EventBus

0. 基本介绍

EventBus is an open-source library for Android and Java using the publisher/subscriber pattern for loose coupling. EventBus enables central communication to decoupled classes with just a few lines of code – simplifying the code, removing dependencies, and speeding up app development.

EventBus是事件总线的实现框架，首先介绍一下事件总线

总线：类比DBus，CBus，总线是流的统一管道路径 事件总线：流动事件消息的统一管道

1. 使用场景

为简化并且更加高质量地在 Activity，Fragment，Thread 和Service等之间的通信，同时解决组件之间高耦合的同时任然继续高效的通行，事件总线的设计出现了。Android常见框架如EventBus，otto

2. 基本使用

2.1 使用之前：名词概念介绍

三要素

Event：事件，可以是任意类型的对象
Subscriber：事件订阅者，事件流可达的对象，我们需要处理对应的事件；在EventBus3.0之前：事件处理的方法只限于onEventXXX；之后：事件处理方法名可任意，但要添加@Subscribe注解
Publisher：事件发布者；可在任意线程的任意位置发送事件：调用Event的post方法

四线程模型（ThreadMode）

POSTING(Default)：事件源所在线程就是事件处理函数所在线程
MAIN：事件处理在主线程处理（显然要注意主线程的操作时间限制，推荐只UI更新操作）
BACKGROUND：事件处理在非主线程；若事件源在主线程，则新建线程，若事件源在非主线程，时间处理位于事件源同线程；事件处理函数不在主线程当然无法进行UI操作
ASYNC：异步模式；事件处理函数必定与事件源所在线程不同，以实现异步操作；事件处理函数所在线程不能为UI线程
2.2 基本使用（5个步骤）
1. Gradle 添加依赖
  1 compile 'org.greenrobot:eventbus:3.0.0'

自定义事件类对应三要素的 Event

  
 public class MessageEvent{
 ...
 }

所在线程订阅事件预注册(对应三要素 Publisher)
1 EventBus.getDefault().register(this);

发送事件

  
 EventBus.getDefault().post(messageEvent);

处理事件（对应三要素 Subscriber）

  
 @Subscribe (threadMode = ThreadMode.MAIN)
 public void randomName(MessageEvent messageEvent){
 // do something
 }

别忘了取消事件订阅
1 EventBus.getDefault().unregister(this);
2.3 特别介绍
EventBus的粘性事件 定义：发送事件之后，再注册事件订阅，订阅者依然能收到之前发送的粘性事件猜想实现：维护了一个粘性事件的队列，保存了所有粘性事件

使用五个步骤没有什么不同，改变的是：发送事件：EventBus.getDefault().postSticky(event)
处理函数注解：@Subscribe(threadMode = ThreadMode.POSTING, sticky=true)

3. 痛点问题解决

4. 其他框架对比

5. 设计模式解析

6. 静态代码文件结构

基本源码阅读

6.1 获得总线实例对象 EventBus.getDefault()：

  
/** Convenience singleton for apps using a process-wide EventBus instance. */
//使用双重检测 实现的 单例模式
    public static EventBus getDefault() {
        EventBus instance = defaultInstance;
        if (instance == null) {
            synchronized (EventBus.class) {
                instance = EventBus.defaultInstance;
                if (instance == null) {
                    instance = EventBus.defaultInstance = new EventBus();//跳到默认构造函数
                }
            }
        }
        return instance;
    }
	// 默认构造函数
	public EventBus() {
        this(DEFAULT_BUILDER);// 使用了 EventBusBuilder 的构造函数
    }
	// EventBusBuilder 的构造函数
	// 这里使用了建造者模式 实现的 EventBus 装配
    EventBus(EventBusBuilder builder) {
        logger = builder.getLogger();
        subscriptionsByEventType = new HashMap<>();
        typesBySubscriber = new HashMap<>();
        // 如我们猜想，这里使用ConcurrentHashMap 来保存粘性事件
        stickyEvents = new ConcurrentHashMap<>();
        mainThreadSupport = builder.getMainThreadSupport();
        mainThreadPoster = mainThreadSupport != null ? mainThreadSupport.createPoster(this) : null;
        backgroundPoster = new BackgroundPoster(this);
        asyncPoster = new AsyncPoster(this);
        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;
        // 可以看出来 使用了Java的ExecutorService线程池
        executorService = builder.executorService;
    }

6.2 订阅者注册EventBus.getDefault().register()

  
/**
     * Registers the given subscriber to receive events. Subscribers must call {@link #unregister(Object)} once they
     * are no longer interested in receiving events.
     * <p/>
     * Subscribers have event handling methods that must be annotated by {@link Subscribe}.
     * The {@link Subscribe} annotation also allows configuration like {@link
     * ThreadMode} and priority.
     */
    public void register(Object subscriber) {
        Class<?> subscriberClass = subscriber.getClass();
        // 1. 查找需要订阅方法
        // 通过@Subscribe注解收集的订阅方法，通过反射 Method，包装为SubscriberMethod对象,并且保存属性，如下
        // SubscriberMethod(Method method, Class<?> eventType, ThreadMode threadMode, int priority, boolean sticky) 
        List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
        // 2. 对方法进行订阅
        // 包装 subsriber 和 subscriberMethod 对象为：Subscription对象
        // 如此便通过 类组合 的方式，建立了事件处理方法 与 subscriber类的关系（subscriber所在线程的关系）
        // 将Subscription 工具 evnetType 存入 SubscriptionsByEventType(Map集合) Map<Class<?>, CopyOnWriteArrayList<Subscription>>
        // 如此便通过map的方式 建立了Event类与相应的订阅方法反射的联系
        synchronized (this) {
            for (SubscriberMethod subscriberMethod : subscriberMethods) {
                subscribe(subscriber, subscriberMethod);
            }
        }
    }

6.3 发送事件 EventBus.getDefault().post(event)

  
/** Posts the given event to the event bus. */
    public void post(Object event) {
    	// currentPostingThreadState 是 线程无关的 threadlocal 数据结构
    	// PostingThreadState:保存着当前线程 的 事件队列 和线程信息状态
    	/**
    	 private final ThreadLocal<PostingThreadState> currentPostingThreadState = new ThreadLocal<PostingThreadState>() {
        @Override
        protected PostingThreadState initialValue() {
            return new PostingThreadState();
        }
    };
    	*/
        PostingThreadState postingState = currentPostingThreadState.get();
        // 取得当前线程的 事件队列
        List<Object> eventQueue = postingState.eventQueue;
        eventQueue.add(event);
		// 修改状态后 对事件队列 全部postSingeEvent
        if (!postingState.isPosting) {
            postingState.isMainThread = isMainThread();
            postingState.isPosting = true;
            if (postingState.canceled) {
                throw new EventBusException("Internal error. Abort state was not reset");
            }
            try {
                while (!eventQueue.isEmpty()) {
                    postSingleEvent(eventQueue.remove(0), postingState);
                }
            } finally {
                postingState.isPosting = false;
                postingState.isMainThread = false;
            }
        }
    }
	// 投递单条 Event 调用 postSingleEventForEventType
	private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
        Class<?> eventClass = event.getClass();
        boolean subscriptionFound = false;
        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) {
            if (logNoSubscriberMessages) {
                logger.log(Level.FINE, "No subscribers registered for event " + eventClass);
            }
            if (sendNoSubscriberEvent && eventClass != NoSubscriberEvent.class &&
                    eventClass != SubscriberExceptionEvent.class) {
                post(new NoSubscriberEvent(this, event));
            }
        }
    }
    // 根据 Evnet类型查找 绑定的订阅者Subscription 
    // 内部调用了 postToSubscription，对每一个订阅者发送事件
	private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass) {
        CopyOnWriteArrayList<Subscription> subscriptions;
        synchronized (this) {
            subscriptions = subscriptionsByEventType.get(eventClass);
        }
        if (subscriptions != null && !subscriptions.isEmpty()) {
            for (Subscription subscription : subscriptions) {
                postingState.event = event;
                postingState.subscription = subscription;
                boolean aborted;
                try {
                    postToSubscription(subscription, event, postingState.isMainThread);
                    aborted = postingState.canceled;
                } finally {
                    postingState.event = null;
                    postingState.subscription = null;
                    postingState.canceled = false;
                }
                if (aborted) {
                    break;
                }
            }
            return true;
        }
        return false;
    }
	// 根据线程模型枚举的值 分情况 加入主线程队列，加入poster队列，加入当前线程队列
    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);
        }
    }

7. 动态代码执行逻辑流

8. 反思思考、

8.1 post event为何要四层调用

决定于事件源与订阅者的路径确定

post（）：将事件添加到本线程待投队列
postSingleEvent（）：对待投队列的每个事件，根据已有Subscription 集合，判断是否有订阅者
postSingleEventForEventType（）：根据 Evnet类型查找绑定的订阅者Subscriptions
postToSubscription（）：根据订阅者绑定ThreadMode判断与Event 投递线程类型

映射关系为：

event 与当前线程队列的关系
当前线程队列与订阅者有无的关系
event 与订阅者集合的关系
event 与订阅者其他线程的关系

9. 开发使用常见问题

10. 常见素材

EventBus 官方网站
EventBus GitHub地址
知乎主题帖
问题1
问题2
优秀博文1
优秀博文2

EventBus框架结构