What?EventBus的核心竟然只是這兩個(gè)Map?

簡單介紹一下EventBus

其實(shí)EventBus大家都很熟悉了,就不過多去說它了。通常我們叫它事件總線,其實(shí)它更像是廣播,觀察者模式,一方發(fā)送消息多方接收。在EventBus的創(chuàng)建訂閱過程中,最重要的就是有兩個(gè)關(guān)鍵的Map,這兩個(gè)鍵值對里面存儲了我們定義的訂閱方法和相關(guān)的類,那到底是具體是怎么操作的呢,來源碼一探究竟。

下面的代碼基于EventBus3.1.1

創(chuàng)建和訂閱消息

注冊

EventBus的注冊很簡單,

 @Override
 public void onStart() {
     super.onStart();
     EventBus.getDefault().register(this);
 }

 @Override
 public void onStop() {
     super.onStop();
     EventBus.getDefault().unregister(this);
 }

這里一定要記住在onStop中注銷,避免在Activity關(guān)閉后還跟EventBus有聯(lián)系,然后造成內(nèi)存泄漏。

接下來進(jìn)入到方法中看一看:

單例模式

EventBus.getDefault

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

這個(gè)方法很簡單,就是雙重校驗(yàn)單例,繼續(xù)看register方法:

register方法

EventBus.register

    public void register(Object subscriber) {
        Class<?> subscriberClass = subscriber.getClass();//注冊的類
        List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);//查找訂閱方法
        synchronized (this) {
            for (SubscriberMethod subscriberMethod : subscriberMethods) {//循環(huán)找到的所有方法
                subscribe(subscriber, subscriberMethod);
            }
        }
    }

在register中,傳入的參數(shù)名為subscriber,意思就是訂閱者,類型為Object。在這個(gè)方法里有個(gè)List<SubscriberMethod>的列表,這個(gè)SubscriberMethod是什么呢?

public class SubscriberMethod {
    final Method method;//對應(yīng)的方法
    final ThreadMode threadMode;//線程模式
    final Class<?> eventType;//消息類型
    final int priority;//優(yōu)先級
    final boolean sticky;//是否支持粘性
    /** Used for efficient comparison */
    String methodString;//用于equal對比
}

其實(shí)這個(gè)類就保存了我們訂閱者里面定義的方法里面的所有信息。

線程模式

其中threadMode的定義如下:

  • ThreadMode.POSTING:默認(rèn)的線程模式,在那個(gè)線程發(fā)送事件就在對應(yīng)線程處理事件,避免了線程切換,效率高。

  • ThreadMode.MAIN:如在主線程(UI線程)發(fā)送事件,則直接在主線程處理事件;如果在子線程發(fā)送事件,則先將事件入隊(duì)列,然后通過 Handler 切換到主線程,依次處理事件。

  • ThreadMode.MAIN_ORDERED:無論在那個(gè)線程發(fā)送事件,都先將事件入隊(duì)列,然后通過 Handler 切換到主線程,依次處理事件。

  • ThreadMode.BACKGROUND:如果在主線程發(fā)送事件,則先將事件入隊(duì)列,然后通過線程池依次處理事件;如果在子線程發(fā)送事件,則直接在發(fā)送事件的線程處理事件。

  • ThreadMode.ASYNC:無論在那個(gè)線程發(fā)送事件,都將事件入隊(duì)列,然后通過線程池處理。

所以在register方法中的這個(gè)列表其實(shí)就是當(dāng)前我們這個(gè)訂閱者類包含所有的接收方法。這里通過subscriberMethodFinder.findSubscriberMethods(subscriberClass)方法獲取:

查找訂閱事件的方法(接收消息的方法)

SubscriberMethodFinder findSubscriberMethods


    private static final Map<Class<?>, List<SubscriberMethod>> METHOD_CACHE = new ConcurrentHashMap<>();

    List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
        List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);//首先從緩存中獲取
        if (subscriberMethods != null) {
            return subscriberMethods;//緩存有的話直接返回
        }

        if (ignoreGeneratedIndex) {//是否忽略注解
            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);//加入到緩存中
            return subscriberMethods;
        }
    }

這里我們注意到有一個(gè)ignoreGeneratedIndex參數(shù),這個(gè)屬性的值是在構(gòu)造方法中獲取的

subscriberMethodFinder = new SubscriberMethodFinder(builder.subscriberInfoIndexes,
                builder.strictMethodVerification, builder.ignoreGeneratedIndex);
    /** Forces the use of reflection even if there's a generated index (default: false). */
    public EventBusBuilder ignoreGeneratedIndex(boolean ignoreGeneratedIndex) {
        this.ignoreGeneratedIndex = ignoreGeneratedIndex;
        return this;
    }

從官方的注釋可以看出,這里默認(rèn)值是false,所以是調(diào)用findUsingInfo方法:


    private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
        FindState findState = prepareFindState();
        findState.initForSubscriber(subscriberClass);//初始化查找輔助類
        while (findState.clazz != null) {//這里就是我們r(jià)egister的類
            findState.subscriberInfo = getSubscriberInfo(findState);
            if (findState.subscriberInfo != null) {//初始狀態(tài)為空,所以執(zhí)行else
                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();//查找父類
        }
        return getMethodsAndRelease(findState);//釋放資源
    }

FindState是查找輔助類,里面存儲了關(guān)于查找到的訂閱方法信息和相關(guān)類的信息:

static class FindState {
        final List<SubscriberMethod> subscriberMethods = new ArrayList<>();//訂閱的方法
        final Map<Class, Object> anyMethodByEventType = new HashMap<>();//通過消息類型獲取方法
        final Map<String, Class> subscriberClassByMethodKey = new HashMap<>();//通過方法獲取訂閱的類
        final StringBuilder methodKeyBuilder = new StringBuilder(128);

        Class<?> subscriberClass;
        Class<?> clazz;
        boolean skipSuperClasses;
        SubscriberInfo subscriberInfo;
}

在這里會執(zhí)行else里面的findUsingReflectionInSingleClass方法,通過反射來查找訂閱方法

SubscriberMethodFinder 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;
        }
        //循環(huán)所有的方法
        for (Method method : methods) {
            int modifiers = method.getModifiers();//獲取修飾符
            if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {//public,非abstract、static
                Class<?>[] parameterTypes = method.getParameterTypes();//獲取參數(shù)類型
                if (parameterTypes.length == 1) {//只能有一個(gè)參數(shù)
                    Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);//獲取Subscribe注解
                    if (subscribeAnnotation != null) {
                        Class<?> eventType = parameterTypes[0];//獲取參數(shù)類型,也就是我們自定義的消息的類型
                        if (findState.checkAdd(method, eventType)) {//是否添加過這個(gè)方法
                            ThreadMode threadMode = subscribeAnnotation.threadMode();//獲取線程模式
                            findState.subscriberMethods.add(new SubscriberMethod(method, eventType, threadMode,
                                    subscribeAnnotation.priority(), subscribeAnnotation.sticky()));//添加到數(shù)組中
                        }
                    }
                } 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");
            }
        }
    }

整個(gè)的過程就是遍歷所有的方法,然后判斷有Subscribe注解的方法,然后將方法的信息加到數(shù)組當(dāng)中。在查找完畢之后,接下來回到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);
            }
        }
    }

循環(huán)訂閱

拿到我們剛才的所有訂閱方法的數(shù)組,然后在循環(huán)中執(zhí)行subscribe方法,這個(gè)方法執(zhí)行訂閱操作:

    private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
        Class<?> eventType = subscriberMethod.eventType;//獲取我們自定義的消息類型
        Subscription newSubscription = new Subscription(subscriber, subscriberMethod);//新建一個(gè)訂閱類,這個(gè)類包含了訂閱的類和方法兩個(gè)屬性
        //subscriptionsByEventType是以eventType為key,Subscription數(shù)組為value的map
        CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
        if (subscriptions == null) {//為空的話創(chuàng)建數(shù)組,并加入到map中
            subscriptions = new CopyOnWriteArrayList<>();
            subscriptionsByEventType.put(eventType, subscriptions);
        } else {
            if (subscriptions.contains(newSubscription)) {//已經(jīng)訂閱過,拋出異常
                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) {//將優(yōu)先級更高的訂閱方法加入到數(shù)組中
                subscriptions.add(i, newSubscription);
                break;
            }
        }
        //typesBySubscriber是以對象為key,訂閱的方法的數(shù)組為value的map
        List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
        if (subscribedEvents == null) {
            subscribedEvents = new ArrayList<>();
            typesBySubscriber.put(subscriber, subscribedEvents);
        }
        subscribedEvents.add(eventType);
        //粘性事件相關(guān)
        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);
            }
        }
    }

其實(shí)EventBus的register方法的內(nèi)部具體做的事情很簡單,總的來說就是兩步:

  1. 通過反射查找我們使用了Subscribe注解的訂閱方法
  2. 循環(huán)遍歷找到的方法,然后加到關(guān)鍵的兩個(gè)map中(subscriptionsByEventTypetypesBySubscriber

subscriptionsByEventType:以eventType為key,Subscription數(shù)組為value

typesBySubscriber:以對象為key,訂閱的方法的數(shù)組為value

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