EventBus3.0源码分析

EventBus源码理解

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EventBus是我们在开发中经常使用的开源库，使用起来比较简单，而且源码看起来不是很吃力。受到广大开发者的喜爱~

综述 ！

上面这张图片很好的解释了EventBus工作流程，简单来说就是事件被提交到EventBus之后进行查找所有订阅该事件的方法然后执行这些方法.

获取EventBus实例（单例模式）

使用了双重判断的方式，防止并发的问题，还能极大的提高效率。

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

构造方法

public EventBus() {        this(DEFAULT_BUILDER);    }

注册

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

这里面其中参数就是订阅者，也就是我们写的this，register方法主要完成两件事,查找订阅者中所有的订阅方法，然后通过遍利订阅着的订阅方法完成订阅操作。我们首先看下findSubscriberMethods这个方法：

//从缓存中获取SubscriberMethod集合List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {        List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);        if (subscriberMethods != null) {            return subscriberMethods;        }//ignoreGeneratedIndex是否忽略注解器生成的MyEventBusIndex        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;        }    }

SubscriberMethod 这个类中主要是用保存订阅方法的Method对象，线程模式，事件类型，优先级，是否粘性事件等属性，主要是两个方法findUsingReflection(subscriberClass)，findUsingInfo(subscriberClass)，这两个方法的区别就是有没有配置subscriberInfo

findUsingInfo

 private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) { //在FindState里面，它保存了一些订阅者的方法以及对订阅方法的校验        FindState findState = prepareFindState();        findState.initForSubscriber(subscriberClass);        // 如果我们通过EventBusBuilder配置了MyEventBusIndex，便会获取到subscriberInfo 通常情况下我们下代码的时候并没有配置~        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 {                //通过反射来查找订阅方法             findUsingReflectionInSingleClass(findState);            }            findState.moveToSuperclass();        }        return getMethodsAndRelease(findState);    }

# findUsingReflectionInSingleClass

• 没有通过EventBusBuilder配置MyEventBusIndex的情况下就执行这个方法了

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();                //定于方法中只能有一个参数                if (parameterTypes.length == 1) {                    Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);                    if (subscribeAnnotation != null) {                     //保存到findState对象当中                        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");            }        }    }

回到register这个方法中，上面我们分析了寻找订阅着方法部分，接下来就是注册了

subscribe

// Must be called in synchronized block    private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {    //获取订阅者方法中的订阅事件        Class<?> eventType = subscriberMethod.eventType;        //创建一个Subscription来保存订阅者和订阅方法        Subscription newSubscription = new Subscription(subscriber, subscriberMethod);        //获取当前订阅事件中Subscription的List集合        CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);        if (subscriptions == null) {         //该事件对应的Subscription的List集合不存在，则重新创建并保存在subscriptionsByEventType中            subscriptions = new CopyOnWriteArrayList<>();            subscriptionsByEventType.put(eventType, subscriptions);        } else {        //肯定药判断订阅者是否已经被注册啦            if (subscriptions.contains(newSubscription)) {                throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event "                        + eventType);            }        }//将newSubscription按照订阅方法的优先级插入到subscriptions中        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;            }        }//通过订阅者获取该订阅者所订阅事件的集合        List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);        if (subscribedEvents == null) {            subscribedEvents = new ArrayList<>();            typesBySubscriber.put(subscriber, subscribedEvents);        }        //将当前的订阅事件添加到subscribedEvents中        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);            }        }    }

这个方法才是真正的注册，上面我们说的知识寻找订阅者订阅事件的方法。概括来说首先会根据subscriber和subscriberMethod来创建一个Subscription集合subscriptions，然后根据事件类型eventType获取事件集合并把他们添加到typesBySubscriber中，然后把Subscription对象添加到subscriptions中。

事件的发送

首先药获取EventBus对象，然后通过Post方法进行事件的发送

“`java

public void post(Object event) {//PostingThreadState保存着事件队列和线程状态信息    PostingThreadState postingState = currentPostingThreadState.get();    //获取事件队列，并将当前事插入到事件队列中    List<Object> eventQueue = postingState.eventQueue;    eventQueue.add(event);    if (!postingState.isPosting) {    //当前线程是否为主线程        postingState.isMainThread = Looper.getMainLooper() == Looper.myLooper();        postingState.isPosting = true;        // 判断是否取消        if (postingState.canceled) {            throw new EventBusException("Internal error. Abort state was not reset");        }        try {        //处理队列中的所有事件            while (!eventQueue.isEmpty()) {                postSingleEvent(eventQueue.remove(0), postingState);            }        } finally {            postingState.isPosting = false;            postingState.isMainThread = false;        }    }}


上面在订阅的时候我们以订阅事件为key，将Subscription的List集合作为Value保存到了一个Map中 ，下面这个方法就是通过key来取出集合
java

 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);    }}

“`

这就是我们在接收信息的时候所用到的几个方法，具体含义就不再啰嗦了。到这里啊管理EventBus所涉及的源码分析的差不多了，虽然还有好多地方没有分析到位，但大体的思路是有的。