淺談EventBus的使用原理

niknowzcd發表於2018-05-21

EventBus這是一個目前被廣泛使用的,用於在不同的介面(Activity/Fragment)之間傳遞資料的三方庫,其簡單的使用深受廣大開發者喜歡。

相比起Bundle或者介面回撥,EventBus使用起來更加簡單快捷,但有一點需要注意,再使用EventBus的時候,你需要對自己業務中的通知分發有很清晰的瞭解，不然很容易導致分發過多的無用通知,導致效能的消耗.

本文會對EventBus做一個簡單的介紹

簡單的使用

1.oncreate()中註冊

EventBus.getDefault().register(this);
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2.onDestroy()銷燬

EventBus.getDefault().unregister(this);
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3.使用@Subscribe註解方法,用來接收事件通知

@Subscribe
public void onMainEvent(String str){
    System.out.println("event = "+str);
}
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4.傳送通知

EventBus.getDefault().post("12456");
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上述4個步驟就能完成一次簡單的事件分發,這也是EventBus被廣泛使用的部分原因。

原始碼解讀

在深入原始碼之前先解釋幾個主要的成員

Subscription.class (訂閱資訊)

final class Subscription {
	//訂閱者物件,一般情況下多為activity/fragment等
    final Object subscriber;
	//訂閱者方法物件,主要包括訂閱的
    final SubscriberMethod subscriberMethod;
}
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SubscriberMethod.class (訂閱者方法物件)

public class SubscriberMethod {
	//方法
    final Method method;
	//執行緒模式
    final ThreadMode threadMode;
	//事件型別
    final Class<?> eventType;
	//優先順序
    final int priority;
	//是否是粘性事件
    final boolean sticky;
}
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FindState.class (查詢狀態,主要用來儲存查詢過程中的變數和結果)

 static class FindState {
    final List<SubscriberMethod> subscriberMethods = new ArrayList<>();
	//事件type為key,method為value
    final Map<Class, Object> anyMethodByEventType = new HashMap<>();
	//method為key,訂閱者class物件為value
    final Map<String, Class> subscriberClassByMethodKey = new HashMap<>();
    final StringBuilder methodKeyBuilder = new StringBuilder(128);
}
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另外還有幾個比較重要的Map物件

  1. subscriptionsByEventType type為key,CopyOnWriteArrayList<Subscription>為value,從這裡可以看出EventBus中一個type型別可以對應很多個訂閱者。
  2. typesBySubscriber 則剛好相反,Subscription為key,Subscription內的所有事件type為value
  3. METHOD_CACHE class類物件為key,List為value,注意這裡的是SubscriberMethod,而上述的是 Subscription， 其中值得注意的是METHOD_CACHE採用的是ConcurrentHashMap()這個資料模型,對於多併發做了一定的優化。

接下來我們以前看看這個強大的函式庫的內部實現原理。

register()

public void register(Object subscriber) {
    Class<?> subscriberClass = subscriber.getClass();
    List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
    synchronized (this) {
        for (SubscriberMethod subscriberMethod : subscriberMethods) {
            subscribe(subscriber, subscriberMethod);
        }
    }
}
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register過程的主要工作
1.獲取訂閱者的類名 (比如MainActivity)
2.通過findSubscriberMethods方法查詢訂閱者的訂閱方法 (@Subscribe註解的並且是Public修飾的)
2.1 查詢訂閱方法

List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
    List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
    if (subscriberMethods != null) {
        return subscriberMethods;
    }
	//ignoreGeneratedIndex屬性表示是否忽略註解器生成的MyEventBusIndex,預設情況下為false
    if (ignoreGeneratedIndex) {
        subscriberMethods = findUsingReflection(subscriberClass);
    } else {
        subscriberMethods = findUsingInfo(subscriberClass);
    }
	//如果訂閱者類中沒有被 @Subscribe且public宣告的方法就會報異常
    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;
    }
}
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findUsingInfo()

 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);
}
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其中以執行緒池的形式獲取FindState物件,並初始化Subscriber訂閱者物件

FindState findState = prepareFindState();
findState.initForSubscriber(subscriberClass);
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正常情況下第一次使用if (findState.subscriberInfo != null)這個判斷會為false,接下來進入findUsingReflectionInSingleClass(findState);流程

另外需要注意的是每一次的迴圈都會呼叫findState.moveToSuperclass()檢索父類的方法,所以對於一個訂閱者來說,子類和父類中的方法都會被檢索到,順序是子類->父類

findUsingReflectionInSingleClass()

 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");
        }
    }
}
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  1. 通過放射獲取所有的類中所有的方法數
  2. 過濾方法 1.public修飾 2.非靜態，非抽象 3.一個引數 4.函式擁有@Subscribe的註解
  3. findState.checkAdd(method, eventType)校驗是否可以加入到list中

findState.checkAdd(method, eventType)這裡分兩種情況

  1. anyMethodByEventType()中沒有的直接返回true
  2. anyMethodByEventType()中有的,做2次校驗,

這次根據 方法名>引數名進行完整校驗,因為同一個類中同名同參的函式是不存在的,而同名不同參的在前一步已經被過濾了，所以這裡確保在一個類中不會重複註冊.

但如果子類重寫父類的方法的話,就會出現相同的methodKey。這時EventBus會做一次驗證,並保留子類的訂閱資訊。由於掃描是由子類向父類的順序，故此時應當保留methodClassOld而忽略methodClass。如果程式碼上的註釋 Revert the put

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
    subscriberClassByMethodKey.put(methodKey, methodClassOld);
    return false;
}
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checkAdd() 之後

ThreadMode threadMode = subscribeAnnotation.threadMode();
findState.subscriberMethods.add(new SubscriberMethod(method, eventType, threadMode,
subscribeAnnotation.priority(), subscribeAnnotation.sticky()));
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新增到findState中,以一個arrayList儲存

接下來回到 getMethodsAndRelease()方法中,return 一個方法集合 List

private List<SubscriberMethod> getMethodsAndRelease(FindState findState) {
    List<SubscriberMethod> subscriberMethods = new ArrayList<>(findState.subscriberMethods);
    findState.recycle();
    synchronized (FIND_STATE_POOL) {
        for (int i = 0; i < POOL_SIZE; i++) {
            if (FIND_STATE_POOL[i] == null) {
                FIND_STATE_POOL[i] = findState;
                break;
            }
        }
    }
    return subscriberMethods;
}
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3.回到開頭的 register()方法中

for (SubscriberMethod subscriberMethod : subscriberMethods) {
    subscribe(subscriber, subscriberMethod);
}
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相對而言註冊流程還是比較簡單的,主要是讓最開始提到的兩個map(subscriptionsByEventType和typesBySubscriber)裡面填加資料,留著事件分發時候使用。最多加一些優先順序/粘性事件的判斷。

 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);
        }
    }
}	
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post流程

同樣先介紹幾個重要的類

PostingThreadState.class (事件分發狀態)

final static class PostingThreadState {
	//事件佇列
    final List<Object> eventQueue = new ArrayList<>();
    //是否正在分發,防止併發狀態下同一個事件發出多次
	boolean isPosting;
	//是否在主執行緒
    boolean isMainThread;
    Subscription subscription;
    Object event;
    boolean canceled;
}
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ThreadLocal.class (執行緒資料的儲存類)
在指定執行緒儲存資料,資料儲存之後，只有在指定執行緒才能獲取到之前儲存過的資料。

post流程

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);
            }
        } finally {
            postingState.isPosting = false;
            postingState.isMainThread = false;
        }
    }
}
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核心部分只有下面這句,遍歷傳送eventQueue佇列中的event,並且移除已經傳送的event

postSingleEvent(eventQueue.remove(0), postingState);
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postSingleEvent.class

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);
    }
    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));
        }
    }
}
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eventInheritance變數預設情況下被賦值為true,其中lookupAllEventTypes函式會遍歷eventclass,得到其父類和介面的class類,事件派發的核心部分在postSingleEventForEventType()

postSingleEventForEventType.class

 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;
}
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這個函式的邏輯也是十分簡單,根據eventClass從subscriptionsByEventType列表中獲取訂閱者列表,接著遍歷訂閱者列表,以此將event回撥。讀了這裡時,發現將eventClass作為key,而不是event作為key,估計是因為class物件能追溯到其父類和介面實現吧。

到此,post流程也結束了,比起註冊流程還要簡單。

執行緒排程 postToSubscription()

幾種poster型別

1.mainThreadPoster 建立於HandlerPoster

HandlerPoster.class

主要變數
//佇列
private final PendingPostQueue queue;
//最大存在秒數 通常為10s,超過則會報錯,這就跟廣播的onReciver回撥10s沒處理完成就會報ANR錯誤有些類似
private final int maxMillisInsideHandleMessage;
private final EventBus eventBus;
//標誌是否活動可用的
private boolean handlerActive;
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核心邏輯 handleMessage

@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;
    }
}
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遍歷佇列,執行eventBus.invokeSubscriber(pendingPost)方法

void invokeSubscriber(Subscription subscription, Object event) {
    try {
		//通過反射的方式，直接呼叫訂閱該事件方法。
        subscription.subscriberMethod.method.invoke(subscription.subscriber, event);
    } catch (InvocationTargetException e) {
        handleSubscriberException(subscription, event, e.getCause());
    } catch (IllegalAccessException e) {
        throw new IllegalStateException("Unexpected exception", e);
    }
}
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**2.BackgroundPoster:**實現 Runnable,會將PendingPostQueue佇列中所有的事件分發出去

**3.AsyncPoster:**同樣實現 Runnable,只會分發一個PendingPostQueue佇列中的事件

postToSubscription.class

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 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);
    }
}
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邏輯也很好理解,相同執行緒時直接invokeSubscriber反射回撥,不同執行緒則發到相同執行緒去回撥。

unregister流程

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());
    }
}
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主要工作分兩步

  1. 移除訂閱者的所有訂閱資訊
  2. 移除訂閱者和其所有Event的關聯

另外看過的一篇博文上寫了有關於EventBus的優化操作,記錄下來

  1. eventInheritance預設為true,會遍歷class對應的父類和介面物件,如果程式中沒有使用到事件的繼承關係,可以將該值設為false
  2. ignoreGeneratedIndex表示是否使用生成的APT程式碼去優化查詢事件的過程,如果專案中沒有接入EventBus的APT，也可以將其設定為false

補充

hashMap.put()

hashMap.put() 會根據key返回對應的value值
如果put的時候沒有對應的key值,則新增到map中,如果有 先返回後新增

如

map.put("222", "222");
String value2=map.put("222","333");
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此時的value2為"222"，而map中 key="222"的value為"333"

instanceof, isinstance,isAssignableFrom的區別

instanceof運算子 只被用於物件引用變數，檢查左邊的被測試物件 是不是 右邊類或介面的 例項化。如果被測物件是null值，則測試結果總是false。 形象地：自身例項或子類例項 instanceof 自身類 返回true

  String s=new String("javaisland"); 
  System.out.println(s instanceof String); //true 
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Class類的isInstance(Object obj)方法，obj是被測試的物件，如果obj是呼叫這個方法的class或介面 的例項，則返回true。這個方法是instanceof運算子的動態等價。 形象地：自身類.class.isInstance(自身例項或子類例項) 返回true

  String s=new String("javaisland"); 
  System.out.println(String.class.isInstance(s)); //true 
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Class類的isAssignableFrom(Class cls)方法，如果呼叫這個方法的class或介面 與 引數cls表示的類或介面相同，或者是引數cls表示的類或介面的父類，則返回true。 形象地：自身類.class.isAssignableFrom(自身類或子類.class) 返回true 例：

System.out.println(ArrayList.class.isAssignableFrom(Object.class));  //false 
System.out.println(Object.class.isAssignableFrom(ArrayList.class));  //true
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參考文章

EventBus原始碼分析 EventBus

另外

個人的github
閒暇之餘寫的故事

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