一文分析EventBus-事件总线的使用方法和实现原理

2023-10-22 17:42:46

前言

本文主要从源码的角度来分析事件总线 EventBus 的实现原理, EventBus 是常用的消息传递的方式之一，其他常见的消息传递的方式还包括 Handler、BroadcastReceiver、Listener。通过本篇你在掌握 EventBus 基本使用的基础之上，能够掌握 EventBus 的实现原理。下面的框架图可以清晰的看到这一点。

一文分析EventBus-事件总线的使用方法和实现原理

一、定义事件类

作为事件的发布者，需要定义所发布的事件的类：

public class MessageEvent {
	private String msg;
	public MessageEvent(String msg) {
    this.msg = msg;
	}

	public String getMsg() {
    	return msg;
	}

	public void setMsg(String msg) {
    	this.msg = msg;
	}
}

二、注册/取消注册响应事件

作为事件的订阅者，需要把响应事件的对象注册到EventBus当中:EventBus.getDefault().register(obj)

当不需要处理某个类型的事件的时候，取消对这个事件的监听:EventBus.getDefault().unregister(obj)

三、声明和注释订阅方法，选择指定线程模式

作为事件的订阅者，需要定义事件的响应方法，方法名称可以随意取，方法的形参类型，必须和监听的事件对象类型一致：

@Subscribe(threadMode = ThreadMode.MAIN)
public void onMessageEvent(MessageEvent event) {
	Toast.makeText(this, event.getMsg (),
	Toast.LENGTH_SHORT).show();
}

3.1 四种线程模式

事件订阅者可以通过注解的方式选择处理事件的方法所在的线程：

  • PostThread： 如果事件处理函数指定了线程模型为PostThread，那么事件的发布和接收处理会在同一个线程当中。

  • BackgroundThread： 如果事件处理函数指定了线程模型为BackgroundThread，那么如果事件是在UI线程中发布出来的，那么该事件处理函数就会在新的子线程中运行，如果事件发布本来就是非UI线程中发布出来 的，那么该事件处理函数直接在发布事件的线程中执行。

  • MainThread: 如果事件处理函数指定了线程模型为MainThread，那么不论事件对象是在哪个线程中发布出来的，该事件处理函数都会在UI线程中执行。

  • Async： 如果事件处理函数指定了线程模型为Async，那么无论事件在哪个线程发布，该事件处理函数都会在新建的子线程中执行。

3.2 黏性事件

通过注解的方式设置sticky为true，那么事件处理函数则可以处理上一次的事件对象：

@Subscribe(sticky = true, threadMode = ThreadMode.MAIN)

四、EventBus 3.0源码详解

4.1 注册流程

/**
* 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) {
//通过注册的对象得到其类的class对象
Class<?> subscriberClass = subscriber.getClass();
//通过类的class对象得到此对象的订阅方法列表
List<SubscriberMethod> subscriberMethods = 	subscriberMethodFinder.findSubscriberMethods(subscriberClass);
	synchronized (this) {
    	for (SubscriberMethod subscriberMethod :subscriberMethods) {
        	//线程同步，遍历订阅方法列表，注册每一个订阅方法
        	subscribe(subscriber, subscriberMethod);
    	}
	}
}

代码subscriberMethodFinder.findSubscriberMethods(subscriberClass)获取订阅方法列表具体如下：

List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
	//在缓存中查找此class对象对应的订阅方法列表
  	List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
  	if (subscriberMethods != null) {
      return subscriberMethods;
  	}
  //是否忽略注解器生成的MyEventBusIndex类
  if (ignoreGeneratedIndex) {
    	//通过反射机制得到订阅者类class对象对应的订阅事件方法列表
    	subscriberMethods = findUsingReflection(subscriberClass);
  } else {
  		//从注解器生成的MyEventBusIndex类中获得订阅类的订阅方法列表
    	subscriberMethods = findUsingInfo(subscriberClass);
	}
  if (subscriberMethods.isEmpty()) {
      throw new EventBusException("Subscriber " + subscriberClass
              + " and its super classes have no public methods with the @Subscribe annotation");
  } else {
      //缓存此class对象的订阅方法列表
      METHOD_CACHE.put(subscriberClass, subscriberMethods);
      return subscriberMethods;
  }
}

通过反射机制获取订阅方法列表：

private List<SubscriberMethod> findUsingReflection(Class<?> subscriberClass) {
  FindState findState = prepareFindState();
  findState.initForSubscriber(subscriberClass);
  while (findState.clazz != null) {
      //遍历当前class对象和其父类中的订阅方法
      findUsingReflectionInSingleClass(findState);
      findState.moveToSuperclass();
  }
  return getMethodsAndRelease(findState);
}

findUsingReflectionInSingleClass方法:

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;
	}
	//遍历此类的方法
	for (Method method : methods) {
    	int modifiers = method.getModifiers();
    	if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
        	Class<?>[] parameterTypes = method.getParameterTypes();
        	//形参只有一个的函数
        	if (parameterTypes.length == 1) { 
        		//得到此函数的注解信息对象
        		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");
    	}
	}
}

至此，我们得到了所有的订阅函数列表，下一步，会对每一个订阅函数进行注册：

// Must be called in synchronized block
private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
	Class<?> eventType = subscriberMethod.eventType;
	Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
	//通过监听的事件对象得到其对应的监听者对象列表
	CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
	if (subscriptions == null) {
    	subscriptions = new CopyOnWriteArrayList<>();
    	subscriptionsByEventType.put(eventType, subscriptions);
	} else {
    	//如果这个对象已经在此事件对象的监听者列表当中，则抛出异常
    	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++) {
    	//遍历监听者列表，根据订阅方法的优先级将新的监听对象插入到列表的指定位置，即订阅方法的注册
    	if (i == size || subscriberMethod.priority > subscriptions.get(i).subscriberMethod.priority) {
        	subscriptions.add(i, newSubscription);
        	break;
    	}
	}
	//通过订阅者对象得到其对应的监听事件类型列表
	List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
	if (subscribedEvents == null) {
    	subscribedEvents = new ArrayList<>();
    	typesBySubscriber.put(subscriber, subscribedEvents);
	}
	//添加事件类型对象到此监听对象对应的事件对象列表当中
	subscribedEvents.add(eventType);
	//在这里处理黏性事件
	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();
                //将黏性事件发送到指定的订阅方法当中处理
                checkPostStickyEventToSubscription(newSubscription, stickyEvent);
            }
        }
    } else {
        Object stickyEvent = stickyEvents.get(eventType);
        //将黏性事件发送到指定的订阅方法当中处理　
        checkPostStickyEventToSubscription(newSubscription, stickyEvent);
    }
}

事件的注册流程实际是从监听者对象和消息事件两个维度，将对方分别添加到自己对应的列表当中，具体可以通过以下流程图总结：

一文分析EventBus-事件总线的使用方法和实现原理

4.2发布流程

/** Posts the given event to the event bus. */
public void post(Object event) {
	//通过ThreadLocal机制得到当前线程的postingState对象
	PostingThreadState postingState = currentPostingThreadState.get();
	List<Object> eventQueue = postingState.eventQueue;
	//在此线程的eventQueue中添加此事件对象
	eventQueue.add(event);
	if (!postingState.isPosting) {
    	//判断当前线程是否UI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 {
        while (!eventQueue.isEmpty()) {
            //遍历此线程消息队列，处理消息队列中的消息事件
            postSingleEvent(eventQueue.remove(0), postingState);
        }
    	} finally {
        postingState.isPosting = false;
        postingState.isMainThread = false;
    	}
	}
}

ThreadLocal机制可以存储各个线程的局部数据；postSingleEvent函数处理此线程消息队列中的消息事件：

private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
		Class<?> eventClass = event.getClass();
boolean subscriptionFound = false;
	if (eventInheritance) {
    	//得到此事件类的所有父类和接口
    	List<Class<?>> eventTypes = lookupAllEventTypes(eventClass);
    	int countTypes = eventTypes.size();
    	for (int h = 0; h < countTypes; h++) {
        	Class<?> clazz = eventTypes.get(h);
        	//通过事件类得到其对应的订阅者对象列表，将事件对象分发到相应的订阅函数中处理，至此实现了事件消息的传递
        	subscriptionFound |= postSingleEventForEventType(event, postingState, clazz);
    	}
	} else {
    	//通过事件类得到其对应的订阅者对象列表，将事件对象分发到相应的订阅函数中处理，至此实现了事件消息的传递
    	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));
    	}
	}
}

事件消息对象具体的分发函数：postToSubscription

private void postToSubscription(Subscription subscription, Object event, boolean isMainThread) {
	//根据注解方式设置的线程模式，在不同的线程中执行订阅函数
	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);
	}
}

至此，我们完成了事件消息对象的分发流程，以下流程图来总结post的过程：

一文分析EventBus-事件总线的使用方法和实现原理

4.3 取消注册流程

/** Unregisters the given subscriber from all event classes. */
public synchronized void unregister(Object subscriber) {
	//根据订阅者对象得到其对应的事件类型列表
	List<Class<?>> subscribedTypes = typesBySubscriber.get(subscriber);
	if (subscribedTypes != null) {
    	for (Class<?> eventType : subscribedTypes) {
        	//遍历事件类型列表，得到每个事件类型对应的订阅者对象列表，遍历这个列表，如果是此观察者对象，则从列表中删除
        	unsubscribeByEventType(subscriber, eventType);
    	}
    	//typesBySubscriber中删除此订阅者对象
    	typesBySubscriber.remove(subscriber);
	} else {
    	Log.w(TAG, "Subscriber to unregister was not registered before: " + subscriber.getClass());
	}
}

unsubscribeByEventType函数：

/** Only updates subscriptionsByEventType, not typesBySubscriber! Caller must update typesBySubscriber. */
private void unsubscribeByEventType(Object subscriber, Class<?> eventType) {
	//根据事件类型得到其对应的订阅者对象列表
	List<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
	if (subscriptions != null) {
    	int size = subscriptions.size();
    	for (int i = 0; i < size; i++) {
        	Subscription subscription = subscriptions.get(i);
        	//遍历列表，如果找到此订阅者对象则从列表中删除
        	if (subscription.subscriber == subscriber) {
            	subscription.active = false;
            	subscriptions.remove(i);
            	i--;
            	size--;
        	}
    	}
	}
}

取消注册的流程总结如下：

  1. 通过观察者类对象通过MAP表得到其对应的事件类class对象列表.

  2. 遍历list列表，通过事件类class对象得到其在MAP表中对应的观察者类对象列表。

  3. 遍历此观察者对象列表，判断如果列表中存在需要取消的注册观察者对象，则从对象列表中删除此观察者对象。

  4. 从第1步中得到的MAP对象中删除以取消注册的观察者对象为key的映射项目。

  5. 完成unregister过程。

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