對於EventBus相信做Android程式設計師們並不陌生，他有很多優點:

• 它可以簡單Android元件之間的通訊
• 它可以避免了Android四大元件複雜的生命週期處理
• 它可以讓你的程式碼更為簡潔。

當我們要呼叫EventBus的功能時，比如註冊或者傳送事件，總會呼叫EventBus.getDefault()來獲取EventBus例項：

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

很明顯是一個單例模式，那麼看看EventBus構造方法：

    public EventBus() {
        this(DEFAULT_BUILDER);
    }

這裡傳入一個EventBusBuilder例項，對EventBus變數進行初始化。
得到到EventBus後，便可以呼叫register方法將訂閱者註冊到EventBus中，看看register方法：

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

看到第三行，根據名字我們也就知道程式碼是去查詢訂閱者的方法，我們進入到裡面看看：

    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;
        }
    }

程式碼的第二行我們看到先從方法快取裡面查詢是否有要查詢的方法，如果返回的不為空則直接返回，如果為空則執行第十行的 subscriberMethods = findUsingInfo(subscriberClass);（這裡因為ignoreGeneratedIndex預設為False）我們進入到方法裡面看個究竟：

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);
    }

這裡通過findState.subscriberInfo = getSubscriberInfo(findState);來得到訂閱者的資訊，然後到indUsingReflectionInSingleClass(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) {
        //得到方法的修飾符
            int modifiers = method.getModifiers();
            if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
                //得到引數型別
                Class<?>[] parameterTypes = method.getParameterTypes();
                //判斷引數型別長度為1
                if (parameterTypes.length == 1) {
                    //得到Subscribe型別註解
                    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");
            }
        }
    }

這個程式碼很簡單，有註釋應該沒有問題，不過到了最裡面有一個findState.checkAdd(method, eventType)；這個根據名字可以理解為檢查新增，這個方法返回一個布林型的，當然我們不能單獨的停留在這個猜想上面，我們要進入去探個究竟：

boolean checkAdd(Method method, Class<?> eventType) {
            // 2 level check: 1st level with event type only (fast), 2nd level with complete signature when required.
            // Usually a subscriber doesn't have methods listening to the same event type.
            Object existing = anyMethodByEventType.put(eventType, method);
            if (existing == null) {
                return true;
            } else {
                if (existing instanceof Method) {
                    if (!checkAddWithMethodSignature((Method) existing, eventType)) {
                        // Paranoia check
                        throw new IllegalStateException();
                    }
                    // Put any non-Method object to "consume" the existing Method
                    anyMethodByEventType.put(eventType, this);
                }
                return checkAddWithMethodSignature(method, eventType);
            }
        }

剛開始我看到這裡感覺這裡寫的好複雜，不知道大家是什麼感受。首先anyMethodByEventType以方法引數為key，儲存方法例項，正常的時候我們不會在父子類中都用相同的方法變數型別作為方法的引數註冊訂閱方法，這時候existing為null所以直接返回True，但是如果父子類中都有註冊訂閱方法，並且方法引數型別相同的話，就會執行到這裡，下面舉個例子來解釋這裡的執行過程：假如有一個頁面存在繼承關係，並且實現了public void onMessageEvent(Object object)，這樣在子類執行到這裡的時候將呼叫anyMethodByEventType.put(eventType, method);將方法變數型別和方法儲存到anyMethodByEventType裡面，然後回到findUsingReflectionInSingleClass(FindState findState)方法裡面儲存到findState中，終於出來了回到findUsingInfo(Class

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

            String methodKey = methodKeyBuilder.toString();
            Class<?> methodClass = method.getDeclaringClass();
            Class<?> methodClassOld = subscriberClassByMethodKey.put(methodKey, methodClass);
            if (methodClassOld == null || methodClassOld.isAssignableFrom(methodClass)) {
                // Only add if not already found in a sub class
                return true;
            } else {
                // Revert the put, old class is further down the class hierarchy
                //保證儲存的是最底層的class
                subscriberClassByMethodKey.put(methodKey, methodClassOld);
                return false;
            }
        }

這時在第一遍呼叫的時候methodClassOld為空所以會返回true，然後回到上層方法中執行anyMethodByEventType.put(eventType, this);繼而再執行返回checkAddWithMethodSignature(method, eventType)；不過這裡請注意，第一次呼叫的時候傳遞的第一個引數型別是子類的Method，而這次卻是父類了，如果這裡沒有注意到的話，理解就會出現各種問題。7行的if中methodClassOld不為空所以判斷後面的條件，當然這裡methodClassOld是methodClassOld的子類所以也是返回空，所以執行Else裡面的程式碼，所以父類的方法不會被加入到訂閱者的註冊方法中。
現在回到register(Object subscriber)方法中，在同步程式碼塊中呼叫了subscribe(subscriber, subscriberMethod);方法程式碼如下：

// 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);
        } 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);
            }
        }
    }

這裡不用管那麼多的程式碼，第三行以傳進來的兩個引數構造了一個Subscription物件然後在後面for迴圈中加入到了儲存所有Subscription的集合中，到此事件的註冊已經告一段落。下面我們來了解事件的傳送。
我們知道事件傳送程式碼是：EventBus.getDefault().post(1);
那麼我們進去看看傳送的程式碼是怎麼實現的：

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

        if (!postingState.isPosting) {
            postingState.isMainThread = Looper.getMainLooper() == Looper.myLooper();
            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);
                }
            } finally {
                postingState.isPosting = false;
                postingState.isMainThread = false;
            }
        }
    }

第二行PostingThreadState儲存著事件佇列和執行緒狀態資訊，得到事件佇列後將傳送的加入到裡面然後進入postSingleEvent(eventQueue.remove(0), postingState);裡面程式碼：

private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
        Class<?> eventClass = event.getClass();
        boolean subscriptionFound = false;
        if (eventInheritance) {
            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);
        }
        //找不到該事件時的異常處理
        if (!subscriptionFound) {
            if (logNoSubscriberMessages) {
                Log.d(TAG, "No subscribers registered for event " + eventClass);
            }
            if (sendNoSubscriberEvent && eventClass != NoSubscriberEvent.class &&
                    eventClass != SubscriberExceptionEvent.class) {
                post(new NoSubscriberEvent(this, event));
            }
        }
    }

eventInheritance預設是true所以執行內部程式碼lookupAllEventTypes(eventClass);則通過lookupAllEventTypes找到所有的父類事件並存在List中，然後通過postSingleEventForEventType方法對事件逐一處理，接下來看看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);
                    aborted = postingState.canceled;
                } finally {
                    postingState.event = null;
                    postingState.subscription = null;
                    postingState.canceled = false;
                }
                if (aborted) {
                    break;
                }
            }
            return true;
        }
        return false;
    }

在這裡我們看到通過傳送訊息的型別得到了所有的觀察者，然後遍歷所有的觀察者，挨個呼叫postToSubscription方法進行訊息傳送postToSubscription方法內部的實現：

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 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);
        }
    }

這裡根據方法的執行緒模式，分別處理，我們可以看到其中一種是直接呼叫，另一種是加入請求對列不過兩種方式最後呼叫的都是invokeSubscriber(subscription, event);裡面實現方式如下：

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);
        }
    }

很明顯是用反射去呼叫觀察者註冊的方法到這裡EventHub原始碼大概的流程已經走完。只是為了做一個筆記不喜勿噴。