EventBus源碼解析

時間 2019-11-06

標籤 eventbus 源碼解析简体版

原文原文鏈接

前面一篇文章講解了EventBus的使用，可是做爲開發人員，不能只停留在僅僅會用的層面上，咱們還須要弄清楚它的內部實現原理。因此本篇博文將分析EventBus的源碼，看看究竟它是如何實現「發佈/訂閱」功能的。 html

相關文章
EventBus使用詳解
 EventBus源碼解析java

事件註冊

根據前一講EventBus使用詳解咱們已經知道EventBus使用首先是須要註冊的，註冊事件的代碼以下：android

EventBus.getDefault().register(this);

EventBus對外提供了一個register方法來進行事件註冊，該方法接收一個Object類型的參數，下面看下register方法的源碼:緩存

public void register(Object subscriber) {
    Class<?> subscriberClass = subscriber.getClass();
    // 判斷該類是不是匿名內部類
    boolean forceReflection = subscriberClass.isAnonymousClass();
    List<SubscriberMethod> subscriberMethods =
            subscriberMethodFinder.findSubscriberMethods(subscriberClass, forceReflection);
    for (SubscriberMethod subscriberMethod : subscriberMethods) {
        subscribe(subscriber, subscriberMethod);
    }
}

該方法首先獲取獲取傳進來參數的Class對象，而後判斷該類是不是匿名內部類。而後根據這兩個參數經過subscriberMethodFinder.findSubscriberMethods方法獲取全部的事件處理方法。安全

List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass, boolean forceReflection) {
    String key = subscriberClass.getName();
    List<SubscriberMethod> subscriberMethods;
    synchronized (METHOD_CACHE) {
        subscriberMethods = METHOD_CACHE.get(key);
    }
    if (subscriberMethods != null) {
        //緩存命中，直接返回
        return subscriberMethods;
    }
    if (INDEX != null && !forceReflection) {
        // 若是INDEX不爲空，而且subscriberClass爲非匿名內部類，
        // 則經過findSubscriberMethodsWithIndex方法查找事件處理函數
        subscriberMethods = findSubscriberMethodsWithIndex(subscriberClass);
        if (subscriberMethods.isEmpty()) {
            //若是結果爲空，則使用findSubscriberMethodsWithReflection方法再查找一次
            subscriberMethods = findSubscriberMethodsWithReflection(subscriberClass);
        }
    } else {
        //INDEX爲空或者subscriberClass未匿名內部類，使用findSubscriberMethodsWithReflection方法查找
        subscriberMethods = findSubscriberMethodsWithReflection(subscriberClass);
    }
    if (subscriberMethods.isEmpty()) {
        throw new EventBusException("Subscriber " + subscriberClass
                + " and its super classes have no public methods with the @Subscribe annotation");
    } else {
        //存入緩存並返回
        synchronized (METHOD_CACHE) {
            METHOD_CACHE.put(key, subscriberMethods);
        }
        return subscriberMethods;
    }
}

經過名字咱們就知道這個方法是獲取subscriberClass類中全部的事件處理方法（即便用了@Subscribe的方法）。該方法首先會從緩存METHOD_CACHE中去獲取事件處理方法，若是緩存中不存在，則須要經過findSubscriberMethodsWithIndex或者findSubscriberMethodsWithReflection方法獲取全部事件處理方法，獲取到以後先存入緩存再返回。app

這個方法裏面有個INDEX對象，咱們看看它是個什麼鬼：異步

/** Optional generated index without entries from subscribers super classes */
private static final SubscriberIndex INDEX;

static {
    SubscriberIndex newIndex = null;
    try {
        Class<?> clazz = Class.forName("de.greenrobot.event.GeneratedSubscriberIndex");
        newIndex = (SubscriberIndex) clazz.newInstance();
    } catch (ClassNotFoundException e) {
        Log.d(EventBus.TAG, "No subscriber index available, reverting to dynamic look-up");
        // Fine
    } catch (Exception e) {
        Log.w(EventBus.TAG, "Could not init subscriber index, reverting to dynamic look-up", e);
    }
    INDEX = newIndex;
}

由上面代碼能夠看出EventBus會試圖加載一個de.greenrobot.event.GeneratedSubscriberIndex類並建立對象賦值給INDEX，可是EventBus3.0 beta並無爲咱們提供該類（可能後續版本會提供）。因此INDEX爲null。async

咱們再返回findSubscriberMethods方法，咱們知道INDEX已經爲null了，因此必然會調用findSubscriberMethodsWithReflection方法查找全部事件處理函數：ide

private List<SubscriberMethod> findSubscriberMethodsWithReflection(Class<?> subscriberClass) {
    List<SubscriberMethod> subscriberMethods = new ArrayList<SubscriberMethod>();
    Class<?> clazz = subscriberClass;
    HashSet<String> eventTypesFound = new HashSet<String>();
    StringBuilder methodKeyBuilder = new StringBuilder();
    while (clazz != null) {
        String name = clazz.getName();
        // 若是查找的類是java、javax或者android包下面的類，則過濾掉
        if (name.startsWith("java.") || name.startsWith("javax.") || name.startsWith("android.")) {
            // Skip system classes, this just degrades performance
            break;
        }

        // Starting with EventBus 2.2 we enforced methods to be public (might change with annotations again)
        // 經過反射查找全部該類中全部方法
        Method[] methods = clazz.getDeclaredMethods();
        for (Method method : methods) {
            int modifiers = method.getModifiers();
            // 事件處理方法必須爲public，這裏過濾掉全部非public方法
            if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
                Class<?>[] parameterTypes = method.getParameterTypes();
                // 事件處理方法必須只有一個參數
                if (parameterTypes.length == 1) {
                    Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);
                    if (subscribeAnnotation != null) {
                        String methodName = method.getName();
                        Class<?> eventType = parameterTypes[0];
                        methodKeyBuilder.setLength(0);
                        methodKeyBuilder.append(methodName);
                        methodKeyBuilder.append('>').append(eventType.getName());

                        String methodKey = methodKeyBuilder.toString();
                        if (eventTypesFound.add(methodKey)) {
                            // Only add if not already found in a sub class
                            // 只有在子類中沒有找到，纔會添加到subscriberMethods
                            ThreadMode threadMode = subscribeAnnotation.threadMode();
                            subscriberMethods.add(new SubscriberMethod(method, eventType, threadMode,
                                    subscribeAnnotation.priority(), subscribeAnnotation.sticky()));
                        }
                    }
                } else if (strictMethodVerification) {
                    // 若是某個方法加了@Subscribe註解，而且不是1個參數，則拋出EventBusException異常
                    if (method.isAnnotationPresent(Subscribe.class)) {
                        String methodName = name + "." + method.getName();
                        throw new EventBusException("@Subscribe method " + methodName +
                                "must have exactly 1 parameter but has " + parameterTypes.length);
                    }
                }
            } else if (strictMethodVerification) {
                // 若是某個方法加了@Subscribe註解，而且不是public修飾，則拋出EventBusException異常
                if (method.isAnnotationPresent(Subscribe.class)) {
                    String methodName = name + "." + method.getName();
                    throw new EventBusException(methodName +
                            " is a illegal @Subscribe method: must be public, non-static, and non-abstract");
                }

            }
        }
        // 會繼續查找父類的方法
        clazz = clazz.getSuperclass();
    }
    return subscriberMethods;
}

該方法主要做用就是找出subscriberClass類以及subscriberClass的父類中全部的事件處理方法（添加了@Subscribe註解，訪問修飾符爲public而且只有一個參數）。值得注意的是：若是子類與父類中同時存在了相同事件處理函數，則父類中的不會被添加到subscriberMethods。函數

好了，查找事件處理函數的過程已經完了，咱們繼續回到register方法中：

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

找到事件處理函數後，會遍歷找到的全部事件處理函數並調用subscribe方法將全部事件處理函數註冊到EventBus中。

private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
    Class<?> eventType = subscriberMethod.eventType;
    // 獲取訂閱了某種類型數據的 Subscription 。 使用了 CopyOnWriteArrayList ，這個是線程安全的，
    // CopyOnWriteArrayList 會在更新的時候，從新生成一份 copy，其餘線程使用的是 
    // copy，不存在什麼線程安全性的問題。
    CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
    Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
    if (subscriptions == null) {
        subscriptions = new CopyOnWriteArrayList<Subscription>();
        subscriptionsByEventType.put(eventType, subscriptions);
    } else {
        //若是已經被註冊過了，則拋出EventBusException異常
        if (subscriptions.contains(newSubscription)) {
            throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event "
                    + eventType);
        }
    }

    // Starting with EventBus 2.2 we enforced methods to be public (might change with annotations again)
    // subscriberMethod.method.setAccessible(true);

    // Got to synchronize to avoid shifted positions when adding/removing concurrently
    // 根據優先級將newSubscription查到合適位置
    synchronized (subscriptions) {
        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;
            }
        }
    }

    //將處理事件類型添加到typesBySubscriber
    List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
    if (subscribedEvents == null) {
        subscribedEvents = new ArrayList<Class<?>>();
        typesBySubscriber.put(subscriber, subscribedEvents);
    }
    subscribedEvents.add(eventType);

    // 若是該事件處理方法爲粘性事件，即設置了「sticky = true」，則須要調用checkPostStickyEventToSubscription
    // 判斷是否有粘性事件須要處理，若是須要處理則觸發一次事件處理函數
    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);
        }
    }
}

若是事件處理函數設置了「sticky = true」，則會調用checkPostStickyEventToSubscription處理粘性事件。

private void checkPostStickyEventToSubscription(Subscription newSubscription, Object stickyEvent) {
    if (stickyEvent != null) {
        // If the subscriber is trying to abort the event, it will fail (event is not tracked in posting state)
        // --> Strange corner case, which we don't take care of here.
        postToSubscription(newSubscription, stickyEvent, Looper.getMainLooper() == Looper.myLooper());
    }
}

若是存在粘性事件，則當即調用postToSubscription觸發該事件的事件處理函數。postToSubscription函數後面講post時會講到。

至此，整個register過程就介紹完了。
總結一下，整個過程分爲3步：

查找註冊的類中全部的事件處理函數（添加了@Subscribe註解且訪問修飾符爲public的方法）
將全部事件處理函數註冊到EventBus
若是有事件處理函數設置了「sticky = true」，則當即處理該事件

post事件

register過程講完後，咱們知道了EventBus如何找到咱們定義好的事件處理函數。有了這些事件處理函數，當post相應事件的時候，EventBus就會觸發訂閱該事件的處理函數。具體post過程是怎樣的呢？咱們看看代碼：

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

    if (!postingState.isPosting) {
        // 標識post的線程是不是主線程
        postingState.isMainThread = Looper.getMainLooper() == Looper.myLooper();
        postingState.isPosting = true;
        if (postingState.canceled) {
            throw new EventBusException("Internal error. Abort state was not reset");
        }
        try {
            // 循環處理eventQueue中的每個event對象
            while (!eventQueue.isEmpty()) {
                postSingleEvent(eventQueue.remove(0), postingState);
            }
        } finally {
            // 處理完以後重置postingState的一些標識信息
            postingState.isPosting = false;
            postingState.isMainThread = false;
        }
    }
}

currentPostingThreadState是一個ThreadLocal類型，裏面存儲了PostingThreadState；

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

/** For ThreadLocal, much faster to set (and get multiple values). */
final static class PostingThreadState {
    final List<Object> eventQueue = new ArrayList<Object>();
    boolean isPosting;
    boolean isMainThread;
    Subscription subscription;
    Object event;
    boolean canceled;
}

PostingThreadState包含了一個事件隊列eventQueue和一些標誌信息。eventQueue存放全部待post的事件對象。

咱們再回到post方法，首先會將event對象添加到事件隊列eventQueue中。而後判斷是否有事件正在post，若是沒有則會遍歷eventQueue中每個event對象，而且調用postSingleEvent方法post該事件。

private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
    Class<?> eventClass = event.getClass();
    boolean subscriptionFound = false;
    if (eventInheritance) {
        // 若是容許事件繼承，則會調用lookupAllEventTypes查找全部的父類和接口類
        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);
    }
    if (!subscriptionFound) {
        if (logNoSubscriberMessages) {
            Log.d(TAG, "No subscribers registered for event " + eventClass);
        }
        if (sendNoSubscriberEvent && eventClass != NoSubscriberEvent.class &&
                eventClass != SubscriberExceptionEvent.class) {
            // 若是post的事件沒有被註冊，則post一個NoSubscriberEvent事件
            post(new NoSubscriberEvent(this, event));
        }
    }
}

若是容許事件繼承，則會調用lookupAllEventTypes查找全部的父類和接口類。

private List<Class<?>> lookupAllEventTypes(Class<?> eventClass) {
    synchronized (eventTypesCache) {
        List<Class<?>> eventTypes = eventTypesCache.get(eventClass);
        if (eventTypes == null) {
            eventTypes = new ArrayList<Class<?>>();
            Class<?> clazz = eventClass;
            while (clazz != null) {
                eventTypes.add(clazz);
                addInterfaces(eventTypes, clazz.getInterfaces());
                clazz = clazz.getSuperclass();
            }
            eventTypesCache.put(eventClass, eventTypes);
        }
        return eventTypes;
    }
}

這個方法很簡單，就是查找eventClass類的全部父類和接口，並將其保存到eventTypesCache中，方便下次使用。
咱們再回到postSingleEvent方法。無論允不容許事件繼承，都會執行postSingleEventForEventType方法post事件。

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postSingleEventForEventType方法中，會已eventClass爲key從subscriptionsByEventType對象中獲取Subscription列表。在上面講register的時候咱們已經看到EventBus在register的時候會將Subscription列表存儲在subscriptionsByEventType中。接下來會遍歷subscriptions列表而後調用postToSubscription方法進行下一步處理。

private void postToSubscription(Subscription subscription, Object event, boolean isMainThread) {
    switch (subscription.subscriberMethod.threadMode) {
        case PostThread:
            // 若是該事件處理函數沒有指定線程模型或者線程模型爲PostThread
            // 則調用invokeSubscriber在post的線程中執行事件處理函數
            invokeSubscriber(subscription, event);
            break;
        case MainThread:
            // 若是該事件處理函數指定的線程模型爲MainThread
            // 而且當前post的線程爲主線程，則調用invokeSubscriber在當前線程（主線程）中執行事件處理函數
            // 若是post的線程不是主線程，將使用mainThreadPoster.enqueue該事件處理函數添加到主線程的消息隊列中
            if (isMainThread) {
                invokeSubscriber(subscription, event);
            } else {
                mainThreadPoster.enqueue(subscription, event);
            }
            break;
        case BackgroundThread:
            // 若是該事件處理函數指定的線程模型爲BackgroundThread
            // 而且當前post的線程爲主線程，則調用backgroundPoster.enqueue
            // 若是post的線程不是主線程，則調用invokeSubscriber在當前線程（非主線程）中執行事件處理函數
            if (isMainThread) {
                backgroundPoster.enqueue(subscription, event);
            } else {
                invokeSubscriber(subscription, event);
            }
            break;
        case Async:
            //添加到異步線程隊列中
            asyncPoster.enqueue(subscription, event);
            break;
        default:
            throw new IllegalStateException("Unknown thread mode: " + subscription.subscriberMethod.threadMode);
    }
}

該方法主要是根據register註冊的事件處理函數的線程模型在指定的線程中觸發事件處理函數。在上一講EventBus使用詳解中已經講過EventBus的線程模型相關概念了，不明白的能夠回去看看。
mainThreadPoster、backgroundPoster和asyncPoster分別是HandlerPoster、BackgroundPoster和AsyncPoster的對象，其中HandlerPoster繼承自Handle，BackgroundPoster和AsyncPoster繼承自Runnable。
咱們主要看看HandlerPoster。

mainThreadPoster = new HandlerPoster(this, Looper.getMainLooper(), 10);

在EventBus的構造函數中，咱們看到mainThreadPoster初始化的時候，傳入的是Looper.getMainLooper()。因此此Handle是運行在主線程中的。
mainThreadPoster.enqueue方法：

void enqueue(Subscription subscription, Object event) {
    PendingPost pendingPost = PendingPost.obtainPendingPost(subscription, event);
    synchronized (this) {
        queue.enqueue(pendingPost);
        if (!handlerActive) {
            handlerActive = true;
            if (!sendMessage(obtainMessage())) {
                throw new EventBusException("Could not send handler message");
            }
        }
    }
}

enqueue方法最終會調用sendMessage方法，因此該Handle的handleMessage方法會被調用。

@Override
public void handleMessage(Message msg) {
    boolean rescheduled = false;
    try {
        long started = SystemClock.uptimeMillis();
        while (true) {
            PendingPost pendingPost = queue.poll();
            if (pendingPost == null) {
                synchronized (this) {
                    // Check again, this time in synchronized
                    pendingPost = queue.poll();
                    if (pendingPost == null) {
                        handlerActive = false;
                        return;
                    }
                }
            }
            eventBus.invokeSubscriber(pendingPost);
            long timeInMethod = SystemClock.uptimeMillis() - started;
            if (timeInMethod >= maxMillisInsideHandleMessage) {
                if (!sendMessage(obtainMessage())) {
                    throw new EventBusException("Could not send handler message");
                }
                rescheduled = true;
                return;
            }
        }
    } finally {
        handlerActive = rescheduled;
    }
}

在該方法中，最終仍是會調用eventBus.invokeSubscriber調用事件處理函數。

BackgroundPoster和AsyncPoster繼承自Runnable，而且會在enqueue方法中調用eventBus.getExecutorService().execute(this);具體run方法你們能夠本身去看源碼，最終都會調用eventBus.invokeSubscriber方法。咱們看看eventBus.invokeSubscriber方法的源碼：

void invokeSubscriber(PendingPost pendingPost) {
    Object event = pendingPost.event;
    Subscription subscription = pendingPost.subscription;
    PendingPost.releasePendingPost(pendingPost);
    if (subscription.active) {
        invokeSubscriber(subscription, event);
    }
}

該方法會調用invokeSubscriber方法進一步處理：

void invokeSubscriber(Subscription subscription, Object event) {
    try {
        // 經過反射調用事件處理函數
        subscription.subscriberMethod.method.invoke(subscription.subscriber, event);
    } catch (InvocationTargetException e) {
        handleSubscriberException(subscription, event, e.getCause());
    } catch (IllegalAccessException e) {
        throw new IllegalStateException("Unexpected exception", e);
    }
}

該方法最終會經過反射來調用事件處理函數。至此，整個post過程分析完了。
總結一下整個post過程，大體分爲3步：

將事件對象添加到事件隊列eventQueue中等待處理
遍歷eventQueue隊列中的事件對象並調用postSingleEvent處理每一個事件
找出訂閱過該事件的全部事件處理函數，並在相應的線程中執行該事件處理函數

取消事件註冊

上面已經分析了EventBus的register和post過程，這兩個過程是EventBus的核心。不須要訂閱事件時須要取消事件註冊：

/** 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) {
            unubscribeByEventType(subscriber, eventType);
        }
        typesBySubscriber.remove(subscriber);
    } else {
        Log.w(TAG, "Subscriber to unregister was not registered before: " + subscriber.getClass());
    }
}

取消事件註冊很簡單，只是將register過程註冊到EventBus的事件處理函數移除掉。

到這裏，EventBus源碼咱們已經分析完了，若有不對的地方還望指點。

本文首發：http://liuling123.com/2016/01/EventBus-source.html