EventBus 是一款针对 Android 优化的发布/订阅事件总线

一、使用方法

1、EventBus 的三要素：

事件、任意类型对象
事件订阅者、任意 @Subscribe 注解方法
事件发布者、EventBus 对象，默认 EventBus.getDefault()

2、代码示例

public class MainActivity extends Activity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        // 1、注册
        EventBus.getDefault().register(this);
    }
    // 3、接收消息
    @Subscribe(threadMode = ThreadMode.MAIN)
    public void onSubscribeEvent(MessageEvent messageEvent) {
        Log.i(TAG, "onSubscribeEvent: " + messageEvent.getMessage());
    }
    @Override
    protected void onDestroy() {
        // 4、注销
        EventBus.getDefault().unregister(this);
        super.onDestroy();
    }
}

// 2、发送消息
EventBus.getDefault().post(new MessageEvent("hello wshunli"));

3、EventBus 的 ThreadMode（线程模型）

POSTING、和发送同一线程
MAIN、主线程
MAIN_ORDERED、主线程顺序执行
BACKGROUND、非主线程；如果主线程发出，在子线程执行，如果子线程发出，在相同线程执行
ASYNC、无论从什么线程发出，总是在新建子线程执行

4、粘性事件（先发送，后注册接收）

EventBus.getDefault().postSticky(new MessageEvent("粘性事件"));
@Subscribe(threadMode = ThreadMode.MAIN, sticky = true)
public void onSubscribeStickyEvent(MessageEvent messageEvent) {
    Log.i(TAG, "onSubscribeStickyEvent: " + messageEvent.getMessage());
}

implementation 'org.greenrobot:eventbus:3.2.0'

二、源码分析

2.1、EventBus 构造

public class EventBus {
    static volatile EventBus defaultInstance;
    private static final EventBusBuilder DEFAULT_BUILDER = new EventBusBuilder();
    // 默认实例，单例、双重校验锁
    public static EventBus getDefault() {
        EventBus instance = defaultInstance;
        if (instance == null) {
            synchronized (EventBus.class) {
                instance = EventBus.defaultInstance;
                if (instance == null) {
                    instance = EventBus.defaultInstance = new EventBus();
                }
            }
        }
        return instance;
    }
    // 构造方法
    public EventBus() {
        this(DEFAULT_BUILDER);
    }
    EventBus(EventBusBuilder builder) {
        // ***
    }
}

EventBus 对象之间，订阅方法是隔离的，一般使用 getDefault()即可。

2.2、注册订阅

// EventBus
private final SubscriberMethodFinder subscriberMethodFinder;
public void register(Object subscriber) {
    Class<?> subscriberClass = subscriber.getClass();
    // （1）获取订阅方法
    List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
    synchronized (this) {
        for (SubscriberMethod subscriberMethod : subscriberMethods) {
            // （2）订阅方法注册
            subscribe(subscriber, subscriberMethod);
        }
    }
}

（1）查找订阅方法

// SubscriberMethodFinder
private static final Map<Class<?>, List<SubscriberMethod>> METHOD_CACHE = new ConcurrentHashMap<>();
private List<SubscriberInfoIndex> subscriberInfoIndexes;
List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
    // 从缓存中获取，订阅方法
    List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
    if (subscriberMethods != null) {
        return subscriberMethods;
    }
    // 是否忽略订阅信息索引
    if (ignoreGeneratedIndex) {
        // 通过反射
        subscriberMethods = findUsingReflection(subscriberClass);
    } else {
        // 通过索引
        subscriberMethods = findUsingInfo(subscriberClass);
    }
    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;
    }
}

Eventbus 支持通过在编译期创建索引（SubscriberInfoIndex）以提高程序运行性能

https://greenrobot.org/eventbus/documentation/subscriber-index/

// 使用方法，编译时生成 MySubscriberInfoIndex 类
EventBus.builder().addIndex(new MySubscriberInfoIndex()).installDefaultEventBus();

// EventBusBuilder,添加索引信息
List<SubscriberInfoIndex> subscriberInfoIndexes;
public EventBusBuilder addIndex(SubscriberInfoIndex index) {
    if (subscriberInfoIndexes == null) {
        subscriberInfoIndexes = new ArrayList<>();
    }
    subscriberInfoIndexes.add(index);
    return this;
}
// EventBus 构造时存入 SubscriberMethodFinder
EventBus(EventBusBuilder builder) {
    //***
    subscriberMethodFinder = new SubscriberMethodFinder(
        builder.subscriberInfoIndexes,
        builder.strictMethodVerification, 
        builder.ignoreGeneratedIndex);
    //***
}
// SubscriberMethodFinder 获取订阅方法
private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
    FindState findState = prepareFindState();
    findState.initForSubscriber(subscriberClass);
    while (findState.clazz != null) {
        // 从索引中中获取 SubscriberInfo 
        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 {
            // 索引中没有，反射获取
            findUsingReflectionInSingleClass(findState);
        }
        findState.moveToSuperclass();
    }
    return getMethodsAndRelease(findState);
}
private SubscriberInfo getSubscriberInfo(FindState findState) {
    // ***
    // 构造时传入 subscriberInfoIndexes 对象
    if (subscriberInfoIndexes != null) {
        for (SubscriberInfoIndex index : subscriberInfoIndexes) {
            SubscriberInfo info = index.getSubscriberInfo(findState.clazz);
            if (info != null) {
                return info;
            }
        }
    }
    return null;
}

反射方式比较简单，不再粘贴代码。

（2）订阅方法注册

// 事件 => Subscription 列表
private final Map<Class<?>, CopyOnWriteArrayList<Subscription>> subscriptionsByEventType;
// 订阅者 => 事件列表
private final Map<Object, List<Class<?>>> typesBySubscriber;
// 存储粘性事件
private final Map<Class<?>, Object> stickyEvents; 

private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
    // 事件，eventType 订阅方法参数类型
    Class<?> eventType = subscriberMethod.eventType;
    Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
    // 事件对应的 Subscription 列表
    CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
    // ***
    int size = subscriptions.size();
    // 遍历，根据优先级插入
    for (int i = 0; i <= size; i++) {
        if (i == size || subscriberMethod.priority > subscriptions.get(i).subscriberMethod.priority) {
            subscriptions.add(i, newSubscription);
            break;
        }
    }
    // 存储订阅者及对应的事件列表
    List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
    if (subscribedEvents == null) {
        subscribedEvents = new ArrayList<>();
        typesBySubscriber.put(subscriber, subscribedEvents);
    }
    subscribedEvents.add(eventType);
    // 粘性事件处理
    if (subscriberMethod.sticky) {
        if (eventInheritance) {
            Set<Map.Entry<Class<?>, Object>> entries = stickyEvents.entrySet();
            for (Map.Entry<Class<?>, 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);
        }
    }
}

2.3、事件的发送、处理

（1）普通事件

// EventBus
private final ThreadLocal<PostingThreadState> currentPostingThreadState = new ThreadLocal<PostingThreadState>() {
    @Override
    protected PostingThreadState initialValue() {
        return new PostingThreadState();
    }
};
// PostingThreadState
final static class PostingThreadState {
    final List<Object> eventQueue = new ArrayList<>();
    boolean isPosting;
    boolean isMainThread;
    Subscription subscription;
    Object event;
    boolean canceled;
}
// 发送普通事件
public void post(Object event) {
    // 当前线程 发送状态
    PostingThreadState postingState = currentPostingThreadState.get();
    List<Object> eventQueue = postingState.eventQueue;
    // 添加至事件队列
    eventQueue.add(event);

    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()) {
                // 发送方法
                postSingleEvent(eventQueue.remove(0), postingState);
            }
        } finally {
            postingState.isPosting = false;
            postingState.isMainThread = false;
        }
    }
}
private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
    // ***
    subscriptionFound = postSingleEventForEventType(event, postingState, eventClass);
    // ***
}
private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass) {
    CopyOnWriteArrayList<Subscription> subscriptions;
    synchronized (this) {
        // 获取事件对应的 subscriptions
        subscriptions = subscriptionsByEventType.get(eventClass);
    }
    if (subscriptions != null && !subscriptions.isEmpty()) {
        for (Subscription subscription : subscriptions) {
            postingState.event = event;
            postingState.subscription = subscription;
            boolean aborted;
            try {
                // 发送方法
                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;
}
private void postToSubscription(Subscription subscription, Object event, boolean isMainThread) {
    switch (subscription.subscriberMethod.threadMode) {
            // 当前线程处理
        case POSTING:
            invokeSubscriber(subscription, event);
            break;
            // 主线程处理
        case MAIN:
            if (isMainThread) {
                invokeSubscriber(subscription, event);
            } else {
                // Handler 发送消息
                mainThreadPoster.enqueue(subscription, event);
            }
            break;
            // 主线程顺序
        case MAIN_ORDERED:
            if (mainThreadPoster != null) {
                mainThreadPoster.enqueue(subscription, event);
            } else {
                invokeSubscriber(subscription, event);
            }
            break;
            // 子线程
        case BACKGROUND:
            if (isMainThread) {
                backgroundPoster.enqueue(subscription, event);
            } else {
                invokeSubscriber(subscription, event);
            }
            break;
            // 在新建子线程
        case ASYNC:
            asyncPoster.enqueue(subscription, event);
            break;
        default:
            throw new IllegalStateException("Unknown thread mode: " + subscription.subscriberMethod.threadMode);
    }
}
// 调用订阅方法
void invokeSubscriber(Subscription subscription, Object event) {
    try {
        subscription.subscriberMethod.method.invoke(subscription.subscriber, event);
    } catch (InvocationTargetException e) {
        handleSubscriberException(subscription, event, e.getCause());
    } catch (IllegalAccessException e) {
        throw new IllegalStateException("Unexpected exception", e);
    }
}

（2）粘性事件

public void postSticky(Object event) {
    synchronized (stickyEvents) {
        stickyEvents.put(event.getClass(), event);
    }
    // 普通事件
    post(event);
}

粘性事件仅将事件存入 stickyEvents 对象，真正调用在注册方法

// EventBus#subscribe
if (subscriberMethod.sticky) {
    if (eventInheritance) {
        Set<Map.Entry<Class<?>, Object>> entries = stickyEvents.entrySet();
        for (Map.Entry<Class<?>, 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);
    }
}
private void checkPostStickyEventToSubscription(Subscription newSubscription, Object stickyEvent) {
    if (stickyEvent != null) {
        // 普通事件相同
        postToSubscription(newSubscription, stickyEvent, isMainThread());
    }
}

2.4、注销订阅

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 {
        logger.log(Level.WARNING, "Subscriber to unregister was not registered before: " + subscriber.getClass());
    }
}

将订阅者、订阅方法从列表中移除

三、总结

优点，组件间通信解耦，预编译生成效率较高

缺点，代码可读性变差，逻辑分散

源码解析：

Android事件总线（一）EventBus3.0用法全解析 | BATcoder - 刘望舒：http://liuwangshu.cn/application/eventbus/1-eventbus.html
Android事件总线（二）EventBus3.0源码解析 | 刘望舒的博客：http://liuwangshu.cn/application/eventbus/2-eventbus-sourcecode.html
Android主流三方库源码分析（九、深入理解EventBus源码） | Deep into Android：链接
【Bugly干货分享】老司机教你 “飙” EventBus 3 - 腾讯bugly - 博客园：https://www.cnblogs.com/bugly/p/5475034.html
EventBus3.0 性能提升之添加索引_zhangke-CSDN博客：https://blog.csdn.net/kaifa1321/article/details/79761507

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一、使用方法

二、源码分析

三、总结

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