EventBus 源码分析

Posted 吴豪杰

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0. 前言

EventBus 是一款针对android优化的发布/订阅事件总线。主要功能是替代 Intent , Handler , BroadCastFragmentActivityService ,线程之间传递消息。优点是开销小,代码更优雅,以及将发送者和接收者解耦。此文将对最新的 EventBus 3.0 的源码进行简要的分析。

1. 用法回顾

EventBus 3.0 的用法较之前的版本有所变化,它使用了最近较为流行的注解形式取代以前的 onEvent 开头作为方法名,但使用步骤大致相同,分为三个步骤:

  • 注册与反注册
    在Activity中的 onCreate() 方法中进行EventBus的注册,在 onDestroy() 方法中进行取消注册。
// 注册
EventBus.getDefault().register(this);
// 取消注册
EventBus.getDefault().unregister(this);
  • 订阅
    在3.0版本中,使用 @Subscribe 注解指定订阅者方法。
@Subscribe(threadMode = ThreadMode.MAIN)
public void function(Params p) 
  • 发布
    发布者通过 post()postSticky() 方法进行发布消息。
EventBus.getDefault().post(new Params());

注意:
1. 注册类经笔者测试,必须要是Activity才可以
2. 订阅者方法可见性不能为 private

2. 从注册入手

2.1 单例

调用 getDefault() 方法可以获取到一个全局的单例对象, 这里采用的是 饿汉式 单例设计模式:

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

2.2 构造方法

单例模式维持了一个唯一的对象,那这个对象包含那些东西呢?接下来让我们看看构造方法:

EventBus(EventBusBuilder builder) 
    subscriptionsByEventType = new HashMap<>();
    typesBySubscriber = new HashMap<>();
    stickyEvents = new ConcurrentHashMap<>();
    mainThreadPoster = new HandlerPoster(this, Looper.getMainLooper(), 10);
    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;
    executorService = builder.executorService;

首先是三张 HashMap ,用来存储订阅信息;然后,有创建了三个 Poster ,这就是用来反馈到订阅者所用的。

2.3 注册订阅

public void register(Object subscriber) 
    Class<?> subscriberClass = subscriber.getClass();
    List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
    synchronized (this) 
        for (SubscriberMethod subscriberMethod : subscriberMethods) 
            subscribe(subscriber, subscriberMethod);
        
    

首先获取订阅类的字节码,然后通过 findSubscriberMethods(...) 方法获取订阅类中包含订阅注解的所有方法,最后将这些方法通过 subscribe() 来实现订阅。

2.4 获取注解方法

为了提高性能,先经过一层存放方法的缓存区,如果缓存区有数据,这直接从缓冲区中获取,没有的话才通过反射机制获取方法,同时将获取到的方法存放一份到缓存区。

List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) 
    List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
    ...
    subscriberMethods = findUsingReflection(subscriberClass);
    ...
    METHOD_CACHE.put(subscriberClass, subscriberMethods);
    return subscriberMethods;

如果没有缓存数据,则通过反射查找所有包含 @Subscribe 注解的方法, findUsingReflection() 方法会查找继承关系中所有包含注解的方法:

private List<SubscriberMethod> findUsingReflection(Class<?> subscriberClass) 
    FindState findState = prepareFindState();
    findState.initForSubscriber(subscriberClass);
    while (findState.clazz != null) 
        findUsingReflectionInSingleClass(findState);
        findState.moveToSuperclass();
    
    return getMethodsAndRelease(findState);

具体的查找过程代码如下:

private void findUsingReflectionInSingleClass(FindState findState) 
    ...
    methods = findState.clazz.getDeclaredMethods();
    ...
    for (Method method : methods) 
        ...
        // 获取 `@Subscribe` 注解
        Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);
                if (subscribeAnnotation != null) 
                    Class<?> eventType = parameterTypes[0];
                    if (findState.checkAdd(method, eventType)) 
                        ThreadMode threadMode = subscribeAnnotation.threadMode();
                        findState.subscriberMethods.add(new SubscriberMethod(method, eventType, threadMode, subscribeAnnotation.priority(), subscribeAnnotation.sticky()));
                    
                
        ...
    
    ...

代码比较长,大体思路就是先通过字节码利用反射获取所有方法,然后遍历查找包含注解的方法,在检查合法性后添加到 findState.subscriberMethods 列表,列表定义如下:

static class FindState 
    final List<SubscriberMethod> subscriberMethods = new ArrayList<>();
    final Map<Class, Object> anyMethodByEventType = new HashMap<>();
    final Map<String, Class> subscriberClassByMethodKey = new HashMap<>();
    ...

至此,订阅者的方法就找到了,回到 register() 方法,了解下一步的操作。

public void register(Object subscriber) 
    ...
    for (SubscriberMethod subscriberMethod : subscriberMethods) 
        subscribe(subscriber, subscriberMethod);
    

哈哈,很明显,接下来就是把包含注解的所有方法进行订阅。

3. 订阅

3.1 存放订阅信息

private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) 
    ...
    Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
    ...
    subscriptions.add(i, newSubscription);
    ...
    subscribedEvents.add(eventType);
    if (subscriberMethod.sticky) 
        ...
        // 获取事件
        Object stickyEvent = stickyEvents.get(eventType);
        checkPostStickyEventToSubscription(newSubscription, stickyEvent);
    

订阅方法中分为两种,一种是立即推送,它被添加至 subscriptions 列表中保存,还有一种 sticky 模式,笔者理解的就是延迟模式,它就调用 checkPostStickyEventToSubscription() 方法。我们先来看后者:

private void checkPostStickyEventToSubscription(Subscription newSubscription, Object stickyEvent) 
    if (stickyEvent != null) 
        postToSubscription(newSubscription, stickyEvent, Looper.getMainLooper() == Looper.myLooper());
    

这里调用了 postToSubscription() 方法,带有三个参数,分别是包含订阅者方法的订阅信息包装类、延迟事件、是否为主线程。笔者这里想说下第三个参数, Looper.getMainLooper() 方法返回的是主线程的Looper对象,Looper.myLooper() 返回的是当前线程的Looper对象,所以判断这两个Looper对象是否相等就可以判断当前线程是否为主线程了,是不是很巧妙呢!好了,继续前进去看看 postToSubscription() 方法。

3.2 提交订阅

private void postToSubscription(Subscription subscription, Object event, boolean isMainThread) 
    switch (subscription.subscriberMethod.threadMode) 
        case POSTING:   // 同一个线程调用
            invokeSubscriber(subscription, event);
            break;
        case MAIN:      // 主线程调用
            if (isMainThread) 
                invokeSubscriber(subscription, event);
             else 
                mainThreadPoster.enqueue(subscription, event);
            
            break;
        case BACKGROUND: // 后台线程调用
            if (isMainThread) 
                backgroundPoster.enqueue(subscription, event);
             else 
                invokeSubscriber(subscription, event);
            
            break;
        case ASYNC:         // 异步线程调用
            asyncPoster.enqueue(subscription, event);
            break;
        default:
            throw new IllegalStateException("Unknown thread mode: " + subscription.subscriberMethod.threadMode);
    

提交订阅也是可以大致分为两种,直接调用或者放入构造方法中提到的 Poster

3.2.1 直接调用

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);
    

直接使用 Method#invoke() 方法调用即可,不多说。

3.2.2 Poster方式

Poster 有三个,原理都基本一致的,这里就以 mainThreadPoster 来说就好,其它两个就靠读者举一反三了:)

 private final PendingPostQueue queue;

 void enqueue(Subscription subscription, Object event) 
    PendingPost pendingPost = PendingPost.obtainPendingPost(subscription, event);
    synchronized (this) 
        queue.enqueue(pendingPost);
        if (!handlerActive) 
            handlerActive = true;
            if (!sendMessage(obtainMessage())) 
                throw new EventBusException("Could not send handler message");
            
        
    

首先,通过 PendingPost.obtainPendingPost() 获取到一个 PendingPost 对象,其包含:

private PendingPost(Object event, Subscription subscription) 
    this.event = event;
    this.subscription = subscription;

然后将其放在存放 PendingPostPendingPostQueue 队列中,并且将标志位 handlerActive 激活。

final class HandlerPoster extends Handler 
    void enqueue(Subscription subscription, Object event)  ... 
    public void handleMessage(Message msg)  ... 

最后利用Handler机制,调用 sendMessage(obtainMessage()) 发送消息,传递到 handleMessage() 中对消息进行处理。

3.3 取出消息

@Override
public void handleMessage(Message msg) 
    boolean rescheduled = false;
    try 
        long started = SystemClock.uptimeMillis();
        while (true) 
            PendingPost pendingPost = queue.poll();
            if (pendingPost == null) 
                synchronized (this) 
                    // 双重检查
                    pendingPost = queue.poll();
                    // 如果没有 pendingPost ,则消息处理完毕,退出
                    if (pendingPost == null) 
                        handlerActive = false;
                        return;
                    
                
            
            // 执行
            eventBus.invokeSubscriber(pendingPost);
            long timeInMethod = SystemClock.uptimeMillis() - started;
            if (timeInMethod >= maxMillisInsideHandleMessage) 
                if (!sendMessage(obtainMessage())) 
                    throw new EventBusException("Could not send handler message");
                
                rescheduled = true;
                return;
            
        
     finally 
        handlerActive = rescheduled;
    

取消息通过循环来取,然后调用 eventBus.invokeSubscriber(pendingPost) 处理消息,另外,如果处理超时,还会重新执行 sendMessage(obtainMessage()) 以重试。

3.4 调用订阅者

接下来看 eventBus.invokeSubscriber(pendingPost) 方法:

void invokeSubscriber(PendingPost pendingPost) 
    Object event = pendingPost.event;
    Subscription subscription = pendingPost.subscription;
    PendingPost.releasePendingPost(pendingPost);
    if (subscription.active) 
        invokeSubscriber(subscription, event);
    

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);
    

思路就是对 pendingPost 解包,取出 subscription ,然后调用其中的方法。但是现在队列中并没有消息,所以上述还不会执行,我们得通过发布者发布消息后才能取出消息执行。

4. 发布者

4.1 发布事件

public void post(Object event) 
    PostingThreadState postingState = currentPostingThreadState.get();
    List<Object> eventQueue = postingState.eventQueue;
    // 添加事件
    eventQueue.add(event);
    if (!postingState.isPosting) 
        postingState.isMainThread = Looper.getMainLooper() == Looper.myLooper();
        postingState.isPosting = true;
        if (postingState.canceled) 
            throw new EventBusException("Internal error. Abort state was not reset");
        
        try 
            while (!eventQueue.isEmpty()) 
                // 执行事件
                postSingleEvent(eventQueue.remove(0), postingState);
            
         finally 
            postingState.isPosting = false;
            postingState.isMainThread = false;
        
    

该方法中,先是获取到事件队列,然后将参数放入队列,刷新 postingState 后执行事件。

4.2 提交事件

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) 
            Log.d(TAG, "No subscribers registered for event " + eventClass);
        
        if (sendNoSubscriberEvent && eventClass != NoSubscriberEvent.class &&
                eventClass != SubscriberExceptionEvent.class) 
            post(new NoSubscriberEvent(this, event));
        
    

大体就是获取到事件类型,然后调用 postSingleEventForEventType() 方法:

private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass) 
    ...
        for (Subscription subscription : subscriptions) 
            postingState.event = event;
            postingState.subscription = subscription;
            boolean aborted = false;
            postToSubscription(subscription, event, postingState.isMainThread);
    ...

同样,先要刷新 postingState 状态,然后调用 postToSubscription() 方法,这个方法是不是有点熟悉呢?

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 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);
    

对,这又回到了前文中的提交订阅部分了,之后 Poster 取出消息,传递消息,调用订阅者。

5. 总结

EventBus 作为安卓消息传递中的一大神器,单凭笔者这篇博文也是分析不透的,不过希望读者阅读此文后能有一个初步的印象,了解具体的流程,对以后的开发工作有所帮助。

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