EventBus源码学习

看了别人的源码文章，一开始有点懵，还要复习下反射和注解器的知识。然后自己也做了下笔记，加深印象。

参考文章：

blog.csdn.net/u012933743/…

www.jianshu.com/p/c4d106419… .

描述

EventBus是Android和 Java 的发布/订阅事件总线。

优缺点：

• 简化了组件之间的通信，将事件发布者和订阅者分离
• 代码简单
• 包体积小缺点：利用反射，性能可能会差一点。可以使用EventBus annotation processor（EventBus注解处理器），在编译期间创建了订阅者索引。（提升EventBus性能： greenrobot.org/eventbus/do…

三要素：

• Event：自定义事件类型
• Subscriber：事件订阅者。在EventBus3.0之前我们必须定义以onEvent开头的那几个方法，分别是onEvent、onEventMainThread、onEventBackgroundThread和onEventAsync，而在3.0之后事件处理的方法名可以随意取，不过需要加上注解@subscribe()，并且指定线程模型，默认是POSTING。
• Publisher：事件发布者。可以在任意线程里发布事件，一般情况下，使用EventBus.getDefault()就可以得到一个EventBus对象，然后再调用post(Object)方法即可。

4个线程：

• POSTING:在发布线程的事件执行
• MAIN：在主线程执行。
• BACKGROUND：如果发布事件的线程为主线程则新建一个线程执行，否则在发布事件的线程执行。
• ASYNC：在新的线程执行

利用Handler切换到主线程。BACKGROUND线程，使用到了线程池ExecutorService。

说说EventBus里面用到的知识点

• 注解处理器：

使用EventBus annotation processor（EventBus注解处理器），在编译期间创建了订阅者索引。（提升EventBus性能： greenrobot.org/eventbus/do… ）。可以看到，会在build包下面生成一个EventBus的索引类。SUBSCRIBER_INDEX是一个Map类型数据，存放着每一个类里面的注解信息

/** This class is generated by EventBus, do not edit. */
public class MyEventBusIndex implements SubscriberInfoIndex {
    private static final Map<Class<?>, SubscriberInfo> SUBSCRIBER_INDEX;

    static {
        SUBSCRIBER_INDEX = new HashMap<Class<?>, SubscriberInfo>();

        putIndex(new SimpleSubscriberInfo(com.example.camera_learning.eventbus.EventBusActivity.class, true,
                new SubscriberMethodInfo[] {
            new SubscriberMethodInfo("onEvent", com.example.camera_learning.eventbus.TestEvent.class),
            new SubscriberMethodInfo("onEvent", com.example.camera_learning.eventbus.TestEvent2.class),
        }));

    }

    private static void putIndex(SubscriberInfo info) {
        SUBSCRIBER_INDEX.put(info.getSubscriberClass(), info);
    }

    @Override
    public SubscriberInfo getSubscriberInfo(Class<?> subscriberClass) {
        SubscriberInfo info = SUBSCRIBER_INDEX.get(subscriberClass);
        if (info != null) {
            return info;
        } else {
            return null;
        }
    }
}

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• 反射

如果索引类中有缓存相关的注解信息的话，那就会直接用索引类里面的注解信息。如果没有的话，就需要利用反射去获取类中的所有方法，然后拿到有@Subscribe的方法。

post方法最后需要利用invoke来调用订阅者对象的订阅方法（具体代码可以看下面）

• 设计模式：单例模式、建造者模式、观察者模式
• ThreadLocal：EventBus会通过ThreadLocal为每个线程维护一个PostingThreadState对象。

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

final static class PostingThreadState {
    //分发队列
    final List<Object> eventQueue = new ArrayList<>();
    //是否正在分发
    boolean isPosting;
    //是否是主线程
    boolean isMainThread;
    //订阅关系
    Subscription subscription;
    //当前正在分发的事件
    Object event;
    //是否取消
    boolean canceled;
}
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EventBus里面的一些数据结构

比较重要的就是下面两个map集合。

//key为事件类型，value值为Subscription的集合。这样可以根据事件的类型，直接获取订阅该事件的所有类中的所有方法
private final Map<Class<?>, CopyOnWriteArrayList<Subscription>> subscriptionsByEventType;
//key为订阅对象，value为该类中的所有订阅事件类型的集合。
private final Map<Object, List<Class<?>>> typesBySubscriber;
//粘性事件
private final Map<Class<?>, Object> stickyEvents;

//Subscription类，表示一个订阅关系，包含订阅对象和订阅方法
final class Subscription {
    final Object subscriber;
    final SubscriberMethod subscriberMethod;
}
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阅读源码

getDefault()方法

• 利用双重校验锁实现单例模式，保证线程安全
• 使用Builder建造者模式，将一个复杂对象的构建和它的表示分离。建造者模式一般用来创建复杂对象，用户可以不用关心其建造过程和细节。在EventBus中，指的是EventBusBuilder类，可以使用自定义参数创建EventBus实例。

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

public EventBus() {
    this(DEFAULT_BUILDER);
}
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;
}

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;
    EventBusBuilder() {
    }

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

register的最终目的是，将注解的信息保存到对应的map中。而获取的过程，优先通过注解器生成的注解信息获取，如果没有的话，再利用反射获取注解方法。

public void register(Object subscriber) {
    //拿到class对象
    Class<?> subscriberClass = subscriber.getClass();
    //利用SubscriberMethodFinder类来找出这个类中的所有订阅方法
    List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
    //拿到类中所有的订阅方法后，调用subscribe方法
    synchronized (this) {
        for (SubscriberMethod subscriberMethod : subscriberMethods) {
            subscribe(subscriber, subscriberMethod);
        }
    }
}

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);
    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);
        }
    }
}


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下面看一下SubscriberMethodFinder类。

//这个map对象，key值为订阅者类，value为该类中的所有订阅方法的list集合。已经register过的话，相应的数据已经在map中存在了，这个时候就可以直接从map中读取就行了，不用再次利用反射进行多余的遍历类中的方法。有点空间换时间的思想。
private static final Map<Class<?>, List<SubscriberMethod>> METHOD_CACHE = new ConcurrentHashMap<>();

//通过这个方法，遍历出某个类中的所有订阅方法
List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
    //根据缓存值，判断是否注册过EventBus
    List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
    if (subscriberMethods != null) {
        return subscriberMethods;
    }
    //默认为false，所以一般是先走findUsingInfo方法，通过自动生成的索引类比如MyEventBusIndex来获取到被注解的办法。这种办法比通过findUsingReflection（）利用反射去获取方法的性能好一点。如果没有获取到索引中存储的订阅者Subscriber信息的话，才去走利用反射的办法。
    if (ignoreGeneratedIndex) {
        subscriberMethods = findUsingReflection(subscriberClass);
    } else {
        subscriberMethods = findUsingInfo(subscriberClass);
    }
    //如果订阅者类和父类中，没有@Subscribe利用监听事件的方法，就会抛这个错误。
    if (subscriberMethods.isEmpty()) {
        throw new EventBusException("Subscriber " + subscriberClass
                + " and its super classes have no public methods with the @Subscribe annotation");
    } else {
        //获取到订阅者类中的所有注解方法后，保存到cache中
        METHOD_CACHE.put(subscriberClass, subscriberMethods);
        return subscriberMethods;
    }
}

//利用注解处理器生成的索引类，来获取类中的订阅方法
private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
    FindState findState = prepareFindState();
    findState.initForSubscriber(subscriberClass);
    while (findState.clazz != null) {
        //在生成的索引类中，获取保存的订阅信息
        findState.subscriberInfo = getSubscriberInfo(findState);
        if (findState.subscriberInfo != null) {
            //获取拿到这个订阅类中的所有的订阅方法
            SubscriberMethod[] array = findState.subscriberInfo.getSubscriberMethods();
            //遍历，检查后，添加到list集合
            for (SubscriberMethod subscriberMethod : array) {
                if (findState.checkAdd(subscriberMethod.method, subscriberMethod.eventType)) {
                    findState.subscriberMethods.add(subscriberMethod);
                }
            }
        } else {
            //如果在索引类中找不到订阅信息，那就通过反射的办法来获取
            findUsingReflectionInSingleClass(findState);
        }
        findState.moveToSuperclass();
    }
    return getMethodsAndRelease(findState);
}

//在生成的索引类中，获取保存的订阅信息
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;
        }
    }
    
    //如果subscriberInfoIndexes为空，也就是没有利用生成索引的办法来提高EventBus性能，直接返回null
    //不为空的话，遍历，拿到这个订阅类中的所有注解方法
    if (subscriberInfoIndexes != null) {
        for (SubscriberInfoIndex index : subscriberInfoIndexes) {
            SubscriberInfo info = index.getSubscriberInfo(findState.clazz);
            if (info != null) {
                return info;
            }
        }
    }
    return null;
}


//使用subscribe方法
private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
    //订阅方法的事件类型
    Class<?> eventType = subscriberMethod.eventType;
    //创建一个Subscription，包含订阅类对象和订阅方法
    Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
    //获取到这种事件类型的list集合
    CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
    
    if (subscriptions == null) {
        //保存到subscriptionsByEventType里面，key为事件类型，value为Subscription
        subscriptions = new CopyOnWriteArrayList<>();
        subscriptionsByEventType.put(eventType, subscriptions);
    } else {
        //处理重复注册的情况
        if (subscriptions.contains(newSubscription)) {
            throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event "
                    + eventType);
        }
    }
    //这部分的作用，是根据事件优先级进行排序
    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;
        }
    }
    //根据订阅对象，获取订阅的所有的事件类型
    List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
    if (subscribedEvents == null) {
        subscribedEvents = new ArrayList<>();
        typesBySubscriber.put(subscriber, subscribedEvents);
    }
    subscribedEvents.add(eventType);
    
    //处理粘性事件
    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);
        }
    }
}




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post方法

post方法最后会走到postSingleEventForEventType方法,拿到注册这个事件的订阅者信息，利用反射调用订阅方法的执行。

public void post(Object event) {
    //获取当前线程的PostingThreadState对象
    PostingThreadState postingState = currentPostingThreadState.get();
    //获取当前的分发队列
    List<Object> eventQueue = postingState.eventQueue;
    //将事件添加到队列中
    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 {
            //当事件队列不为空的时候，一个一个进行分发
            while (!eventQueue.isEmpty()) {
                postSingleEvent(eventQueue.remove(0), postingState);
            }
        } finally {
            postingState.isPosting = false;
            postingState.isMainThread = false;
        }
    }
}

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private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass) {
    CopyOnWriteArrayList<Subscription> subscriptions;
    synchronized (this) {
        //去map集合中，根据事件类型，获取到所有的订阅方法
        subscriptions = subscriptionsByEventType.get(eventClass);
    }
    if (subscriptions != null && !subscriptions.isEmpty()) {
        //遍历订阅该事件的list集合
        for (Subscription subscription : subscriptions) {
            postingState.event = event;
            postingState.subscription = subscription;
            boolean aborted = false;
            try {
                postToSubscription(subscription, event, postingState.isMainThread);
                aborted = postingState.canceled;
            } finally {
                postingState.event = null;
                postingState.subscription = null;
                postingState.canceled = false;
            }
            if (aborted) {
                break;
            }
        }
        return true;
    }
    return false;
}

//根据线程进行分发事件
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);
    }
}


//最后是利用反射，调用订阅者对象的订阅方法
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);
    }
}



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unregister方法

unregister方法就是要把这个类中的注解信息都给移除掉。

public synchronized void unregister(Object subscriber) {
    //获取这个订阅者种的所有订阅事件类型list
    List<Class<?>> subscribedTypes = typesBySubscriber.get(subscriber);
    if (subscribedTypes != null) {
        for (Class<?> eventType : subscribedTypes) {
            //遍历事件类型list，移除subscriptionsByEventType中包含这个订阅者的所有订阅关系
            unsubscribeByEventType(subscriber, eventType);
        }
        //移除这个订阅对象
        typesBySubscriber.remove(subscriber);
    } else {
        logger.log(Level.WARNING, "Subscriber to unregister was not registered before: " + subscriber.getClass());
    }
}

//移除subscriptionsByEventType中包含这个订阅者的所有订阅关系
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--;
            }
        }
    }
}

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