項目中用到了很多優(yōu)秀的框架,但是一直停留在會用的階段是，沒有硬著頭皮去看看它們是如何實現(xiàn)的漩氨，趁著活終于不多了饭耳，打算把項目中的用到的框架都研究研究，學(xué)習(xí)一下優(yōu)秀的代碼睛约。今天就看看EventBus吧屏镊，優(yōu)秀的框架一般使用起來都特別簡單，EventBus也不例外痰腮。EventBus的具體使用這里就不介紹了而芥，無外乎register注冊；unregister注銷膀值；@subscriber在合適的線程關(guān)心相應(yīng)的事件棍丐；post/postSticky發(fā)送事件，我們就從EventBus的register開始吧沧踏。

大多數(shù)情況下我們都是通過EventBus的靜態(tài)方法getDefault獲得默認的EventBus對象歌逢，然后register注冊。

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

很常見的單例設(shè)計模式,繼續(xù)看看EventBus初始化做了什么

public EventBus() {
    this(DEFAULT_BUILDER);
}

EventBus(EventBusBuilder builder) {
    // subscriptionsByEventType是一個map集合,存儲著每個事件對應(yīng)的subscription集合 (key:EventType value: List<subscription>)
    // subscription是對訂閱者還有訂閱方法SubscriberMethod的包裝
    // SubscriberMethod又是對method翘狱，優(yōu)先級秘案，線程類型，是否接受粘性事件的包裝
    subscriptionsByEventType = new HashMap<>();
    // typesBySubscriber也是一個map集合,存儲著每個訂閱者訂閱的事件集合( key：訂閱者 value：List<EventType>)        
    typesBySubscriber = new HashMap<>();
    // 粘性事件集合 
    stickyEvents = new ConcurrentHashMap<>(); 
    // 需要在主線程處理的事件的poster
    mainThreadPoster = new HandlerPoster(this, Looper.getMainLooper(), 10);
    // 需要Background線程處理的事件的poster
    backgroundPoster = new BackgroundPoster(this);
     // 需要異步處理的事件的poster
    asyncPoster = new AsyncPoster(this);
    // 索引個數(shù)
    indexCount = builder.subscriberInfoIndexes != null ? builder.subscriberInfoIndexes.size() : 0;
    // 通過subscriberMethodFinder尋找訂閱者的訂閱方法 可以通過反射和索引兩種方式
    subscriberMethodFinder = new SubscriberMethodFinder(builder.subscriberInfoIndexes,
            builder.strictMethodVerification, builder.ignoreGeneratedIndex);
    // 以下都是是否打印或者拋出異常的標(biāo)志位
    logSubscriberExceptions = builder.logSubscriberExceptions;
    logNoSubscriberMessages = builder.logNoSubscriberMessages;
    sendSubscriberExceptionEvent = builder.sendSubscriberExceptionEvent;
    sendNoSubscriberEvent = builder.sendNoSubscriberEvent;
    throwSubscriberException = builder.throwSubscriberException;
    // 是否支持事件繼承
    eventInheritance = builder.eventInheritance;
    // 異步和 BackGround 處理方式的線程池
    executorService = builder.executorService;
}

又是很常見的Builder設(shè)計模式對EventBus進行初始化潦匈。

接下來就是register了

public void register(Object subscriber) {
    // 訂閱者的類型
    Class<?> subscriberClass = subscriber.getClass();
    // 通過SubscriberMethodFinder類尋找該subscriber所有帶@subscriber注解方法相關(guān)信息(SubscriberMethod)的集合阱高。
    // SubscriberMethod實際上是對帶@subscriber注解的方法一些信息的封裝。
    // 包括該方法的Method對象茬缩，事件回調(diào)的線程ThreadMode赤惊，優(yōu)先級priority，是否響應(yīng)粘性事件sticky凰锡，以及最重要的所關(guān)心的事件類型eventType未舟。
    List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
    synchronized (this) {
        for (SubscriberMethod subscriberMethod : subscriberMethods) {
            subscribe(subscriber, subscriberMethod);
        }
    }
}

先看看上面findSubscriberMethods方法是如何獲取到SubscriberMethod集合的:

List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
    // 先看看緩存中有沒有
    List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
    if (subscriberMethods != null) {
        return subscriberMethods;
    }
    // 是否忽略index,默認是false
    if (ignoreGeneratedIndex) {
        //通過反射查找
        subscriberMethods = findUsingReflection(subscriberClass);
    } else {
        //嘗試從索引中查找
        subscriberMethods = findUsingInfo(subscriberClass);
    }
    // 某些類調(diào)用了register方法，卻沒有帶@Subscribe注解的方法掂为，就會拋出如下異常(是不是很眼熟裕膀？)
    if (subscriberMethods.isEmpty()) {
        throw new EventBusException("Subscriber " + subscriberClass
                + " and its super classes have no public methods with the @Subscribe annotation");
    } else {
        // 放入緩存并且返回subscriberMethods
        METHOD_CACHE.put(subscriberClass, subscriberMethods);
        return subscriberMethods;
    }
}

從上面的代碼中可以看到有兩種方式來獲取訂閱者中帶@Subscribe注解的方法，這里其實是EventBus3.0的優(yōu)化勇哗，之前都是通過反射,而新版如果啟用了索引的話昼扛，會在編譯時期，將所有訂閱者及其帶@Subscribe注解的方法相關(guān)信息通過硬編碼的方式存放到map中智绸，這樣的話就可以直接通過key來獲取了野揪，下面是編譯時期生成的索引類的部分代碼：

 //存放所有的訂閱者及其帶`@Subscribe`注解的方法相關(guān)信息
private static final Map<Class<?>, SubscriberInfo> SUBSCRIBER_INDEX;

static {
    SUBSCRIBER_INDEX = new HashMap<Class<?>, SubscriberInfo>();
    // 將所有的訂閱者及其關(guān)心的事件存放到map中
    putIndex(new SimpleSubscriberInfo(MainActivity.class, true, new SubscriberMethodInfo[] {
        new SubscriberMethodInfo("onEventHanlder1", String.class, ThreadMode.MAIN),
        new SubscriberMethodInfo("onEventHanlder2", String.class, ThreadMode.MAIN),
        new SubscriberMethodInfo("onEventHanlder3", String.class, ThreadMode.MAIN),
        new SubscriberMethodInfo("onEventHanlderInteger", Integer.class, ThreadMode.MAIN),
    }));

    putIndex(new SimpleSubscriberInfo(SecondActivity.class, true, new SubscriberMethodInfo[] {
        new SubscriberMethodInfo("onSecondEvent1", String.class, ThreadMode.MAIN),
        new SubscriberMethodInfo("onSecondEvent2", String.class, ThreadMode.MAIN, 0, true),
        new SubscriberMethodInfo("onSecondEvent3", Integer.class, ThreadMode.MAIN),
        new SubscriberMethodInfo("onSecondEvent4", Integer.class, ThreadMode.MAIN),
        new SubscriberMethodInfo("onSecondEvent5", Integer.class, ThreadMode.MAIN),
        new SubscriberMethodInfo("onSecondEvent6", Integer.class, ThreadMode.MAIN),
    }));

}

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

先來看看通過反射的方式獲取List<SubscriberMethod>集合

private List<SubscriberMethod> findUsingReflection(Class<?> subscriberClass) {
    // 從對象池中取一個FindState 
    FindState findState = prepareFindState();
    // 初始化FindState
    findState.initForSubscriber(subscriberClass);
    while (findState.clazz != null) {
        // 通過反射尋找訂閱方法相關(guān)信息
        findUsingReflectionInSingleClass(findState);
        // 有父類的話 findState.clazz 設(shè)置為父類
        // 沒有的話findState.clazz = null访忿， 結(jié)束尋找
        findState.moveToSuperclass();
    }
    // 釋放資源并且方法findState中的subscriberMethods
    return getMethodsAndRelease(findState);
}

//通過反射查找
private void findUsingReflectionInSingleClass(FindState findState) {
    Method[] methods;
    try {
        // This is faster than getMethods, especially when subscribers are fat classes like Activities
        // 之所以比getMethods快,是因為getDeclaredMethods不包括繼承的方法
        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();
        // 判斷權(quán)限修飾符是否是public，是否是抽象方法斯稳，是否靜態(tài)方法
        if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
            Class<?>[] parameterTypes = method.getParameterTypes();
            // 是否只有一個參數(shù)
            if (parameterTypes.length == 1) {
                Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);
                // 帶注解
                if (subscribeAnnotation != null) {
                    // 關(guān)心的事件類型
                    Class<?> eventType = parameterTypes[0];
                    // 校驗是否需要添加
                    if (findState.checkAdd(method, eventType)) {
                        // 事件回調(diào)的線程
                        ThreadMode threadMode = subscribeAnnotation.threadMode();
                        // 將該方法相關(guān)信息(method海铆，eventType，threadMode挣惰，priority卧斟，sticky)包裝下添加到subscriberMethods集合中
                        findState.subscriberMethods.add(new SubscriberMethod(method, eventType, threadMode,
                                subscribeAnnotation.priority(), subscribeAnnotation.sticky()));
                    }
                }
            } else if (strictMethodVerification && method.isAnnotationPresent(Subscribe.class)) {
                // 帶注解的方法參數(shù)必須是一個參數(shù)
                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)) {
            // 拋異常，帶注解的方法必須是public憎茂，非靜態(tài)珍语，非抽象（是不是很眼熟，有時候手快少寫public或者寫成了private）
            String methodName = method.getDeclaringClass().getName() + "." + method.getName();
            throw new EventBusException(methodName +
                    " is a illegal @Subscribe method: must be public, non-static, and non-abstract");
        }
    }
}

再來看看通過索引的方式吧

private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
    FindState findState = prepareFindState();
    findState.initForSubscriber(subscriberClass);
    while (findState.clazz != null) {
        // 取出自動生成的索引中的subscriberInfo 
        findState.subscriberInfo = getSubscriberInfo(findState);
        // 如果僅僅啟用了索引竖幔，但未將索引添加到eventbus中板乙，還是采用反射的方式
        if (findState.subscriberInfo != null) {
            // subscriberInfo中包含除了method以外的subscriberMethods所需要的字段
            // 所以通過subscriberInfo的getSubscriberMethods方法進而通過createSubscriberMethod的方法獲取method對象
            // 并且創(chuàng)建SubscriberMethod對象
            SubscriberMethod[] array = findState.subscriberInfo.getSubscriberMethods();
            for (SubscriberMethod subscriberMethod : array) {
                if (findState.checkAdd(subscriberMethod.method, subscriberMethod.eventType)) {
                    findState.subscriberMethods.add(subscriberMethod);
                }
            }
        } else {
            // 反射的方式
            findUsingReflectionInSingleClass(findState);
        }
        // 有父類的話 findState.clazz 設(shè)置為父類
        // 沒有的話findState.clazz = null， 結(jié)束尋找
        findState.moveToSuperclass();
    }
     // 釋放資源并且方法findState中的subscriberMethods
    return getMethodsAndRelease(findState);
}

通過上述的兩種方式找出該訂閱者所有的訂閱方法信息拳氢， 我們繼續(xù)回到egister方法中募逞，走完剩下的邏輯

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

private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
    // 事件類型
    Class<?> eventType = subscriberMethod.eventType;
    // 用Subscription類包裝訂閱者以及訂閱方法
    Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
    // 取出所有訂閱了該事件的Subscription集合
    CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
    // 可能此eventType是第一次出現(xiàn),初始化一下
    if (subscriptions == null) {
        subscriptions = new CopyOnWriteArrayList<>();
        subscriptionsByEventType.put(eventType, subscriptions);
    } else {
         // 該訂閱者register了兩次會出現(xiàn)此異常
         // 可以自行查看Subscription equals方法 細節(jié)還是挺多的
        if (subscriptions.contains(newSubscription)) {
            throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event "
                    + eventType);
        }
    }
  
   // 根據(jù)優(yōu)先級重新排序下
    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;
        }
    }
    // 該訂閱者所有的關(guān)心的事件類型集合
    List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
    // 第一次先初始化
    if (subscribedEvents == null) {
        subscribedEvents = new ArrayList<>();
        typesBySubscriber.put(subscriber, subscribedEvents);
    }
    subscribedEvents.add(eventType);
    
    // 如果當(dāng)前訂閱方法接受粘性事件，并且訂閱方法關(guān)心的事件在粘性事件集合中馋评，那么將該event事件post給subscriber
    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);
        }
    }
}

register的流程大致分析完了放接，概況下就是通過合適的方式找到訂閱者中所有的訂閱方法及其信息、關(guān)心的事件留特，然后存到對應(yīng)的集合中纠脾，如果存在關(guān)心粘性事件的訂閱方法，那么在判斷是否有之前post過的sticky事件蜕青，有則立即post給該訂閱者苟蹈。最后來張流程圖裝裝逼吧：

register流程圖