EventBus3.0源码浅析

一、构建

1、EventBus实例创建采用了DCL单例：

static volatile EventBus defaultInstance;

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

2、其构造器相关代码如下:

private static final EventBusBuilder DEFAULT_BUILDER = new EventBusBuilder();
// 基于事件类型, 订阅列表构成的集合
private final Map, CopyOnWriteArrayList> subscriptionsByEventType;
private final Map>> typesBySubscriber;

public EventBus() {
        this(DEFAULT_BUILDER);
 }

EventBus(EventBusBuilder builder) {
        subscriptionsByEventType = new HashMap<>();
        typesBySubscriber = new HashMap<>();
        stickyEvents = new ConcurrentHashMap<>();
        // 线程发送器
        mainThreadPoster = new HandlerPoster(this, Looper.getMainLooper(), 10);
        backgroundPoster = new BackgroundPoster(this);
        asyncPoster = new AsyncPoster(this);

        // 设置build选项
        indexCount = builder.subscriberInfoIndexes != null ? builder.subscriberInfoIndexes.size() : 0;
        subscriberMethodFinder = new SubscriberMethodFinder(builder.subscriberInfoIndexes,
                builder.strictMethodVerification, builder.ignoreGeneratedIndex);
        logSubscriberExceptions = builder.logSubscriberExceptions;
        logNoSubscriberMessages = builder.logNoSubscriberMessages;
        sendSubscriberExceptionEvent = builder.sendSubscriberExceptionEvent;
        sendNoSubscriberEvent = builder.sendNoSubscriberEvent;
        throwSubscriberException = builder.throwSubscriberException;
        eventInheritance = builder.eventInheritance;
        executorService = builder.executorService;
 }

也就是说，采用的是默认的建造者EventBusBuilder对象来构建EventBus对象。
其中初始化的subscriptionsByEventType 、typesBySubscriber 是两个很重要的map集合。

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二、注册

1、注册订阅者对象

public void register(Object subscriber) {
        Class subscriberClass = subscriber.getClass();
        // 获得订阅者方法
        List subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
        synchronized (this) {
            for (SubscriberMethod subscriberMethod : subscriberMethods) {
                // 订阅
                subscribe(subscriber, subscriberMethod);
            }
        }
 }

可以看出，subscriberMethodFinder根据订阅者Class实例来获得subscriberMethod集合。SubscriberMethod是对注册方法以及相关属性的封装：

public class SubscriberMethod {
    final Method method;
    final ThreadMode threadMode;
    final Class eventType;
    final int priority;
    final boolean sticky;
    /** Used for efficient comparison */
    String methodString;

    ...

}

到此，我们可以联想到Method对象之后应该会执行invoke方法，这也是EventBus传递消息的根本。ThreadMode是线程模式，我们都知道EventBus可以通过注解来指定执行线程。

2、查找订阅者方法

// 方法缓存集合，线程安全
private static final Map, List> METHOD_CACHE = new ConcurrentHashMap<>();

List findSubscriberMethods(Class subscriberClass) {
        List subscriberMethods = METHOD_CACHE.get(subscriberClass);
        if (subscriberMethods != null) {
            return subscriberMethods;
        }

        // 默认为false
        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 {
            // 添加进缓存
            METHOD_CACHE.put(subscriberClass, subscriberMethods);
            return subscriberMethods;
        }
 }

这是一个获得数据并添加到缓存的过程，ignoreGeneratedIndex是在EventBusBuilder中设置的，默认为false，也就是说返回的结果是由findUsingInfo()方法得到的。

private List 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);
 }

findState对象是对查找状态的封装。调用initForSubscriber方法后，findState.clazz为subscriberClass，findState.subscriberInfo为null，故而调用findUsingReflectionInSingleClass方法，这个方法是查找功能的具体实现。

private void findUsingReflectionInSingleClass(FindState findState) {
        Method[] methods;
        try {
            // This is faster than getMethods, especially when subscribers are fat classes like Activities
            // 获得所有声明的方法
            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) {
            // 修饰符
            int modifiers = method.getModifiers();
            // 必须是public，且不是Modifier.ABSTRACT | Modifier.STATIC | BRIDGE | SYNTHETIC;
            if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
                // 参数类型
                Class[] parameterTypes = method.getParameterTypes();
                if (parameterTypes.length == 1) {
                    // 获得注解
                    Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);
                    if (subscribeAnnotation != null) {
                        Class eventType = parameterTypes[0];
                        if (findState.checkAdd(method, eventType)) {
                            ThreadMode threadMode = subscribeAnnotation.threadMode();
                            // 添加到集合
                            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");
            }
        }
}

由上可以看到方法查找的逻辑，最终把满足要求的方法以及相关参数封装起来，添加到 findState.subscriberMethods集合中。

接下来回到findUsingInfo方法中，又调用了moveToSuperclass方法。

 void moveToSuperclass() {
            if (skipSuperClasses) {
                clazz = null;
            } else {
                clazz = clazz.getSuperclass();
                String clazzName = clazz.getName();
                /** Skip system classes, this just degrades performance. */
                if (clazzName.startsWith("java.") || clazzName.startsWith("javax.") || clazzName.startsWith("android.")) {
                    clazz = null;
                }
            }
}

这是一个把遍历目标转移到父类的方法，skipSuperClasses默认为false。该方法不停的循环，一直到系统类，将clazz对象设为null，从而结束循环。

最终返回订阅方法集合。

 private List getMethodsAndRelease(FindState findState) {
        List subscriberMethods = new ArrayList<>(findState.subscriberMethods);
        // 回收findState
        findState.recycle();
        synchronized (FIND_STATE_POOL) {
            for (int i = 0; i < POOL_SIZE; i++) {
                if (FIND_STATE_POOL[i] == null) {
                    FIND_STATE_POOL[i] = findState;
                    break;
                }
            }
        }
        return subscriberMethods;
}

3、订阅关系确立
之前查找到的订阅方法集合仅仅是把SubscriberMethod 对象累积起来，并不能根据事件类型，快速得到订阅对象（post方法发送的是事件对象），故而需要遍历订阅方法对象，建立订阅关系。

private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
        Class eventType = subscriberMethod.eventType;
        // 封装订阅对象、订阅方法为订阅关系
        Subscription newSubscription = new Subscription(subscriber, subscriberMethod);

        CopyOnWriteArrayList subscriptions = subscriptionsByEventType.get(eventType);
        if (subscriptions == null) {
            subscriptions = new CopyOnWriteArrayList<>();
            // 建立事件类型-订阅关系映射
            subscriptionsByEventType.put(eventType, subscriptions);
        } else {
            // 重复注册抛出异常
            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) {
                // 添加订阅关系到集合中
                subscriptions.add(i, newSubscription);
                break;
            }
        }
        
        // 根据订阅对象获得事件类型
        List> subscribedEvents = typesBySubscriber.get(subscriber);
        if (subscribedEvents == null) {
            subscribedEvents = new ArrayList<>();
            typesBySubscriber.put(subscriber, subscribedEvents);
        }
        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).
                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);
            }
        }
 }

由上述方法可得到两个集合subscriptionsByEventType、typesBySubscriber，建立了事件类型与订阅对象之间的双重映射。前者用于发送事件处理，后者用于取消注册。

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三、发送

1、发送事件对象

 public void post(Object event) {
        PostingThreadState postingState = currentPostingThreadState.get();
        List