如上圖所示，簡而言之，EventBus是一個用於Android/Java的釋出/訂閱事件匯流排，可以簡化元件間的資料傳輸。它的優勢在於解耦事件傳送者和接受者，在Activities、Fragments以及後臺執行緒和主執行緒間的資料傳輸方面表現不錯，避免了複雜和容易出錯的依賴關係和生命週期問題。

       從原始碼分析的角度來看，EventBus的資料傳輸機制，主要是依賴於Java反射機制和自定義註解處理器。對於後者，本章暫且按下不表，集中精力分析EventBus是如何通過反射機制傳輸資料的。

一、EventBus的例項獲取

/** 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;
}
 

public EventBus() {
    this(DEFAULT_BUILDER);
}

private static final EventBusBuilder DEFAULT_BUILDER = new EventBusBuilder();

       從上述程式碼可知，EventBus的getDefault()方法就是一個採用了單例模式的同步方法，同時分析EventBus的構造器方法可知，EventBus內部的一些引數屬性是由EventBusBuilder這個類物件來負責定義的。

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

    ...

    MainThreadSupport mainThreadSupport;

    ...

    MainThreadSupport getMainThreadSupport() {
        if (mainThreadSupport != null) {
            return mainThreadSupport;
        } else if (AndroidLogger.isAndroidLogAvailable()) {
            Object looperOrNull = getAndroidMainLooperOrNull();
            return looperOrNull == null ? null :
                    new MainThreadSupport.AndroidHandlerMainThreadSupport((Looper) looperOrNull);
        } else {
            return null;
        }
    }

    //獲取主執行緒Looper
    Object getAndroidMainLooperOrNull() {
        try {
            return Looper.getMainLooper();
        } catch (RuntimeException e) {
            // Not really a functional Android (e.g. "Stub!" maven dependencies)
            return null;
        }
    }


    public EventBus installDefaultEventBus() {
        synchronized (EventBus.class) {
            if (EventBus.defaultInstance != null) {
                throw new EventBusException("Default instance already exists." +
                        " It may be only set once before it's used the first time to ensure consistent behavior.");
            }
            EventBus.defaultInstance = build();
            return EventBus.defaultInstance;
        }
    }


    public EventBus build() {
        return new EventBus(this);
    }

}
 

       針對EventBusBuilder類，可以簡單過一遍，主要是設定了一些屬性和生成一些工具，暫時知道它是配合EventBus的構造器方法就好。接著，咱們將注意力轉向register方法。

二、EventBus的register方法流程

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

// Must be called in synchronized block
private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
    Class<?> eventType = subscriberMethod.eventType;
    Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
    CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
    if (subscriptions == null) {
        subscriptions = new CopyOnWriteArrayList<>();
        subscriptionsByEventType.put(eventType, subscriptions);  //儲存事件型別和訂閱物件們
    } else {
       ...
    }

    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) { //對粘性事件的處理
       ...
    }
}

Map<Class<?>, CopyOnWriteArrayList<Subscription>> subscriptionsByEventType;

      register方法其實就做了兩件事，即獲取訂閱者的訂閱方法、儲存訂閱者和訂閱者方法以及事件型別等等資訊。首先通過findSubscriberMethods方法獲取訂閱者的訂閱方法，接著通過遍歷，呼叫subsribe訂閱方法，儲存訂閱資訊。

List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
    List<SubscriberMethod> 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;
    }
}

private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
        ...
        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);
}

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

由上述程式碼可知，findSubscriberMethods方法是由findUsingReflectionInSingleClass方法具體實現的，接下來就是重點了。

private void findUsingReflectionInSingleClass(FindState findState) {    //利用反射獲取訂閱者方法
    Method[] methods;
    try {
        methods = findState.clazz.getDeclaredMethods(); //獲取本類中的所有方法
    } catch (Throwable th) {
        ...
    }
    for (Method method : methods) {
        int modifiers = method.getModifiers();  //返回此類或介面以整數編碼的 Java語言修飾符(PUBLIC、PRIVATE...)
        //是否屬於PUBLIC方法且不屬於抽象、靜態等方法
        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();
                        //儲存訂閱方法資訊
                        findState.subscriberMethods.add(new SubscriberMethod(method, eventType, threadMode,
                                subscribeAnnotation.priority(), subscribeAnnotation.sticky()));
                    }
                }
            //處於嚴格方法驗證模式以及註解無誤，但是缺少方法引數時
            } 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");
        }
    }
}

public class SubscriberMethod {
    final Method method;
    final ThreadMode threadMode;
    final Class<?> eventType;
    final int priority;
    final boolean sticky;
    String methodString;

    public SubscriberMethod(Method method, Class<?> eventType, ThreadMode threadMode, int priority, boolean sticky) 
   {      
        this.method = method;
        this.threadMode = threadMode;
        this.eventType = eventType;
        this.priority = priority;
        this.sticky = sticky;
    }
    ...
}

       由上述程式碼可知，通過findSubscriberMethods方法，利用反射機制，將訂閱者方法、傳遞的事件型別、優先順序、是否為粘性事件等資訊都儲存好了。小結一下，其實register方法就是用來獲取訂閱者的訂閱方法、儲存訂閱資訊的。

三、Unregister方法

/** Unregisters the given subscriber from all event classes. */
public synchronized void unregister(Object subscriber) {
    List<Class<?>> subscribedTypes = typesBySubscriber.get(subscriber);
    if (subscribedTypes != null) {
        for (Class<?> eventType : subscribedTypes) {
            unsubscribeByEventType(subscriber, eventType);
        }
        typesBySubscriber.remove(subscriber);
    } else {
        logger.log(Level.WARNING, "Subscriber to unregister was not registered before: " + subscriber.getClass());
    }
}

unregister方法就是用來取消訂閱的，就不再贅述細節了。

四、Post方法

EventBus是通過Post方法進行事件傳遞（釋出）的，在此對post方法原始碼進行簡單分析。

public void post(Object event) {
  PostingThreadState postingState = currentPostingThreadState.get();
  List<Object> eventQueue = postingState.eventQueue;
  eventQueue.add(event);
    ...

   while (!eventQueue.isEmpty()) { postSingleEvent(eventQueue.remove(0), postingState); } //核心方法
    ...
}

private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
    Class<?> eventClass = event.getClass();
    boolean subscriptionFound = false;
    if (eventInheritance) { //是否釋出有繼承關係型別的事件
        List<Class<?>> eventTypes = lookupAllEventTypes(eventClass);
        int countTypes = eventTypes.size();
        for (int h = 0; h < countTypes; h++) {
            Class<?> clazz = eventTypes.get(h);
            subscriptionFound |= postSingleEventForEventType(event, postingState, clazz);
        }
    } else {
        subscriptionFound = postSingleEventForEventType(event, postingState, eventClass); //核心方法
    }
    ...
}

//傳遞單獨事件的方法
private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass) {
    CopyOnWriteArrayList<Subscription> subscriptions;
    synchronized (this) {
        subscriptions = subscriptionsByEventType.get(eventClass); //獲取訂閱物件集合
    }
    if (subscriptions != null && !subscriptions.isEmpty()) {
        for (Subscription subscription : subscriptions) {
            postingState.event = event;
            postingState.subscription = subscription;
            boolean aborted = false;
            try {
                postToSubscription(subscription, event, postingState.isMainThread); //核心方法
                aborted = postingState.canceled;
            } finally {
                postingState.event = null;
                postingState.subscription = null;
                postingState.canceled = false;
            }
            if (aborted) {
                break;
            }
        }
        return true;
    }
    return false;
}

          從Post方法的內部邏輯分析，post方法主要是對eventQueue進行遍歷，然後呼叫postSingleEvent方法進行處理；接著postSingleEvent方法通過遍歷事件型別eventTypes，再呼叫postSingleEventForEventType方法，通過遍歷訂閱物件集合subscriptions, 將subscription訂閱物件, event事件, postingState.isMainThread是否為主執行緒等引數傳遞給postToSubscription方法。而postToSubscription方法根據不同的執行緒模式threadMode對subsription和event進行不同儲存操作，最終呼叫invokeSubscriber方法，即利用反射機制呼叫處理訂閱方法。

//將事件傳遞給訂閱者
private void postToSubscription(Subscription subscription, Object event, boolean isMainThread) {
    switch (subscription.subscriberMethod.threadMode) { //根據訂閱者設定的執行緒模式threadMode進行處理
        case POSTING:
            invokeSubscriber(subscription, event); //通過反射機制呼叫處理訂閱方法
            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);
    }
}

void invokeSubscriber(Subscription subscription, Object event) {    //利用反射機制呼叫處理訂閱方法
    try {
        subscription.subscriberMethod.method.invoke(subscription.subscriber, event); //利用反射機制呼叫處理訂閱方法
    } catch (InvocationTargetException e) {
        handleSubscriberException(subscription, event, e.getCause());
    } catch (IllegalAccessException e) {
        throw new IllegalStateException("Unexpected exception", e);
    }
}

         事實上，invokeSubscriber方法就是處理訂閱方法被呼叫的關鍵程式碼，是利用java反射機制呼叫類方法的一個例項。至此，EventBus的普通事件釋出處理流程簡單地被梳理了一遍。至於粘性等事件的釋出流程，本章並未涉及。