EventBus -- 源码分析

概述

介绍完了 EventBus 的基本用法之后，照例要进行一下源码分析。
本文基于 EventBus V3.0.0 进行分析，并尽量保持同步更新。

github 源码地址

未完待续……

消息订阅

public void register(Object subscriber) {
    // 获取订阅对象的 class 对象
    Class<?> subscriberClass = subscriber.getClass();
    List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
    synchronized (this) {
        for (SubscriberMethod subscriberMethod : subscriberMethods) {
            subscribe(subscriber, subscriberMethod);
        }
    }
}

SubscriberMethodFinder

SubscriberMethodFinder 作用是解析并找出订阅对象中所有的订阅方法。

SubscriberMethodFinder(List<SubscriberInfoIndex> subscriberInfoIndexes, boolean strictMethodVerification,
                       boolean ignoreGeneratedIndex) {
    this.subscriberInfoIndexes = subscriberInfoIndexes;
    this.strictMethodVerification = strictMethodVerification;
    this.ignoreGeneratedIndex = ignoreGeneratedIndex;
}

List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
    // 先找出是否有缓存
    List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
    // 如果有缓存直接返回缓存的订阅方法
    if (subscriberMethods != null) {
        return subscriberMethods;
    }
    
    if (ignoreGeneratedIndex) {
        // 如果设置了忽略生成的索引，关于索引，后面专门介绍
        subscriberMethods = findUsingReflection(subscriberClass);
    } else {
        // 没有设置会走到这里，默认是没有设置的
        subscriberMethods = findUsingInfo(subscriberClass);
    }
    if (subscriberMethods.isEmpty()) {
        // 如果没有找到订阅方法，抛出异常
        throw new EventBusException("Subscriber " + subscriberClass
                + " and its super classes have no public methods with the @Subscribe annotation");
    } else {
        // 找到订阅方法，加入缓存
        METHOD_CACHE.put(subscriberClass, subscriberMethods);
    }
}

获取订阅对象中的订阅方法列表：

private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
    FindState findState = prepareFindState();
    findState.initForSubscriber(subscriberClass);
    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 {
            findUsingReflectionInSingleClass(findState);
        }
        findState.moveToSuperclass();
    }
    return getMethodsAndRelease(findState);
}

解析订阅对象的注解信息。
因为订阅方法以及它的一些属性都是通过注解来标识的，只有解析到了这些方法才能向它们发布消息事件。

private void findUsingReflectionInSingleClass(FindState findState) {
    Method[] methods;
    try {
        // 获取订阅者对象的所有方法
        methods = findState.clazz.getDeclaredMethods();
    } catch (Throwable th) {
        methods = findState.clazz.getMethods();
        findState.skipSuperClasses = true;
    }
    for (Method method : methods) {
        int modifiers = method.getModifiers();
        // 方法的修饰符必须符合下面的类型
        if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
            // 获取方法的参数
            Class<?>[] parameterTypes = method.getParameterTypes();
            // 订阅方法必须接受一个参数，也就是表示消息事件的类
            if (parameterTypes.length == 1) {
                // 获取该方法的 Subscribe 注解信息
                Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);
                // 如果有这个注解
                if (subscribeAnnotation != null) {
                    // 获取参数的 class 对象
                    Class<?> eventType = parameterTypes[0];
                    if (findState.checkAdd(method, eventType)) {
                        // 获取 ThreadMode 类型
                        ThreadMode threadMode = subscribeAnnotation.threadMode();
                        // 保存到subscriberMethods
                        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");
        }
    }
}

消息发布

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 MAIN_ORDERED:
            if (mainThreadPoster != null) {
                mainThreadPoster.enqueue(subscription, event);
            } else {
                // temporary: technically not correct as poster not decoupled from subscriber
                invokeSubscriber(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);
    }
}

未完待续……