别是Class<?>, List<SubscriberMethod>,也就是findSubscriberMethods函数会查找订阅方法，然后缓存到METHOD_CACHE中，下次优先通过通过缓存查找。初次注册，那肯定是没有缓存。ignoreGeneratedIndex 默认情况下是flase，所以会调用findUsingInfo(subscriberClass)查找类中的注册方法。

findUsingInfo

findUsingInfo函数会将相关的信息封装到FindState对象，最后获取到订阅方法SubscriberMethod列表后释放FindState对象。

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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);
}

通过prepareFindState函数返回了FindState对象，该函数主要遍历大小为4的FIND_STATE_POOL数组，取第一个不为null的FindState对象,并将数组该位置引用置为null，如果数组不存在元素，则直接新建一个FindState对象。

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private FindState prepareFindState() {
    synchronized (FIND_STATE_POOL) {
        for (int i = 0; i < POOL_SIZE; i++) {
            FindState state = FIND_STATE_POOL[i];
            if (state != null) {
                FIND_STATE_POOL[i] = null;
                return state;
            }
        }
    }
    return new FindState();
}

然后调用FindState对象的initForSubscriber函数。将当前订阅类类型赋值FindState对象的subscriberClass和clazz变量。

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void initForSubscriber(Class<?> subscriberClass) {
    this.subscriberClass = clazz = subscriberClass;
    skipSuperClasses = false;
    subscriberInfo = null;
}

回到findUsingInfo函数，由于clazz被赋值为subscriberClass,所以while循环为ture。接着调用了getSubscriberInfo(findState)函数。

getSubscriberInfo

在getSubscriberInfo(findState)函数中，由于第一次注册该类，所以FindState对象的subscriberInfo=null。而subscriberInfoIndexes在SubscriberMethodFinder实例创建的时候传递进来。subscriberInfoIndexes是通过EventBusBuilder传递进来的，默认为null。所以getSubscriberInfo函数返回了null。

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private SubscriberInfo getSubscriberInfo(FindState findState) {
    if (findState.subscriberInfo != null && findState.subscriberInfo.getSuperSubscriberInfo() != null) {
        SubscriberInfo superclassInfo = findState.subscriberInfo.getSuperSubscriberInfo();
        if (findState.clazz == superclassInfo.getSubscriberClass()) {
            return superclassInfo;
        }
    }
    if (subscriberInfoIndexes != null) {
        for (SubscriberInfoIndex index : subscriberInfoIndexes) {
            SubscriberInfo info = index.getSubscriberInfo(findState.clazz);
            if (info != null) {
                return info;
            }
        }
    }
    return null;
}

findUsingReflectionInSingleClass

回到[findUsingInfo](# findUsingInfo)函数中，由于findState.subscriberInfo=null,将执行findUsingReflectionInSingleClass(findState)函数。

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private void findUsingReflectionInSingleClass(FindState findState) {
    Method[] methods;
    try {
        methods = findState.clazz.getDeclaredMethods();
    } catch (Throwable th) {
        try {
            methods = findState.clazz.getMethods();
        } catch (LinkageError error) { 
           ...
        }
        findState.skipSuperClasses = true;//该标志位会在moveToSuperclass函数用到
    }
    for (Method method : methods) {
      	//获取方法修饰符
        int modifiers = method.getModifiers();
      	//public修饰且非编译器生成
        if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
          	//获取所有参数类型
            Class<?>[] parameterTypes = method.getParameterTypes();
            if (parameterTypes.length == 1) {//参数长度为1
                Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);
                if (subscribeAnnotation != null) {//方法被Subscribe注解
                    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");
        }
    }
}

getDeclaredMethods函数是获取订阅类自身声明的所有方法，包括public、protected、private方法,而getMethods函数是类所有公共方法，自身、父类和接口的公共方法。如果前者抛出异常，将采用后者，并设置findState.skipSuperClasses = true。

接着遍历所有获取的方法，访问方法的修饰符。判断条件(modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0表示当前方法是public修饰符且不是编译器生成的。然后通过method.getParameterTypes()函数，获取方法的参数类型列表，这里只是用来判断参数的数量，当参数数量为1才继续下一步。

然后通过method.getAnnotation(Subscribe.class)函数判断方法被Subscribe注解着。然后调用了FindState对象的checkAdd函数，将方法method和事件类型eventType（参数类型）作为参数传递 。

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boolean checkAdd(Method method, Class<?> eventType) {
    Object existing = anyMethodByEventType.put(eventType, method);
    if (existing == null) {
        return true;
    } else {
        if (existing instanceof Method) {
            if (!checkAddWithMethodSignature((Method) existing, eventType)) {
                throw new IllegalStateException();
            }
            anyMethodByEventType.put(eventType, this);
        }
        return checkAddWithMethodSignature(method, eventType);
    }
}

anyMethodByEventType是HashMap类型，键值对是<Class, Object>。将注解类型和方法put到anyMethodByEventType，如果返回null，说明该类只有一个参数的类型为该eventType的方法，没有冲突，直接返回ture。如果返回了existing对象不为null，说明类中已经找到了相同参数类型的方法，进一步检查方法签名。

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private boolean checkAddWithMethodSignature(Method method, Class<?> eventType) {
    methodKeyBuilder.setLength(0);
    methodKeyBuilder.append(method.getName());
    methodKeyBuilder.append('>').append(eventType.getName());

    String methodKey = methodKeyBuilder.toString();//methodName>paramTypeName
    Class<?> methodClass = method.getDeclaringClass();//获取方法所在的类
    Class<?> methodClassOld = subscriberClassByMethodKey.put(methodKey, methodClass);
    if (methodClassOld == null || methodClassOld.isAssignableFrom(methodClass)) {
        return true;
    } else {
        subscriberClassByMethodKey.put(methodKey, methodClassOld);
        return false;
    }
}

checkAddWithMethodSignature函数通过put元素到subscriberClassByMethodKey来判断前面是否有已存在的方法，subscriberClassByMethodKey也是HashMap类型，键值对是Map<String, Class>。其put函数返回null表明前面没有签名（methodName>paramTypeName）相同的方法,或者存在且其值是当前订阅类的父类，父接口或者相同类。

因此checkAdd函数也称为第二层检查EventBus中类的合法注册。checkAdd函数返回true之后，将相关信息封装到SubscriberMethod对象中，并添加到findState.subscriberMethods列表中。

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ThreadMode threadMode = subscribeAnnotation.threadMode();
findState.subscriberMethods.add(new SubscriberMethod(method, eventType, threadMode,
     subscribeAnnotation.priority(), subscribeAnnotation.sticky()));

**到这里确定了如何注解一个合法的方法： **

  • 注解方法必须是public修饰且非编译器生成，只有一个参数,并且Subscribe注解该方法
  • 不存在相同签名（methodName>paramTypeName）
  • 若已存在的注册类则得是方法所在的类或者父类

moveToSuperclass

回到findUsingInfo函数。调用了FindState对象的moveToSuperclass函数。

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void moveToSuperclass() {
    if (skipSuperClasses) {//findUsingReflectionInsingleClass函数，默认情况getDeclaredMethods函数获取当前类自身声明方法，发生异常情况下调用getMethods函数获取所有方法，会将skipSuperClasses设为true
        clazz = null;
    } else {
        clazz = clazz.getSuperclass();
        String clazzName = clazz.getName();
        //如果当前类是系统类，直接跳过
        if (clazzName.startsWith("java.") || clazzName.startsWith("javax.") ||
                clazzName.startsWith("android.") || clazzName.startsWith("androidx.")) {
            clazz = null;
        }
    }
}

注意到这里还在while循环中，正常情况skipSuperClasses=true,clazz将赋值为当前订阅类的父类，然后重复寻找父类的订阅方法。

findUsingInfo函数最后调用了getMethodsAndRelease函数返回SubscriberMethod列表，将FindState对象进行回收。

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private List<SubscriberMethod> getMethodsAndRelease(FindState findState) {
    List<SubscriberMethod> subscriberMethods = new ArrayList<>(findState.subscriberMethods);
    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;
}

subscribe

回到register函数。寻找到SubscriberMethod列表后，会遍历每个SubscribeMethod，并调用subscribe函数，将订阅者与订阅方法绑定。

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private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
    Class<?> eventType = subscriberMethod.eventType;
    Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
  	//通过参数类型获取已存在的订阅信息
    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);
        }
    }
		//将方法的订阅信息添加到subscriptions列表合适的位置，
    int size = subscriptions.size();
    for (int i = 0; i <= size; i++) {
      	//最后的位置或者根据优先级排序
        if (i == size || subscriberMethod.priority > subscriptions.get(i).subscriberMethod.priority) {
            subscriptions.add(i, newSubscription);
            break;
        }
    }
		//typesBySubscriber保存当前类的订阅事件（方法的参数类型）
    List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
    if (subscribedEvents == null) {
        subscribedEvents = new ArrayList<>();
        typesBySubscriber.put(subscriber, subscribedEvents);
    }
    subscribedEvents.add(eventType);
		//判断当前方法订阅的是黏性事件
    if (subscriberMethod.sticky) {
        if (eventInheritance) {//默认true,表示黏性事件可以从父类继承
            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);
        }
    }
}

subscribe函数相对来说比较简单，已经在上面代码中注释一些信息。看下最后的checkPostStickyEventToSubscription函数，如何发送黏性事件。

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private void checkPostStickyEventToSubscription(Subscription newSubscription, Object stickyEvent) {
    if (stickyEvent != null) {
        postToSubscription(newSubscription, stickyEvent, isMainThread());
    }
}

可见调用了postToSubscription函数，发送了一次事件，可见，如果订阅的事黏性事件，会出发最后一次黏性消息事件。这里不继续分析，因为和后面的重叠。

unregister

在register函数注册中，会把当前类添加到typesBySubscriber中，而unregister函数则是从typesBySubscriber移除该对象，并调用unsubscribeByEventType函数。

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public synchronized void unregister(Object subscriber) {
    List<Class<?>> subscribedTypes = typesBySubscriber.get(subscriber);
    if (subscribedTypes != null) {
        for (Class<?> eventType : subscribedTypes) {//遍历当前类注册的所有事件，在事件列表里移除本注册类对应的订阅信息
            unsubscribeByEventType(subscriber, eventType);
        }
        typesBySubscriber.remove(subscriber);
    } else {
        logger.log(Level.WARNING, "Subscriber to unregister was not registered before: " + subscriber.getClass());
    }
}

看看unsubscribeByEventType函数，通过subscriptionsByEventType获取注册事件eventType的注册信息列表，遍历列表，判断每个订阅信息是否注册类是否与当前解绑注册相同，相同的话则从订阅信息列表移除该订阅信息。

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private void unsubscribeByEventType(Object subscriber, Class<?> eventType) {
  	//订阅类型的所有订阅信息
    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) {//订阅信息中的订阅类与解绑订阅类一致
                subscription.active = false;//该标志位在处理订阅方法会很有用
                subscriptions.remove(i);
                i--;
                size--;
            }
        }
    }
}

也就是说，解绑的过程是将订阅者和订阅信息分别从typesBySubscriber和subscriptions集合中移除。

Post

分析了注册与解注册，接下来分析发送事件的post函数。

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public void post(Object event) {
  	//将事件添加到当前线程的事件队列中
    PostingThreadState postingState = currentPostingThreadState.get();
    List<Object> eventQueue = postingState.eventQueue;
    eventQueue.add(event);

    if (!postingState.isPosting) {
      	//判断是否Android主线程
        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);//此时队列只有我们一个事件
            }
        } finally {
            postingState.isPosting = false;
            postingState.isMainThread = false;
        }
    }
}

currentPostingThreadState是ThreadLocal类型，缓存PostingThreadState。post函数的 主要功能是将事件添加到eventQueue队列中，如果当前postingState.isPosting!=ture,则调用postSingleEvent发布该事件。

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private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
    Class<?> eventClass = event.getClass();
    boolean subscriptionFound = false;
    if (eventInheritance) {
        List<Class<?>> eventTypes = lookupAllEventTypes(eventClass);//查询该EventType所有父类和接口
        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));
        }
    }
}

调用postSingleEventForEventType函数。

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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;
}

postSingleEventForEventType函数遍历eventClass对应的Subscription列表，然后调用了postToSubscription函数发送事件。前面分析到，当类订阅黏性事件也会调用该方法。

postToSubscription

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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);
    }
}

这里就涉及到EventBus中线程的一些知识点，在注册类中，我们通常通过这样方式去订阅一个事件。

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@Subscribe(threadMode = ThreadMode.MAIN)
fun get(event:Int){}

其中threadMode表示当事件来临时，执行get函数的方法体的线程。默认情况下是ThreadMode.POSTING,表示与事件发布的线程一致，即发布线程，所以可能在主线程，也有可能在后台线程。而ThreadMode.MAIN表示在Android的主线程，如果发布线程也在主线程，那么方法体会被直接执行，所以可能会被主线程阻塞，可以通过MAIN_ORDERED按序在主线程执行，这样就不会阻塞主线程。ThreadMode.MAIN_ORDERED后台线程，即Android主线程外的其他线程。ThreadMode.ASYNC即异步线程，即发布线程和订阅线程是独立线程，不会直接运行和等待订阅方法。

回到postToSubscription函数。由于我们发布事件和订阅事件都在主线程，所以直接执行了invokeSubscriber函数。也就是直接通过反射调用了我们的订阅方法。

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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);
    }
}

假设我们在子线程发布了事件，此时调用的是mainThreadPoster.enqueue(subscription, event)。mainThreadPoster对象的类型MainThreadSupport，在EventBus对象创建的时候被创建。

MainThreadPoster

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mainThreadSupport = builder.getMainThreadSupport();
mainThreadPoster = mainThreadSupport != null ? mainThreadSupport.createPoster(this) : null;
//EventBusBuilder
MainThreadSupport getMainThreadSupport() {
    if (mainThreadSupport != null) {
        return mainThreadSupport;
    } else if (AndroidLogger.isAndroidLogAvailable()) {// Class.forName("android.util.Log") != null
        Object looperOrNull = getAndroidMainLooperOrNull();//Looper.getMainLooper()
        return looperOrNull == null ? null :
                new MainThreadSupport.AndroidHandlerMainThreadSupport((Looper) looperOrNull);
    } else {
        return null;
    }
}

所以MainThreadSupport其实很简单。直接创建了MainThreadSupport的实现类AndroidHandlerMainThreadSupport,内部持有主线程的Looper对象。

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public interface MainThreadSupport {

    boolean isMainThread();

    Poster createPoster(EventBus eventBus);

    class AndroidHandlerMainThreadSupport implements MainThreadSupport {

        private final Looper looper;

        public AndroidHandlerMainThreadSupport(Looper looper) {
            this.looper = looper;
        }

        @Override
        public boolean isMainThread() {
            return looper == Looper.myLooper();
        }

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

}

而mainPoster则是HandlerPoster,HandlerPoster是Handler的子类，并且实现Poster接口。查看其enqueue函数。

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public void enqueue(Subscription subscription, Object event) {
    //利用旧的或直接新建PendingPost对象
    PendingPost pendingPost = PendingPost.obtainPendingPost(subscription, event);
    synchronized (this) {
        queue.enqueue(pendingPost);//加入队列
        if (!handlerActive) {
            handlerActive = true;
            if (!sendMessage(obtainMessage())) {
                throw new EventBusException("Could not send handler message");
            }
        }
    }
}

将信息包装成PendingPost对象，然后加入到PendingPostQueue队列queue中，如handler未激活，则调用handle的sendMessage函数。

handleMessage

我们看看其handleMessage函数。

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public void handleMessage(Message msg) {
    boolean rescheduled = false;
    try {
        long started = SystemClock.uptimeMillis();
        while (true) {
          	//从队列取PendingPost对象
            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;
    }
}

调用了invokeSubscriber函数。可见最终调用了invokeSubscriber的重载函数，并释放PendingPost对象。

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void invokeSubscriber(PendingPost pendingPost) {
    Object event = pendingPost.event;
    Subscription subscription = pendingPost.subscription;
    PendingPost.releasePendingPost(pendingPost);
    if (subscription.active) {//调用unregister,该变量=false。
        invokeSubscriber(subscription, event);
    }
}

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);
    }
}

这样一分析，EventBus的订阅与解订阅，简单事件发布流程就分析完毕了。

黏性事件

所谓的黏性事件，就是在订阅方法的Subscribe注解，将其sticky属性设置为ture。

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@Subscribe(threadMode = ThreadMode.MAIN,sticky = true)
fun get(event:Int){

}

这样在类注册的时候，[subscribe](# subscribe)函数会发布上次黏性事件。如果没有黏性事件，则不会发布该事件。

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private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
	  ...
		//判断当前方法订阅的是黏性事件
    if (subscriberMethod.sticky) {
        if (eventInheritance) {//默认true,表示黏性事件可以从父类继承
            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);
        }
    }
}

我们通过postSticky函数来发布黏性事件。它会将黏性事件保存到stickyEvents中，这种后面订阅就可以取出上次黏性事件。然后再调用post进行事件发布。

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public void postSticky(Object event) {
    synchronized (stickyEvents) {
        stickyEvents.put(event.getClass(), event);
    }
    post(event);
}

优先权

在订阅事件时候，我们也可以通过priority属性设置同线程内的同事件不同订阅类接收该订阅事件的优先权。值越大表示优先权越高，会优先接收到订阅事件，这个通过[subscribe](# subscribe)函数得知。

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@Subscribe(threadMode = ThreadMode.MAIN,sticky = true,priority = 1)
fun get(event:Int){

}

线程切换

前面分析中，分析了主线程的切换[MainThreadPoster](# MainThreadPoster)。回到postToSubscription函数，了解其他线程的切换。

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private void postToSubscription(Subscription subscription, Object event, boolean isMainThread) {
    switch (subscription.subscriberMethod.threadMode) {
        case POSTING://订阅线程为POSTING，表示与发布线程一致，不需要切换，直接调用订阅方法。
            invokeSubscriber(subscription, event);
            break;
        case MAIN:
            if (isMainThread) {//先判断发布线程是否在主线程，是的话，直接调用
                invokeSubscriber(subscription, event);
            } else {//否则，通过mainThreadPoster切换到主线程
                mainThreadPoster.enqueue(subscription, event);
            }
            break;
        case MAIN_ORDERED:
            if (mainThreadPoster != null) {//mainThreadPoster进队列按序发布事件
                mainThreadPoster.enqueue(subscription, event);
            } else {
                //技术错误，直接调用
                invokeSubscriber(subscription, event);
            }
            break;
        case BACKGROUND:
            if (isMainThread) {//后台线程队列，在线程池执行
                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);
    }
}

这里主要看下backgroundPoster和asyncPoster的工作原理。

BackgroundPoster

BackgroundPoster实现Runnable和Poster接口。其enqueue函数。

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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);
        }
    }
}

先把订阅信息包装类PendingPost添加到队列中，executorRunning=false,则调用eventBus.getExecutorService().execute(this)执行自身。这里的eventBus.getExecutorService()返回的线程池是Executors.newCachedThreadPool()对象,此时查看run函数。

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public void run() {
    try {
        try {
            while (true) {
                PendingPost pendingPost = queue.poll(1000);
                if (pendingPost == null) {
                    synchronized (this) {
                        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;
    }
}

可见于MainPoster的逻辑是一致的，只不过这里在线程池执行订阅方法，从而实现切换到后台线程，且在一条后台线程会处理完所有的事件的订阅方法，。

AsyncPoster

AsyncPoster也实现Runnable和Poster接口。查看其enqueue函数和run函数。也就是说AsyncPoster实现逻辑与BackgroundPoster一致的。只不过AsyncPoster对象中一条线程只执行一个事件的订阅方法。

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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);
}

【相关知识点连接】

ThreadLocal

源码分析EventBus