EventBus

EventBus是一個為Android和Java平臺設計的發布/訂閱事件總線

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EventBus有以下特點：

• 簡化組件之間的通信
  • 將事件發送方和事件接收方解耦
  • 很好的使用于Activitis、Fragments和后臺線程
  • 避免復雜、易出錯的依賴
• 簡化你的代碼
• 快
• 體積小(~50K jar)
• 有100,000,000+次相關APP安裝，證明EventBus的可靠性
• 還有其他額外的功能

集成到項目

gradle配置：

implementation 'org.greenrobot:eventbus:3.1.1'

使用(只需3步)

1. 定義事件

   public static class MessageEvent { /* 如果有需要，可以聲明相應的屬性 */ }

2. 聲明訂閱方法

   @Subscribe(threadMode = ThreadMode.MAIN)
   public void onMessageEvent(MessageEvent event) {/* Do something */};

需要@Subscribe注解，默認的threadMode是ThreadMode.MAIN

注冊和反注冊訂閱，比如在Activities和Fragments中根據生命周期方法去注冊和反注冊

@Override
 public void onStart() {
     super.onStart();
     EventBus.getDefault().register(this);
 }

 @Override
 public void onStop() {
     super.onStop();
     EventBus.getDefault().unregister(this);
 }

3. 發送事件

   EventBus.getDefault().post(new MessageEvent());

源碼解析

register

/**
 * Registers the given subscriber to receive events. Subscribers must call {@link #unregister(Object)} once they
 * are no longer interested in receiving events.
 * <p/>
 * Subscribers have event handling methods that must be annotated by {@link Subscribe}.
 * The {@link Subscribe} annotation also allows configuration like {@link
 * ThreadMode} and priority.
 */
public void register(Object subscriber) {
    Class<?> subscriberClass = subscriber.getClass();
    List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);// -> 1
    synchronized (this) {
        for (SubscriberMethod subscriberMethod : subscriberMethods) {
            subscribe(subscriber, subscriberMethod);// -> 2
        }
    }
}

注釋：
注冊subscriber用來接收事件，當不再希望接收到事件發送時必須要調用unregister取消注冊
subscriber必須包含帶Subscribe注解的方法，Subscribe注解可配置ThreadMode和priority

• 注釋1處代碼：

  List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);

根據sbuscriber的class去查找帶有@subscribe注解的方法，findSubscriberMethods方法：

List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
    List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);// -> 3
    if (subscriberMethods != null) {
        return subscriberMethods;
    }

    if (ignoreGeneratedIndex) {// -> 4
        subscriberMethods = findUsingReflection(subscriberClass);// -> 5
    } else {
        subscriberMethods = findUsingInfo(subscriberClass);// -> 6
    }
    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;
    }
}

• 注釋3處代碼

先從緩存中去查找該subscriber中訂閱的方法，如果查到，直接返回。
METHOD_CACHE緩存中的數據是在register后添加的，但是unregister的時候并不會清掉，這樣當一個subscriber注冊完，然后反注冊，下次再注冊的時候，不需要再去查找其中訂閱的方法。

• 注釋4處的代碼

如果上一步沒有查到相應的方法，就會走到這一步，先根據ignoreGeneratedIndex判斷是否忽略索引查找,如果忽略，則執行注釋5處的代碼，不管你是否添加了index所有，都利用反射去查找subscriber中訂閱的方法列表；如果不忽略，則執行6處的代碼，先從索引index中去查找，如果沒有索引或者索引中沒有找到，再考慮利用反射的方式去查找。
關于索引index的使用后面會提到。

• 注釋5處的代碼

調用findUsingReflection方法：

private List<SubscriberMethod> findUsingReflection(Class<?> subscriberClass) {
    FindState findState = prepareFindState();
    findState.initForSubscriber(subscriberClass);
    while (findState.clazz != null) {// -> 7
        findUsingReflectionInSingleClass(findState);// -> 8
        findState.moveToSuperclass();
    }
    return getMethodsAndRelease(findState);
}


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;
    }
    for (Method method : methods) {
        int modifiers = method.getModifiers();
        if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
            Class<?>[] parameterTypes = method.getParameterTypes();
            if (parameterTypes.length == 1) {
                Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);
                if (subscribeAnnotation != null) {
                    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");
        }
    }
}

• 注釋7處的代碼

這里之所以使用while循環而不用if判斷，是因為除了查找當前subscriber還需要查找其向上的繼承鏈，findState.moveToSuperclass();就是用來控制向上查找的，EventBus中所有的繼承鏈查找都是用這種方式，不要迷惑就好。

• 注釋8處的代碼

這里就是調用了findUsingReflectionInSingleClass去用反射的方式查找subscriber中訂閱的方法列表，代碼很簡單，就是普通的反射使用，看不懂的問我吧。

反射的邏輯就這些

再回到注釋4處的代碼繼續分析另外一種查找方法列表的方式：

• 注釋6處的代碼

調用findUsingInfo方法：

private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
    FindState findState = prepareFindState();
    findState.initForSubscriber(subscriberClass);
    while (findState.clazz != null) {
        findState.subscriberInfo = getSubscriberInfo(findState);// -> 9
        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);// -> 10
        }
        findState.moveToSuperclass();
    }
    return getMethodsAndRelease(findState);
}

• 注釋9處代碼

調用getSubscriberInfo方法：

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;
}

這里有兩個分支，第一次register的時候，第一個分支條件是不會成立的，我們看第二個分支：

這里的subscriberInfoIndexes就是我們通過EventBus的注解生成的index索引，然后調用EventBus.builder().addIndex(new MyEventBusIndex()).build();添加的，這個索引的作用就是把本來運行時通過反射查找訂閱方法的邏輯換成了編譯器自動生成文件，然后運行時在該文件中直接去查找的方式，從而提高了效率，index索引的方式后面會介紹。

• 注釋10處的代碼

如果我們沒有添加index索引，那么代碼9處得到的findState.subscriberInfo就是null，代碼就會走到注釋10處，注釋10處的邏輯和上面注釋8處的邏輯一樣，都是通過反射去查找訂閱方法。

到這里，不管通過哪種方式，訂閱的方法列表已經找到了，我們回到注釋1處接著往下看：

• 注釋2處的代碼：

private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
    //獲取事件類型
    Class<?> eventType = subscriberMethod.eventType;
    // 將訂閱者和訂閱者中的訂閱方法打包成一個Subscription對象
    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();
    // 遍歷訂閱了該event類型的方法，根據priority把新訂閱的方法放到合適的位置
    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);
        }
    }
}

至此，register流程已經走完。

post事件流程

post事件的流程很簡單，就是通過反射調用相應的方法(訂閱的方法在上一步已經找到并存到了相應的集合中)即可。

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);// -> 1
            }
        } finally {
            postingState.isPosting = false;
            postingState.isMainThread = false;
        }
    }
}

• 注釋1處代碼

調用postSingleEvent方法：

private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
    Class<?> eventClass = event.getClass();
    boolean subscriptionFound = false;
    if (eventInheritance) { // -> 2
        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); // -> 3
        }
    } else {
        subscriptionFound = postSingleEventForEventType(event, postingState, eventClass); // -> 4
    }
    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));
        }
    }
}

• 注釋2處代碼

eventInheritance：該屬性可以在EventBus的Build中初始化，默認是true。

true的情況舉個例子：
A和B是兩個Event，并且A extends B
X和Y是兩個方法，X訂閱了事件A，Y訂閱了事件B
post A事件，同時會post B事件，那么X和Y都會收到事件

fasle的情況就是：post哪個事件，就只會發送這一個事件，不會發送它的父類事件，上面這個例子，如果eventInheritance為false，post A的時候Y就不會收到事件

• 注釋3和注釋4代碼

都調用了postSingleEventForEventType方法：

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 = false;
            try {
                postToSubscription(subscription, event, postingState.isMainThread);// -> 5
                aborted = postingState.canceled;
            } finally {
                postingState.event = null;
                postingState.subscription = null;
                postingState.canceled = false;
            }
            if (aborted) {
                break;
            }
        }
        return true;
    }
    return false;
}

• 注釋5處代碼：

這里就是遍歷到該event中的所有訂閱方法，挨個給他們發送事件，具體的發送邏輯就是代碼5處的postToSubscription方法：

private void postToSubscription(Subscription subscription, Object event, boolean isMainThread) {
    switch (subscription.subscriberMethod.threadMode) {
        case POSTING://直接在當前線程發送
            invokeSubscriber(subscription, event);
            break;
        case MAIN:
            if (isMainThread) {//如果發送事件是在主線程，直接發送
                invokeSubscriber(subscription, event);
            } else {//如果發送事件不是在主線程，加入到主線程的事件隊列里面等待發送
                mainThreadPoster.enqueue(subscription, event);
            }
            break;
        case MAIN_ORDERED:// 沒太看懂和MAIN有啥區別
            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) {// 如果發送事件是在主線程，加入到子線程的事件隊列里面，等待發送
                backgroundPoster.enqueue(subscription, event);
            } else {// 如果發送事件不是在主線程，直接發送
                invokeSubscriber(subscription, event);
            }
            break;
        case ASYNC:// 不管發送線程是在哪個線程，都加入到異步線程的隊列里面等待發送
            asyncPoster.enqueue(subscription, event);
            break;
        default:
            throw new IllegalStateException("Unknown thread mode: " + subscription.subscriberMethod.threadMode);
    }
}

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);
    }
}

這里就是根據不同的ThreadMode去執行不同的邏輯了，invokeSubscriber就是通過反射的方式去調用方法，沒什么可說的。

unregister

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());
    }
}

當unregister的時候，會把該subscriber中的訂閱方法都從集合中移除掉，同時也會把該subscriber從相應的集合中移除掉。

索引index

EventBus提供一個注解處理器，可以在編譯期把你項目中所有regisger的class和所有的 @Subscribe 方法給整合起來，生成一個特殊的類，類似下面：

/** 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>();// key就是register的對象， value就是該對象中@subscribe的方法集合

        putIndex(new SimpleSubscriberInfo(cn.jerry.webviewdemo.FirstActivity.class, true, new SubscriberMethodInfo[] {
            new SubscriberMethodInfo("onEventArrived", cn.jerry.webviewdemo.CustomEvent.class),
        }));

    }

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

    @Override
    // 根據class對象獲取該對象中@Subscribe注解的方法列表
    public SubscriberInfo getSubscriberInfo(Class<?> subscriberClass) {
        SubscriberInfo info = SUBSCRIBER_INDEX.get(subscriberClass);
        if (info != null) {
            return info;
        } else {
            return null;
        }
    }
}

代碼運行的時候，再遇到register邏輯，會直接從這個索引中查找相應的方法列表，從而避免了相應的反射操作，上面提到的register流程的注釋9處：

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;
        }
    }
    // 這里就是從剛剛的index處去查找
    if (subscriberInfoIndexes != null) {
        for (SubscriberInfoIndex index : subscriberInfoIndexes) {
            // 根據相應的class對象，去查找該對象中@subscribe的方法列表
            SubscriberInfo info = index.getSubscriberInfo(findState.clazz);
            if (info != null) {
                return info;
            }
        }
    }
    return null;
}