EventBus原理详解

在EventBus 3.0 使用介绍 这篇博客中介绍了关于EventBus的一些使用方法，下面我们就来看看它内部的具体实现吧！

流程图

先来一张整体流程图：

思路图

在看看register 和 post 的过程思路图：

register

post

通过上面的几张图，我们可以大致了解eventbus的工作流程，下面我们在来介绍一下这个流程中比较重要的几个方法

重要的方法

register中重要的方法

    List findSubscriberMethods(Class subscriberClass) {
        List subscriberMethods = METHOD_CACHE.get(subscriberClass);//先从缓存中查找是否存在订阅者的方法
        if (subscriberMethods != null) {
            return subscriberMethods;
        }

        if (ignoreGeneratedIndex) { 
            subscriberMethods = findUsingReflection(subscriberClass);//使用反射方法拿到订阅者中的订阅方法
        } else {
            subscriberMethods = findUsingInfo(subscriberClass);//使用apt处理器拿到订阅者中的订阅方法
        }
        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);//将查到的方法放到缓存中，以便下次使用
            return subscriberMethods;
        }
    }

通过反射获取方法

//准备一个FindState 来装订阅者的方法
    private FindState prepareFindState() {
        synchronized (FIND_STATE_POOL) {
            for (int i = 0; i < POOL_SIZE; i++) {
                FindState state = FIND_STATE_POOL[i];
                if (state != null) { //得到一个state并将相应的引用变量置为null
                    FIND_STATE_POOL[i] = null;
                    return state;
                }
            }
        }
        return new FindState();
    }

//将FindState 里面的方法放入subscriberMethods list中，并将FindState 放回去继续使用
    private List getMethodsAndRelease(FindState findState) {
        List 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) {//如果引用变量为null将当前这个findState赋值过去，和上面的方法相对应
                    FIND_STATE_POOL[i] = findState;
                    break;
                }
            }
        }
        return subscriberMethods;
    }

    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();//通过反射方法得到订阅者类里面的订阅方法
        } 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) { //判断方法是不是public
                Class[] parameterTypes = method.getParameterTypes();
                if (parameterTypes.length == 1) {//方法参数是不是一个
                    Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);
                    if (subscribeAnnotation != null) {//是否有注解
                        Class eventType = parameterTypes[0];
                        if (findState.checkAdd(method, eventType)) { //检查方法和参数（事件）类型是否合法
                            ThreadMode threadMode = subscribeAnnotation.threadMode();
                            findState.subscriberMethods.add(new SubscriberMethod(method, eventType, threadMode,
                                    subscribeAnnotation.priority(), subscribeAnnotation.sticky())); //都满足将方法加入findState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");
            }
        }
    }

通过apt的到方法

    private List findUsingInfo(Class subscriberClass) {
        FindState findState = prepareFindState();
        findState.initForSubscriber(subscriberClass);
        while (findState.clazz != null) {
            findState.subscriberInfo = getSubscriberInfo(findState);//通过apt获取方法
            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);//如果没有获取到再通过反射获取方法
            }
            findState.moveToSuperclass();
        }
        return getMethodsAndRelease(findState);
    }

    private SubscriberInfo getSubscriberInfo(FindState findState) {
        if (findState.subscriberInfo != null && findState.subscriberInfo.getSuperSubscriberInfo() != null) {
            SubscriberInfo superclassInfo = findState.subscriberInfo.getSuperSubscriberInfo();
            if (findState.clazz == superclassInfo.getSubscriberClass()) {//先判断缓存中是否存在
                return superclassInfo;
            }
        }
        if (subscriberInfoIndexes != null) {//subscriberInfoIndexes 是通过apt处理器自动生成的文件
            for (SubscriberInfoIndex index : subscriberInfoIndexes) {
                SubscriberInfo info = index.getSubscriberInfo(findState.clazz);
                if (info != null) {
                    return info;
                }
            }
        }
        return null;
    }

订阅

    private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
        Class eventType = subscriberMethod.eventType;
        Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
        CopyOnWriteArrayList subscriptions = subscriptionsByEventType.get(eventType);
        if (subscriptions == null) {
            subscriptions = new CopyOnWriteArrayList<>();
            subscriptionsByEventType.put(eventType, subscriptions);//获取subscriptionsByEventType map对象，用于post的时候通过eventtype获取订阅者的信息
        } 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> subscribedEvents = typesBySubscriber.get(subscriber);
        if (subscribedEvents == null) {
            subscribedEvents = new ArrayList<>();
            typesBySubscriber.put(subscriber, subscribedEvents);//获取typesBySubscriber map对象，unregister时会用到
        }
        subscribedEvents.add(eventType);

        if (subscriberMethod.sticky) {//如果是sticky事件，马上post
            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).
                Set, Object>> entries = stickyEvents.entrySet();
                for (Map.Entry, 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);
            }
        }
    }

发布事件

//通过threadMode来确认使用哪个线程执行订阅者的订阅函数
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);
        }
    }

其中mainThreadPoster继承自Handler、backgroundPoster和asyncPoster都继承自Runnable