EventBus3.0源码解析

看了好几次EventBus源码了,这次总算基本上搞清楚了,简单来说就是调用 EventBus.getDefault().register(this),将订阅者类(如下MainActivity)中所有订阅方法(如下onMessageEvent方法)存下来,然后某个对象在调用EventBus.getDefault().post(new MsgEvent())时,实际上是通过反射调用之前存好的订阅方法,本文分为以下几个部分:注册、销毁、发送事件来解读EventBus。

基本用法

我把基本用法的代码贴出来,方便我们查看,并根据代码来理解其逻辑:
消息事件:

public class MsgEvent {
}

应用类:

public class MainActivity extends Activity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        EventBus.getDefault().register(this);
        EventBus.getDefault().post(new MsgEvent());
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        EventBus.getDefault().unregister(this);
    }

    @Subscribe(threadMode = ThreadMode.MAIN)
    public void onMessageEvent(MsgEvent event) {
        Log.d("tag","******just a event****");
    };
}

我们主要从注册事件EventBus.getDefault().register(this);发送事件EventBus.getDefault().post(new MsgEvent()); 取消注册事件EventBus.getDefault().unregister(this);来开始分析EventBus。

重要的数据结构

private final Map, CopyOnWriteArrayList> subscriptionsByEventType;
private final Map>> typesBySubscriber;
private final Map, Object> stickyEvents;
final class Subscription {
    final Object subscriber;
    final SubscriberMethod subscriberMethod;
    volatile boolean active;
    Subscription(Object subscriber, SubscriberMethod subscriberMethod) {
        this.subscriber = subscriber;
        this.subscriberMethod = subscriberMethod;
        active = true;
    }
...此处省略
}

subscriptionsByEventType:以event(即事件类)为key,以订阅列表(Subscription)为value,事件发送之后,在这里寻找订阅者,而Subscription又是一个CopyOnWriteArrayList,这是一个线程安全的容器。你们看会好奇,Subscription到底是什么,其实看下去就会了解的,现在先提前说下:Subscription是一个封装类,封装了订阅者、订阅方法这两个类。
typesBySubscriber:以订阅者类为key,以event事件类为value,在进行register或unregister操作的时候,会操作这个map。
stickyEvents:保存的是粘性事件,粘性事件上一篇文章有详细描述。
回到EventBus的构造方法,接下来实例化了三个Poster,分别是mainThreadPoster、backgroundPoster、asyncPoster等,这三个Poster是用来处理粘性事件的,我们下面会展开讲述。接着,就是对builder的一系列赋值了,这里使用了建造者模式。
Subscription:以订阅者为key(类比上面的MainActivity),以订阅者中方法(类比上面的onMessageEvent方法)为value。

注册事件

要想注册成为订阅者,必须在一个类中调用如下:

EventBus.getDefault().register(this);

那么,我们看一下getDefault()的源码是怎样的,EventBus#getDefault():
getDefault()

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

可以看出,这里使用了单例模式,而且是双重校验的单例,确保在不同线程中也只存在一个EvenBus的实例。
register

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

先获取了订阅者类的class,接着交给SubscriberMethodFinder.findSubscriberMethods()处理,返回结果保存在List(即上面的onMessageEvent)中,由此可推测通过上面的方法把订阅方法找出来了,并保存在集合中。

为了方便读者理解,具体findSubscriberMethods()分析在最后一节。

在找到订阅者中所有的方法集合subscriberMethods后,将执行subscribe(subscriber, subscriberMethod),生成subscription,将所有subscription都添加到subscriptionsByEventType变量中,获取subscriberMethod的类型,并赋值给typesBySubscriber。
subscribe

private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
    Class eventType = subscriberMethod.eventType;
    //将subscriber和subscriberMethod封装成 Subscription
    Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
    //根据事件类型获取特定的 Subscription
    CopyOnWriteArrayList subscriptions = subscriptionsByEventType.get(eventType);
    //如果为null,说明该subscriber尚未注册该事件
    if (subscriptions == null) {
        subscriptions = new CopyOnWriteArrayList<>();
        subscriptionsByEventType.put(eventType, subscriptions);
    } else {
        //如果不为null,并且包含了这个subscription 那么说明该subscriber已经注册了该事件,抛出异常
        if (subscriptions.contains(newSubscription)) {
            throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event "
                    + eventType);
        }
    }

    //根据优先级来设置放进subscriptions的位置,优先级高的会先被通知
    int size = subscriptions.size();
    for (int i = 0; i <= size; i++) {
        if (i == size || subscriberMethod.priority > subscriptions.get(i).subscriberMethod.priority) {
            subscriptions.add(i, newSubscription);
            break;
        }
    }

    //根据subscriber(订阅者)来获取它的所有订阅事件
    List> subscribedEvents = typesBySubscriber.get(subscriber);
    if (subscribedEvents == null) {
        subscribedEvents = new ArrayList<>();
        //把订阅者、事件放进typesBySubscriber这个Map中
        typesBySubscriber.put(subscriber, subscribedEvents);
    }
    subscribedEvents.add(eventType);

    //下面是对粘性事件的处理
    if (subscriberMethod.sticky) {
        //从EventBusBuilder可知,eventInheritance默认为true.
        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 {
            //根据eventType,从stickyEvents列表中获取特定的事件
            Object stickyEvent = stickyEvents.get(eventType);
            //分发事件
            checkPostStickyEventToSubscription(newSubscription, stickyEvent);
        }
    }
}

到目前为止,注册流程基本分析完毕。

发送事件

EventBus.getDefault().post(new MessageEvent());`

post

public void post(Object event) {
    //1、 获取一个postingState
    PostingThreadState postingState = currentPostingThreadState.get();
    List eventQueue = postingState.eventQueue;
    //将事件加入队列中
    eventQueue.add(event);

    if (!postingState.isPosting) {
        //判断当前线程是否是主线程
        postingState.isMainThread = Looper.getMainLooper() == Looper.myLooper();
        postingState.isPosting = true;
        if (postingState.canceled) {
            throw new EventBusException("Internal error. Abort state was not reset");
        }
        try {
            //只要队列不为空,就不断从队列中获取事件进行分发
            while (!eventQueue.isEmpty()) {
                postSingleEvent(eventQueue.remove(0), postingState);
            }
        } finally {
            postingState.isPosting = false;
            postingState.isMainThread = false;
        }
    }
}
 
 

首先是获取一个PostingThreadState,那么PostingThreadState是什么呢?我们来看看它的类结构:
PostingThreadState

final static class PostingThreadState {
    final List eventQueue = new ArrayList();
    boolean isPosting;
    boolean isMainThread;
    Subscription subscription;
    Object event;
    boolean canceled;
}
 
 

可以看出,该PostingThreadState主要是封装了当前线程的信息,以及订阅者、订阅事件等。那么怎么得到这个PostingThreadState呢?让我们回到post()方法,看①号代码,这里通过currentPostingThreadState.get()来获取PostingThreadState。那么currentPostingThreadState又是什么呢?

private final ThreadLocal currentPostingThreadState = new ThreadLocal() {
    @Override
    protected PostingThreadState initialValue() {
        return new PostingThreadState();
    }
};

原来 currentPostingThreadState是一个ThreadLocal,而ThreadLocal是每个线程独享的,其数据别的线程是不能访问的,因此是线程安全的。我们再次回到Post()方法,继续往下走,是一个while循环,这里不断地从队列中取出事件,并且分发出去,调用的是EventBus#postSingleEvent方法。
postSingleEvent

private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
    Class eventClass = event.getClass();
    boolean subscriptionFound = false;
    //该eventInheritance上面有提到,默认为true,即EventBus会考虑事件的继承树
    //如果事件继承自父类,那么父类也会作为事件被发送
    if (eventInheritance) {
        //查找该事件的所有父类
        List> eventTypes = lookupAllEventTypes(eventClass);
        int countTypes = eventTypes.size();
        //遍历所有事件
        for (int h = 0; h < countTypes; h++) {
            Class clazz = eventTypes.get(h);
            subscriptionFound |= postSingleEventForEventType(event, postingState, clazz);
        }
    } else {
        subscriptionFound = postSingleEventForEventType(event, postingState, eventClass);
    }
    //如果没找到订阅该事件的订阅者
    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));
        }
    }
}

postSingleEventForEventType

private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class eventClass) {
    CopyOnWriteArrayList subscriptions;
    synchronized (this) {
        //从subscriptionsByEventType获取响应的subscriptions
        subscriptions = subscriptionsByEventType.get(eventClass);
    }
    if (subscriptions != null && !subscriptions.isEmpty()) {
        for (Subscription subscription : subscriptions) {
            postingState.event = event;
            postingState.subscription = subscription;
            boolean aborted = false;
            try {
                //发送事件
                postToSubscription(subscription, event, postingState.isMainThread);
                aborted = postingState.canceled;
            } 
            //...
        }
        return true;
    }
    return false;
}

接着,进一步调用了EventBus#postToSubscription,可以发现,这里把订阅列表作为参数传递了进去,显然,订阅列表内部保存了订阅者以及订阅方法,那么可以猜测,这里应该是通过反射的方式来调用订阅方法。具体怎样的话,我们看源码。
postToSubscription

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);
    }
}

首先获取threadMode,即订阅方法运行的线程,如果是POSTING,那么直接调用invokeSubscriber()方法即可,如果是MAIN,则要判断当前线程(post事件的线程)是否是MAIN线程,如果是也是直接调用invokeSubscriber()方法,否则会交给mainThreadPoster来处理,其他情况相类似。
invokeSubscriber
主要利用反射的方式来调用订阅方法,这样就实现了事件发送给订阅者,订阅者调用订阅方法这一过程。如下所示:

void invokeSubscriber(Subscription subscription, Object event) {
    try {
        subscription.subscriberMethod.method.invoke(subscription.subscriber, event);
    } 
    //...
}

HandlerPoster
我们首先看mainThreadPoster,它的类型是HandlerPoster继承自Handler:

final class HandlerPoster extends Handler {

    //PendingPostQueue队列,待发送的post队列
    private final PendingPostQueue queue;
    //规定最大的运行时间,因为运行在主线程,不能阻塞主线程
    private final int maxMillisInsideHandleMessage;
    private final EventBus eventBus;
    private boolean handlerActive;
    //省略...
}

可以看到,该handler内部有一个PendingPostQueue,这是一个队列,保存了PendingPost,即待发送的post,该PendingPost封装了event和subscription,方便在线程中进行信息的交互。在postToSubscription方法中,当前线程如果不是主线程的时候,会调用HandlerPoster#enqueue方法:

void enqueue(Subscription subscription, Object event) {
    //将subscription和event打包成一个PendingPost
    PendingPost pendingPost = PendingPost.obtainPendingPost(subscription, event);
    synchronized (this) {
        //入队列
        queue.enqueue(pendingPost);
        if (!handlerActive) {
            handlerActive = true;
            //发送消息,在主线程运行
            if (!sendMessage(obtainMessage())) {
                throw new EventBusException("Could not send handler message");
            }
        }
    }
}

首先会从PendingPostPool中获取一个可用的PendingPost,接着把该PendingPost放进PendingPostQueue,发送消息,那么由于该HandlerPoster在初始化的时候获取了UI线程的Looper,所以它的handleMessage()方法运行在UI线程:

@Override
public void handleMessage(Message msg) {
    boolean rescheduled = false;
    try {
        long started = SystemClock.uptimeMillis();
        //不断从队列中取出pendingPost
        while (true) {
            //省略...
            eventBus.invokeSubscriber(pendingPost);        
            //..
        }
    } finally {
        handlerActive = rescheduled;
    }
}

里面调用到了EventBus#invokeSubscriber方法,在这个方法里面,将PendingPost解包,进行正常的事件分发,这上面都说过了,就不展开说了。

BackgroundPoster
BackgroundPoster继承自Runnable,与HandlerPoster相似的,它内部都有PendingPostQueue这个队列,当调用到它的enqueue的时候,会将subscription和event打包成PendingPost:

public void enqueue(Subscription subscription, Object event) {
    PendingPost pendingPost = PendingPost.obtainPendingPost(subscription, event);
    synchronized (this) {
        queue.enqueue(pendingPost);
        //如果后台线程还未运行,则先运行
        if (!executorRunning) {
            executorRunning = true;
            //会调用run()方法
            eventBus.getExecutorService().execute(this);
        }
    }
}

该方法通过Executor来运行run()方法,run()方法内部也是调用到了EventBus#invokeSubscriber方法。
AsyncPoster
与BackgroundPoster类似,它也是一个Runnable,实现原理与BackgroundPoster大致相同,但有一个不同之处,就是它内部不用判断之前是否已经有一条线程已经在运行了,它每次post事件都会使用新的一条线程。

注销

与注册相对应的是注销,当订阅者不再需要事件的时候,我们要注销这个订阅者,即调用以下代码:

EventBus.getDefault().unregister(this);

那么我们来分析注销流程是怎样实现的,首先查看EventBus#unregister:
unregister

public synchronized void unregister(Object subscriber) {
    //根据当前的订阅者来获取它所订阅的所有事件
    List> subscribedTypes = typesBySubscriber.get(subscriber);
    if (subscribedTypes != null) {
        //遍历所有订阅的事件
        for (Class eventType : subscribedTypes) {
            unsubscribeByEventType(subscriber, eventType);
        }
        //从typesBySubscriber中移除该订阅者
        typesBySubscriber.remove(subscriber);
    } else {
        Log.w(TAG, "Subscriber to unregister was not registered before: " + subscriber.getClass());
    }
}

上面调用了EventBus#unsubscribeByEventType,把订阅者以及事件作为参数传递了进去,那么应该是解除两者的联系。
unsubscribeByEventType

private void unsubscribeByEventType(Object subscriber, Class eventType) {
    //根据事件类型从subscriptionsByEventType中获取相应的 subscriptions 集合
    List subscriptions = subscriptionsByEventType.get(eventType);
    if (subscriptions != null) {
        int size = subscriptions.size();
        //遍历所有的subscriptions,逐一移除
        for (int i = 0; i < size; i++) {
            Subscription subscription = subscriptions.get(i);
            if (subscription.subscriber == subscriber) {
                subscription.active = false;
                subscriptions.remove(i);
                i--;
                size--;
            }
        }
    }
}

可以看到,上面两个方法的逻辑是非常清楚的,都是从typesBySubscriber或subscriptionsByEventType移除相应与订阅者有关的信息,注销流程相对于注册流程简单了很多,其实注册流程主要逻辑集中于怎样找到订阅方法上。

继续分析findSubscriberMethods

findSubscriberMethods

List findSubscriberMethods(Class subscriberClass) {
    //首先从缓存中取出subscriberMethodss,如果有则直接返回该已取得的方法
    List subscriberMethods = METHOD_CACHE.get(subscriberClass);
    if (subscriberMethods != null) {
        return subscriberMethods;
    }
    //从EventBusBuilder可知,ignoreGenerateIndex一般为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 {
        //将获取的subscriberMeyhods放入缓存中
        METHOD_CACHE.put(subscriberClass, subscriberMethods);
        return subscriberMethods;
    }
}

findSubscriberMethods通过调用findUsingInfo来找到订阅者中的所有方法集合。

findUsingInfo

private List findUsingInfo(Class subscriberClass) {
    //准备一个FindState,该FindState保存了订阅者类的信息
    FindState findState = prepareFindState();
    //对FindState初始化
    findState.initForSubscriber(subscriberClass);
    while (findState.clazz != null) {
        //获得订阅者的信息,一开始会返回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 {
            //1 、到了这里
            findUsingReflectionInSingleClass(findState);
        }
        //移动到父类继续查找
        findState.moveToSuperclass();
    }
    return getMethodsAndRelease(findState);
}

上面用到了FindState这个内部类来保存订阅者类的信息,我们看看它的内部结构:
FindState

static class FindState {
    //订阅方法的列表
    final List subscriberMethods = new ArrayList<>();
    //以event为key,以method为value
    final Map anyMethodByEventType = new HashMap<>();
    //以method的名字生成一个method为key,以订阅者类为value
    final Map subscriberClassByMethodKey = new HashMap<>();
    final StringBuilder methodKeyBuilder = new StringBuilder(128);

    Class subscriberClass;
    Class clazz;
    boolean skipSuperClasses;
    SubscriberInfo subscriberInfo;

    //对FindState初始化
    void initForSubscriber(Class subscriberClass) {
        this.subscriberClass = clazz = subscriberClass;
        skipSuperClasses = false;
        subscriberInfo = null;
    }

    //省略...
}

可以看出,该内部类保存了订阅者及其订阅方法的信息,用Map一一对应进行保存,接着利用initForSubscriber进行初始化,这里subscriberInfo初始化为null,因此在SubscriberMethodFinder#findUsingInfo里面,会直接调用到①号代码,即调用SubscriberMethodFinder#findUsingReflectionInSingleClass,这个方法非常重要!!!在这个方法内部,利用反射的方式,对订阅者类进行扫描,找出订阅方法,并用上面的Map进行保存,我们来看这个方法。
findUsingReflectionInSingleClass

  private void findUsingReflectionInSingleClass(FindState findState) {
    Method[] methods;
    try {
        // This is faster than getMethods, especially when subscribers are fat classes like Activities
        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) {
        //获取方法的修饰符
        int modifiers = method.getModifiers();
        if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
            //获取方法的参数类型
            Class[] parameterTypes = method.getParameterTypes();
            //如果参数个数为一个,则继续
            if (parameterTypes.length == 1) {
                //获取该方法的@Subscribe注解
                Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);
                if (subscribeAnnotation != null) {
                    //参数类型 即为 事件类型
                    Class eventType = parameterTypes[0];
                    // 2 、调用checkAdd方法
                    if (findState.checkAdd(method, eventType)) {
                        //从注解中提取threadMode
                        ThreadMode threadMode = subscribeAnnotation.threadMode();
                        //新建一个SubscriberMethod对象,并添加到findState的subscriberMethods这个集合内
                        findState.subscriberMethods.add(new SubscriberMethod(method, eventType, threadMode,
                                subscribeAnnotation.priority(), subscribeAnnotation.sticky()));
                    }
                }
            //如果开启了严格验证,同时当前方法又有@Subscribe注解,对不符合要求的方法会抛出异常
            } 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");
        }
    }
}

虽然该方法比较长,但是逻辑非常清晰,逐一扫描订阅者内所有的方法,并判断订阅者类中是否存在订阅方法,如果符合要求,并且②号代码调用FindState#checkAdd方法返回true的时候,才会把方法保存在findState的subscriberMethods内。而SubscriberMethod则是用于保存订阅方法的一个类。

回到findUsingReflectionInSingleClass方法,当遍历完当前类的所有方法后,会回到findUsingInfo方法,接着会执行最后一行代码,即return getMethodsAndRelease(findState);
getMethodsAndRelease

private List getMethodsAndRelease(FindState findState) {
    //从findState获取subscriberMethods,放进新的ArrayList
    List subscriberMethods = new ArrayList<>(findState.subscriberMethods);
    //把findState回收
    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;
}

通过该方法,把subscriberMethods返回。

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