EventBus 事件总线 原理

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目录

目录
EventBus
注册 register
findSubscriberMethods
suscribe 方法
发布 post
粘性事件 sticky

EventBus

一句话描述：调用【register】方法后，EventBus会把当前类中【onEvent开头的方法】，存入一个【单例】内部维持着一个【map】中，KEY就是【方法中的参数】，Value就是此【onEvent开头的方法】；而后在post的时候，EventBus就会根据我们传入的参数的类型去map中查找匹配的方法(可能有很多个)，然后逐个【反射】调用这些方法。

收不到消息的原因

• 方法名不对（必须以onEvent开头）
• 参数不对（参数只能有一个，且不能是基本类型）
• 可能没有注册。发送不需要注册，接收需要注册。父类如果注册的话，子类调用super方法后不能再注册，否则异常退出
• 注册的位置可能不对（试一下改为在onCreate方法中注册，在onDestroy中1取消）
• 可能需要加注解（在发送和接收的方法上加上@Subscribe注解）
• 可能你的onEvent方法运行时有错误（比如空指针），此时EventBus不会提示任何异常，这会让你误以为是没有回调到
• 如果在BActivity中发了一条消息，AActivity中收到消息后时不能显示吐司的

注册 register

简单说，EventBus.getDefault().register(this)所做的事情就是：在当前类中遍历所有的方法，找到onEvent开头的然后进行存储。

EventBus.getDefault()其实就是个单例，register公布给我们使用的有4个，本质上调用的是同一个：

public void register(Object subscriber) { register(subscriber, DEFAULT_METHOD_NAME, false, 0); }
public void register(Object subscriber, int priority) { register(subscriber, DEFAULT_METHOD_NAME, false, priority); }
public void registerSticky(Object subscriber) { register(subscriber, DEFAULT_METHOD_NAME, true, 0);    }
public void registerSticky(Object subscriber, int priority) { register(subscriber, DEFAULT_METHOD_NAME, true, priority); }

private synchronized void register(Object subscriber, String methodName, boolean sticky, int priority) {
    List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriber.getClass(), methodName);
    for (SubscriberMethod subscriberMethod : subscriberMethods) {
        subscribe(subscriber, subscriberMethod, sticky, priority);
    }
}

四个参数

• subscriber：需要扫描的类，也就是我们代码中常见的this
• methodName：用于确定扫描什么开头的方法，这个是写死的"onEvent"，可见我们的类中都是以这个开头
• sticky：是否是粘性事件，最后面再单独说
• priority：优先级，优先级越高，在调用的时候会越先调用

findSubscriberMethods

首先会调用findSubscriberMethods方法，实际是去遍历该类内部所有方法，然后根据methodName去匹配，匹配成功的就封装成SubscriberMethod对象，最后存储到一个List并返回。

List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass, String eventMethodName) {
    String key = subscriberClass.getName() + '.' + eventMethodName;
    List<SubscriberMethod> subscriberMethods;
    synchronized (methodCache) {
        subscriberMethods = methodCache.get(key);
    }
    if (subscriberMethods != null) {
        return subscriberMethods;
    }
    subscriberMethods = new ArrayList<SubscriberMethod>();
    Class<?> clazz = subscriberClass;
    HashSet<String> eventTypesFound = new HashSet<String>();
    StringBuilder methodKeyBuilder = new StringBuilder();
    while (clazz != null) {
        String name = clazz.getName();
        if (name.startsWith("java.") || name.startsWith("javax.") || name.startsWith("android.")) {
            // Skip system classes, this just degrades performance  降低性能
            break;
        }
        // Starting with EventBus 2.2 we enforced 强制 methods to be public (might change with annotations again)
        Method[] methods = clazz.getMethods();//去得到所有的方法
        for (Method method : methods) {//开始遍历每一个方法，去匹配封装
            String methodName = method.getName();
            if (methodName.startsWith(eventMethodName)) {//分别判断了是否以onEvent开头，是否是public且非static和abstract方法，是否是一个参数。如果都符合，才进入封装的部分。
                int modifiers = method.getModifiers();
                if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
                    Class<?>[] parameterTypes = method.getParameterTypes();
                    if (parameterTypes.length == 1) {
                        String modifierString = methodName.substring(eventMethodName.length());
                        ThreadMode threadMode;
                        if (modifierString.length() == 0) {//根据方法的后缀，来确定threadMode，threadMode是个枚举类型
                            threadMode = ThreadMode.PostThread;
                        } else if (modifierString.equals("MainThread")) {
                            threadMode = ThreadMode.MainThread;
                        } else if (modifierString.equals("BackgroundThread")) {
                            threadMode = ThreadMode.BackgroundThread;
                        } else if (modifierString.equals("Async")) {
                            threadMode = ThreadMode.Async;
                        } else {
                            if (skipMethodVerificationForClasses.containsKey(clazz)) {
                                continue;
                            } else {
                                throw new EventBusException("Illegal onEvent method, check for typos: " + method);
                            }
                        }
                        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
                            subscriberMethods.add(new SubscriberMethod(method, threadMode, eventType));//将method, threadMode, eventType传入构造SubscriberMethod对象，添加到List
                        }
                    }
                } else if (!skipMethodVerificationForClasses.containsKey(clazz)) {
                    Log.d(EventBus.TAG, "Skipping method (not public, static or abstract): " + clazz + "." + methodName);
                }
            }
        }
        clazz = clazz.getSuperclass();//会扫描所有的父类，不仅仅是当前类
    }
    if (subscriberMethods.isEmpty()) {
        throw new EventBusException("Subscriber " + subscriberClass + " has no public methods called " + eventMethodName);//最常见的这个异常是在这里抛出来的
    } else {
        synchronized (methodCache) {
            methodCache.put(key, subscriberMethods);
        }
        return subscriberMethods;
    }
}

suscribe 方法

然后for循环扫描到的方法，然后调用suscribe方法

// Must be called in synchronized block
private void subscribe(Object subscriber, SubscriberMethod subscriberMethod, boolean sticky, int priority) {
    subscribed = true;
    Class<?> eventType = subscriberMethod.eventType; //eventType是我们方法参数的Class，是Map的key
    //根据subscriberMethod.eventType，去subscriptionsByEventType查找一个CopyOnWriteArrayList<Subscription> ，如果没有则创建。
    CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType); // Map的value，这里的subscriptionsByEventType是个Map，这个Map其实就是EventBus存储方法的地方
    Subscription newSubscription = new Subscription(subscriber, subscriberMethod, priority);//Map的value中保存的是Subscription
    if (subscriptions == null) {
        subscriptions = new CopyOnWriteArrayList<Subscription>();
        subscriptionsByEventType.put(eventType, subscriptions);
    } else {
        for (Subscription subscription : subscriptions) {
            if (subscription.equals(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);
    int size = subscriptions.size();
    for (int i = 0; i <= size; i++) { //按照优先级添加newSubscription，优先级越高，会插到在当前List的前面。
        if (i == size || newSubscription.priority > subscriptions.get(i).priority) {
            subscriptions.add(i, newSubscription);
            break;
        }
    }
    List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber); //根据subscriber存储它所有的eventType
    if (subscribedEvents == null) {
        subscribedEvents = new ArrayList<Class<?>>();
        typesBySubscriber.put(subscriber, subscribedEvents);
    }
    subscribedEvents.add(eventType);
    if (sticky) { //判断sticky；如果为true，从stickyEvents中根据eventType去查找有没有stickyEvent，如果有则立即发布去执行
        Object stickyEvent;
        synchronized (stickyEvents) {
            stickyEvent = stickyEvents.get(eventType);
        }
        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());
        }
    }
}

你只要记得一件事：扫描了所有的方法，把匹配的方法最终保存在subscriptionsByEventType(Map，key：eventType ； value：CopyOnWriteArrayList<Subscription> )中；

eventType是我们方法参数的Class，Subscription中则保存着subscriber, subscriberMethod, priority；包含了执行改方法所需的一切。

发布 post

调用很简单，一句话，你也可以叫发布，只要把这个param发布出去，EventBus会在它内部存储的方法中，进行扫描，找到参数匹配的，就使用反射进行调用。

/** Posts the given event to the event bus. */
public void post(Object event) {
    PostingThreadState postingState = currentPostingThreadState.get();
    List<Object> eventQueue = postingState.eventQueue;
    eventQueue.add(event);
    if (postingState.isPosting) {//防止每次post都会去调用整个队列
        return;
    } else {
        postingState.isMainThread = Looper.getMainLooper() == Looper.myLooper();//判断当前是否是UI线程（我去，竟然是通过这种方式判断的！）
        postingState.isPosting = true;
        if (postingState.canceled) {
            throw new EventBusException("Internal error. Abort state was not reset");
        }
        try {
            while (!eventQueue.isEmpty()) {//遍历队列中的所有的event，调用postSingleEvent方法。
                postSingleEvent(eventQueue.remove(0), postingState);
            }
        } finally {
            postingState.isPosting = false;
            postingState.isMainThread = false;
        }
    }
}

private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
    //得到event当前对象的Class，以及父类和接口的Class类型；主要用于匹配，比如你传入Dog extends Animal，他会把Animal也装到该List中。
    Class<? extends Object> eventClass = event.getClass();
    List<Class<?>> eventTypes = findEventTypes(eventClass);
    boolean subscriptionFound = false;
    int countTypes = eventTypes.size();
    for (int h = 0; h < countTypes; h++) { //遍历所有的Class，到subscriptionsByEventType去查找subscriptions，register时我们就是把方法存在了这个Map里
        Class<?> clazz = eventTypes.get(h);
        CopyOnWriteArrayList<Subscription> subscriptions;
        synchronized (this) {
            subscriptions = subscriptionsByEventType.get(clazz);
        }
        if (subscriptions != null && !subscriptions.isEmpty()) {
            for (Subscription subscription : subscriptions) {
                postingState.event = event;
                postingState.subscription = subscription;
                boolean aborted = false;
                try {
                    postToSubscription(subscription, event, postingState.isMainThread); //反射执行方法
                    aborted = postingState.canceled;
                } finally {
                    postingState.event = null;
                    postingState.subscription = null;
                    postingState.canceled = false;
                }
                if (aborted) {
                    break;
                }
            }
            subscriptionFound = true;
        }
    }
    if (!subscriptionFound) {
        Log.d(TAG, "No subscribers registered for event " + eventClass);
        if (eventClass != NoSubscriberEvent.class && eventClass != SubscriberExceptionEvent.class) {
            post(new NoSubscriberEvent(this, event));
        }
    }
}

private void postToSubscription(Subscription subscription, Object event, boolean isMainThread) {
    switch (subscription.subscriberMethod.threadMode) { //根据threadMode去判断应该在哪个线程去执行该方法
    case PostThread:
        invokeSubscriber(subscription, event); //直接【反射】调用；也就是说在当前的线程直接调用该方法
        break;
    case MainThread:
        if (isMainThread) {//如果是UI线程，则直接调用
            invokeSubscriber(subscription, event);
        } else {//否则把当前的方法加入到队列，然后直接通过【handler】去发送一个消息，并在handler的handleMessage中去执行我们的方法
            mainThreadPoster.enqueue(subscription, event);
        }
        break;
    case BackgroundThread:
        if (isMainThread) {//如果是UI线程，则将任务加入到后台的一个【队列】，最终由Eventbus中的一个【线程池】去调用executorService = Executors.newCachedThreadPool();
            backgroundPoster.enqueue(subscription, event);
        } else {//如果当前非UI线程，则直接调用
            invokeSubscriber(subscription, event);
        }
        break;
    case Async: //将任务加入到后台的一个【队列】，最终由Eventbus中的一个【线程池】去调用；线程池与BackgroundThread用的是【同一个】
        asyncPoster.enqueue(subscription, event); //BackgroundThread中的任务，【一个接着一个去调用】，中间使用了一个布尔型变量进行的控制。Async则会【动态控制并发】
        break;
    default:
        throw new IllegalStateException("Unknown thread mode: " + subscription.subscriberMethod.threadMode);
    }
}

粘性事件 sticky

介绍了register和post；大家获取还能想到一个词sticky，在register中，如果sticky为true，会去stickyEvents去查找事件，然后立即去post；

那么这个stickyEvents何时进行保存事件呢？

其实evevntbus中，除了post发布事件，还有一个方法也可以：

public void postSticky(Object event) {
    synchronized (stickyEvents) {
        stickyEvents.put(event.getClass(), event);
    }
    // Should be posted after it is putted, in case the subscriber wants to remove immediately
    post(event);
}

和post功能类似，但是会把方法存储到stickyEvents中去；

大家再去看看EventBus中所有的public方法，无非都是一些状态判断，获取事件，移除事件的方法；没什么好介绍的，基本见名知意。

2016-9-7