EventBus 3.1.1 源码解析

* 本篇文章已授权微信公众号 guolin_blog(郭霖)独家发布

一、本文需要解决的问题

我研究EventBus源码的目的是解决以下几个我在使用过程中所思考的问题:

  1. 这个框架涉及到一种设计模式叫做观察者模式,什么是观察者模式?
  2. 事件如何进行定义,有没有相关限制?
  3. 观察者绑定观察事件的时候,绑定方法的命名有限制吗?
  4. 事件发送和接收的原理?

二、初步使用

为了研究源码的方便,我写了一个简单的demo。

定义事件

TestEvent.java:

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public class TestEvent {
private String msg;
public String getMsg() {
return msg;
}
public void setMsg(String msg) {
this.msg = msg;
}
}

主Activity

MainActivity.java:

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public class MainActivity extends AppCompatActivity {
private Button button;
private TextView textView;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
button = findViewById(R.id.button);
textView = findViewById(R.id.text);
EventBus.getDefault().register(this);
button.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
TestEvent event = new TestEvent();
event.setMsg("已接收到事件!");
EventBus.getDefault().post(event);
}
});
}
@Subscribe(threadMode = ThreadMode.MAIN)
public void onTestEvent(TestEvent event) {
textView.setText(event.getMsg());
}
@Override
protected void onDestroy() {
EventBus.getDefault().unregister(this);
super.onDestroy();
}
}

运行效果

demo.gif

三、源码分析

关于观察者模式
  • 简介:观察者模式是设计模式中的一种。它是为了定义对象间的一种一对多的依赖关系,即当一个对象的状态发生改变时,所有依赖于它的对象都得到通知并被自动更新。
  • 如何使用:这里传送门有相关的demo,这里不再详述。
  • 重点:在这个模式中主要包含两个重要的角色:发布者订阅者(又称观察者)。对应EventBus来说,发布者即发送消息的一方(即调用EventBus.getDefault().post(event)的一方),订阅者即接收消息的一方(即调用EventBus.getDefault().register()的一方)。
    我们已经解决了第一个问题~
关于事件

这里指的事件其实是一个泛泛的统称,指的是一个概念上的东西(当时我还以为一定要以啥Event命名…),通过查阅官方文档,我知道事件的命名格式并没有任何要求,你可以定义一个对象作为事件,也可以发送基本数据类型如int,String等作为一个事件。后续的源码分析我也会再次证明一下。

具体分析

从函数入口开始分析:
1.EventBus#getDefault():

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

这里就是采用双重校验并加锁的单例模式生成EventBus实例。

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

由于我们传入的为this,即MainActivity的实例,所以第一行代码获取了订阅者的class对象,然后会找出所有订阅的方法。我们看一下第二行的逻辑。
SubscriberMethodFinder#findSubscriberMethods():

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List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
if (subscriberMethods != null) {
return subscriberMethods;
}
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 {
METHOD_CACHE.put(subscriberClass, subscriberMethods);
return subscriberMethods;
}
}

分析:

  • 如果缓存中有对应class的订阅方法列表,则直接返回,这里我们是第一次创建,所以此时subscriberMethods为空;
  • 接下来会有一个参数判断,通过查看前面的创建过程,ignoreGeneratedIndex默认为false,进入else代码块,后面生成subscriberMethods成功的话会加入到缓存中,失败的话会throw异常。

2.SubscriberMethodFinder#findUsingInfo():

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private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
// 2.1
FindState findState = prepareFindState();
findState.initForSubscriber(subscriberClass);
// 2.2
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 {
// 2.3
findUsingReflectionInSingleClass(findState);
}
findState.moveToSuperclass();
}
// 2.4
return getMethodsAndRelease(findState);
}

2.1 SubscriberMethodFinder#prepareFindState():

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private static final FindState[] FIND_STATE_POOL = new FindState[POOL_SIZE];
private FindState prepareFindState() {
synchronized(FIND_STATE_POOL) {
for (int i = 0; i < POOL_SIZE; i++) {
FindState state = FIND_STATE_POOL[i];
if (state != null) {
FIND_STATE_POOL[i] = null;
return state;
}
}
}
return new FindState();
}

这个方法是创建一个新的FindState类,通过两种方法获取,一种是从FIND_STATE_POOL即FindState池中取出可用的FindState,如果没有的话,则通过第二种方式:直接new一个新的FindState对象。
FindState#initForSubscriber():

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static class FindState {
// 省略代码
void initForSubscriber(Class<?> subscriberClass) {
this.subscriberClass = clazz = subscriberClass;
skipSuperClasses = false;
subscriberInfo = null;
}
// 省略代码
}

FindState类是SubscriberMethodFinder的内部类,这个方法主要做一个初始化的工作。

2.2 SubscriberMethodFinder#getSubscriberInfo():

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private SubscriberInfo getSubscriberInfo(FindState findState) {
if (findState.subscriberInfo != null && findState.subscriberInfo.getSuperSubscriberInfo() != null) {
SubscriberInfo superclassInfo = findState.subscriberInfo.getSuperSubscriberInfo();
if (findState.clazz == superclassInfo.getSubscriberClass()) {
return superclassInfo;
}
}
if (subscriberInfoIndexes != null) {
for (SubscriberInfoIndex index: subscriberInfoIndexes) {
SubscriberInfo info = index.getSubscriberInfo(findState.clazz);
if (info != null) {
return info;
}
}
}
return null;
}

这里由于初始化的时候,findState.subscriberInfo和subscriberInfoIndexes为空,所以这里直接返回null,后续我们可以再回到这里看看subscriberInfo有什么作用。

2.3 SubscriberMethodFinder#findUsingReflectionInSingleClass():

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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) {
int modifiers = method.getModifiers();
if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
Class<?> [] parameterTypes = method.getParameterTypes();
if (parameterTypes.length == 1) {
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()));
}
}
} 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");
}
}
}

这个方法的逻辑是:
通过反射的方式获取订阅者类中的所有声明方法,然后在这些方法里面寻找以@Subscribe作为注解的方法进行处理(!!!部分的代码),先经过一轮检查,看看findState.subscriberMethods是否存在,如果没有的话,将方法名,threadMode,优先级,是否为sticky方法封装为SubscriberMethod对象,添加到subscriberMethods列表中。

什么是sticky event?

sticky event,中文名为粘性事件。普通事件是先注册,然后发送事件才能收到;而粘性事件,在发送事件之后再订阅该事件也能收到。此外,粘性事件会保存在内存中,每次进入都会去内存中查找获取最新的粘性事件,除非你手动解除注册。

在这里我们解决了第二个和第三个问题,方法的命名并没有任何要求,只是加上@Subscribe注解即可!同时事件的命名也没有任何要求!

之后这个while循环会继续检查父类,当然遇到系统相关的类时会自动跳过,以提升性能。

2.4 SubscriberMethodFinder#getMethodsAndRelease

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private List<SubscriberMethod> getMethodsAndRelease(FindState findState) {
List<SubscriberMethod> subscriberMethods = new ArrayList<>(findState.subscriberMethods);
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;
}

这里将subscriberMethods列表直接返回,同时会把findState做相应处理,存储在FindState池中,方便下一次使用,提高性能。

  1. EventBus#subscribe():
    返回subscriberMethods之后,register方法的最后会调用subscribe方法:
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    public void register(Object subscriber) {
    Class<?> subscriberClass = subscriber.getClass();
    List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
    synchronized (this) {
    for (SubscriberMethod subscriberMethod : subscriberMethods) {
    subscribe(subscriber, subscriberMethod);
    }
    }
    }
    private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
    Class<?> eventType = subscriberMethod.eventType;
    Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
    CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
    if (subscriptions == null) {
    subscriptions = new CopyOnWriteArrayList <> ();
    subscriptionsByEventType.put(eventType, subscriptions);
    } else {
    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) {
    subscriptions.add(i, newSubscription);
    break;
    }
    }
    List<Class<?>> 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<Class>).
    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);
    }
    }
    }

分析:

  • 首先,根据subscriberMethod.eventType(在Demo里面指的是TestEvent),在subscriptionsByEventType去查找一个CopyOnWriteArrayList ,如果没有则创建一个新的CopyOnWriteArrayList;
  • 然后将这个CopyOnWriteArrayList放入subscriptionsByEventType中,这里的subscriptionsByEventType是一个Map,key为eventType,value为CopyOnWriteArrayList,这个Map非常重要,后续还会用到它;
  • 接下来,就是添加newSubscription,它属于Subscription类,里面包含着subscriber和subscriberMethod等信息,同时这里有一个优先级的判断,说明它是按照优先级添加的。优先级越高,会插到在当前List靠前面的位置;
  • typesBySubscriber这个类也是一个Map,key为subscriber,value为subscribedEvents,即所有的eventType列表,这个类我找了一下,发现在EventBus#isRegister()方法中有用到,应该是用来判断这个Subscriber是否已被注册过。然后将当前的eventType添加到subscribedEvents中;
  • 最后,判断是否是sticky。如果是sticky事件的话,到最后会调用checkPostStickyEventToSubscription()方法。

这里其实就是将所有含@Subscribe注解的订阅方法最终保存在subscriptionsByEventType中。

  1. EventBus#checkPostStickyEventToSubscription():
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    private void checkPostStickyEventToSubscription(Subscription newSubscription, Object stickyEvent) {
    if (stickyEvent != null) {
    // If the subscriber is trying to abort the event, it will fail (event is not tracked in posting state)
    // --> Strange corner case, which we don't take care of here.
    postToSubscription(newSubscription, stickyEvent, isMainThread());
    }
    }

接下来,我们重点看post()和postToSubscription()方法。post事件相当于把事件发送出去,我们看看订阅者是如何接收到事件的。

  1. EventBus#post():
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    /** Posts the given event to the event bus. */
    public void post(Object event) {
    // 5.1
    PostingThreadState postingState = currentPostingThreadState.get();
    List <Object> eventQueue = postingState.eventQueue;
    eventQueue.add(event);
    // 5.2
    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;
    }
    }
    }

5.1 代码段分析

  • currentPostingThreadState是一个ThreadLocal类型的,里面存储了PostingThreadState,而PostingThreadState中包含了一个eventQueue和其他一些标志位;
  • 然后把传入的event,保存到了当前线程中的一个变量PostingThreadState的eventQueue中。
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    private final ThreadLocal <PostingThreadState> currentPostingThreadState = new ThreadLocal <PostingThreadState> () {
    @Override
    protected PostingThreadState initialValue() {
    return new PostingThreadState();
    }
    };
    /** For ThreadLocal, much faster to set (and get multiple values). */
    final static class PostingThreadState {
    final List <Object> eventQueue = new ArrayList<>();
    boolean isPosting;
    boolean isMainThread;
    Subscription subscription;
    Object event;
    boolean canceled;
    }

5.2 代码段分析

  • 这里涉及到两个标志位,第一个是isMainThread,判断是否为UI线程;第二个是isPosting,作用是防止方法多次调用。
  • 最后调用到postSingleEvent()方法
  1. EventBus#postSingleEvent():
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    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));
    }
    }
    }
    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 = false;
    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;
    }
  • 这里会首先取出Event的class类型,然后有一个标志位eventInheritance判断,默认为true,作用在相关代码注释有说,如果设为true的话,它会拿到Event父类的class类型,设为false,可以在一定程度上提高性能;
  • 接下来是lookupAllEventTypes()方法,就是取出Event及其父类和接口的class列表,当然重复取的话会影响性能,所以它也有做一个eventTypesCache的缓存,这样不用都重复调用getClass()方法。
  • 然后是postSingleEventForEventType()方法,这里就很清晰了,就是直接根据Event类型从subscriptionsByEventType中取出对应的subscriptions,与之前的代码对应,最后调用postToSubscription()方法。
  1. EventBus#postToSubscription():
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    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);
    }
    }

这里会根据threadMode来判断应该在哪个线程中去执行方法:

  • POSTING:执行invokeSubscriber()方法,就是直接反射调用;
  • MAIN:首先去判断当前是否在UI线程,如果是的话则直接反射调用,否则调用mainThreadPoster#enqueue(),即把当前的方法加入到队列之中,然后通过handler去发送一个消息,在handler的handleMessage中去执行方法。具体逻辑在HandlerPoster.java中;
  • MAIN_ORDERED:与上面逻辑类似,顺序执行我们的方法;
  • BACKGROUND:判断当前是否在UI线程,如果不是的话直接反射调用,是的话通过backgroundPoster.enqueue()将方法加入到后台的一个队列,最后通过线程池去执行;
  • ASYNC:与BACKGROUND的逻辑类似,将任务加入到后台的一个队列,最终由Eventbus中的一个线程池去调用,这里的线程池与BACKGROUND逻辑中的线程池用的是同一个。

补充:BACKGROUND和ASYNC有什么区别呢?
BACKGROUND中的任务是一个接着一个的去调用,而ASYNC则会即时异步运行,具体的可以对比AsyncPoster.java和BackgroundPoster.java两者代码实现的区别。

到这里,我们就解决了第四个问题,事件的发送和接收,主要是通过subscriptionsByEventType这个非常重要的列表,我们将订阅即接收事件的方法存储在这个列表,发布事件的时候在列表中查询出相对应的方法并执行~

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