EventBus 及一些思考

xiaorulou

2020-03-01

EventBus 是 Android 开发的一种常用框架,其解耦的思维令人赞叹

从特性上来讲,其与 Android SDK中的BroadcastReceiver很像,二者都是注册,发送事件,反注册,都可以定义事件的优先级,且都支持粘性(sticky)事件,只是EventBus 使用起来简单得多,而且不能跨进程

Android SDK其实也有一个不能跨进程的BroadcastReceiver机制——LocalBroadcastManager,其发送和接受的广播只能在本进程,相比传统的 registerBroadcastReceiver,其有着更高的安全性,与EventBus的相似度也更高

关于 EventBus 基础,请参考:

EventBus

使用教程高级用法源码解析

这里结合源码,记录几个在 EventBus 学习和使用中值得思考和注意的地方:

哪些方法会被注册 ?
多次注册同一对象会如何?
注册对象没有被 Subscribe 注解的方法会如何 ?
注册对象时父类中被 Subscribe 注解的方法会被注册吗?
如果子类B 重写了父类A 的方法 fun,注册子类B 的对象会发生什么?调用时会调用哪个类的方法?

上述问题,我们需要分析 register方法:

public void register(Object subscriber) {
    Class<?> subscriberClass = subscriber.getClass();
    // 1. 找 subscriber 中被需要被注册的方法
    List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
    synchronized (this) {
        // 2. 注册这些方法
        for (SubscriberMethod subscriberMethod : subscriberMethods) {
            subscribe(subscriber, subscriberMethod);
        }
    }
}

先来看上面代码第一点;

List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
    // 缓存中取
    List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
    if (subscriberMethods != null) {
        return subscriberMethods;
    }
    if (ignoreGeneratedIndex) {
        // 通过反射去找
        subscriberMethods = findUsingReflection(subscriberClass);
    } else {
        // 通过 Subscriber Index 去找,这里如果没找到,也会通过反射去找
        subscriberMethods = findUsingInfo(subscriberClass);
    }
    // 如果没有 找到被 Subscribe 注解的方法,抛出异常  ———— 问题 3
    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;
    }
}

// 继续分析注册方法的查找过程findUsingReflection
private List<SubscriberMethod> findUsingReflection(Class<?> subscriberClass) {
    FindState findState = prepareFindState();
    findState.initForSubscriber(subscriberClass);
    while (findState.clazz != null) {
        findUsingReflectionInSingleClass(findState);
        // 继续去 subscriberClass 的父类中找,但是这里有异常情况(几乎不会发生),往下看 ———— 问题 4
        findState.moveToSuperclass();
    }
    return getMethodsAndRelease(findState);
}
// findState.moveToSuperclass();
void moveToSuperclass() {
    if (skipSuperClasses) { // 异常情况,至于在哪里设置该标志位呢,继续往下看
        clazz = null;
    }
    // ...
}

// 继续分析注册方法的查找过程 findUsingReflectionInSingleClass
private void findUsingReflectionInSingleClass(FindState findState) {
    Method[] methods;
    // 先通过getDeclaredMethods查找,再通过getMethods查找
    try {
        methods = findState.clazz.getDeclaredMethods();
    } catch (Throwable th) {
        methods = findState.clazz.getMethods();
        // ...
        // 问题 4 的异常情况,在 getDeclaredMethods 发生异常时,跳过父类的注册方法查找
        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)) {
                        // 方法被注册的条件: public,被 Subscribe 注解,参数列表只有1个参数(也就是事件类型) ———— 问题 1
                        ThreadMode threadMode = subscribeAnnotation.threadMode();
                        findState.subscriberMethods.add(new SubscriberMethod(method, eventType, threadMode,
                                subscribeAnnotation.priority(), subscribeAnnotation.sticky()));
                    }
                }
            } else if (strictMethodVerification && method.isAnnotationPresent(Subscribe.class)) {
                // 在设置了 strictMethodVerification 时,如果 Subscribe 注解的方法参数个数不是 1,抛出异常,默认 strictMethodVerification标志是false,可以通过 EventBusBuilder 设置
                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)) {
            // 在设置了 strictMethodVerification 时,如果被 Subscribe 注解的方法不是 public,抛出异常
            String methodName = method.getDeclaringClass().getName() + "." + method.getName();
            throw new EventBusException(methodName +
                    " is a illegal @Subscribe method: must be public, non-static, and non-abstract");
        }
    }
}

再来看 register方法中的第2 点:

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 {
        // 已经注册过了该 subscriber,再次注册抛出异常  ———— 问题 2
        if (subscriptions.contains(newSubscription)) {
            throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event "
                    + eventType);
        }
    }
    // ...
}

上述问题 5 ,代码中没写,但是其实在回答问题 4 时已经回答了,在寻找注册方法时,会注册父类的合格的方法,那么在post 调用时,调用的是哪个方法呢,EventBus 通过反射调用方法,自然也就是实现类的方法;

事件触发

post时,如果没有找到 eventType 对应的注册方法会如何?
事件的 eventType 调用时符合多态吗?

private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
    Class<?> eventClass = event.getClass();
    boolean subscriptionFound = false;
    // 设置了 eventInheritance 标志,查找 eventType 的 父类 和 父接口 的对应的事件类型,eventInheritance 标志默认为 true  ———— 问题 2
    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) {
        // 没有找到对应 eventType 的注册方法,先打个日志
        if (logNoSubscriberMessages) {
            logger.log(Level.FINE, "No subscribers registered for event " + eventClass);
        }
        // 如果设置了 sendNoSubscriberEvent 标志,post 一个 NoSubscriberEvent 事件  ———— 问题 1
        if (sendNoSubscriberEvent && eventClass != NoSubscriberEvent.class &&
                eventClass != SubscriberExceptionEvent.class) {
            post(new NoSubscriberEvent(this, event));
        }
    }
}

反注册

多次反注册同一对象,或者反注册一个未被注册过的对象会如何?

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 {
        // 没找到,就打了个日志  ———— 问题 1
        logger.log(Level.WARNING, "Subscriber to unregister was not registered before: " + subscriber.getClass());
    }
}

eventbus