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浅谈EventBus的使用原理

EventBus这是一个目前被广泛使用的,用于在不同的界面(Activity/Fragment)之间传递数据的三方库,其简单的使用深受广大开发者喜欢。

相比起Bundle或者接口回调,EventBus使用起来更加简单快捷,但有一点需要注意,再使用EventBus的时候,你需要对自己业务中的通知分发有很清晰的了解,不然很容易导致分发过多的无用通知,导致性能的消耗.

本文会对EventBus做一个简单的介绍

简单的使用

1.oncreate()中注册

EventBus.getDefault().register(this);
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2.onDestroy()销毁

EventBus.getDefault().unregister(this);
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3.使用@Subscribe注解方法,用来接收事件通知

@Subscribe
public void onMainEvent(String str){
    System.out.println("event = "+str);
}
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4.发送通知

EventBus.getDefault().post("12456");
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上述4个步骤就能完成一次简单的事件分发,这也是EventBus被广泛使用的部分原因。

源码解读

在深入源码之前先解释几个主要的成员

Subscription.class (订阅信息)

final class Subscription {
	//订阅者对象,一般情况下多为activity/fragment等
    final Object subscriber;
	//订阅者方法对象,主要包括订阅的
    final SubscriberMethod subscriberMethod;
}
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SubscriberMethod.class (订阅者方法对象)

public class SubscriberMethod {
	//方法
    final Method method;
	//线程模式
    final ThreadMode threadMode;
	//事件类型
    final Class<?> eventType;
	//优先级
    final int priority;
	//是否是粘性事件
    final boolean sticky;
}
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FindState.class (查找状态,主要用来保存查找过程中的变量和结果)

 static class FindState {
    final List<SubscriberMethod> subscriberMethods = new ArrayList<>();
	//事件type为key,method为value
    final Map<Class, Object> anyMethodByEventType = new HashMap<>();
	//method为key,订阅者class对象为value
    final Map<String, Class> subscriberClassByMethodKey = new HashMap<>();
    final StringBuilder methodKeyBuilder = new StringBuilder(128);
}
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另外还有几个比较重要的Map对象

  1. subscriptionsByEventType type为key,CopyOnWriteArrayList<Subscription>为value,从这里可以看出EventBus中一个type类型可以对应很多个订阅者。
  2. typesBySubscriber 则刚好相反,Subscription为key,Subscription内的所有事件type为value
  3. METHOD_CACHE class类对象为key,List为value,注意这里的是SubscriberMethod,而上述的是 Subscription, 其中值得注意的是METHOD_CACHE采用的是ConcurrentHashMap()这个数据模型,对于多并发做了一定的优化。

接下来我们以前看看这个强大的函数库的内部实现原理。

register()

public void register(Object subscriber) {
    Class<?> subscriberClass = subscriber.getClass();
    List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
    synchronized (this) {
        for (SubscriberMethod subscriberMethod : subscriberMethods) {
            subscribe(subscriber, subscriberMethod);
        }
    }
}
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register过程的主要工作
1.获取订阅者的类名 (比如MainActivity)
2.通过findSubscriberMethods方法查找订阅者的订阅方法 (@Subscribe注解的并且是Public修饰的)
2.1 查找订阅方法

List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
    List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
    if (subscriberMethods != null) {
        return subscriberMethods;
    }
	//ignoreGeneratedIndex属性表示是否忽略注解器生成的MyEventBusIndex,默认情况下为false
    if (ignoreGeneratedIndex) {
        subscriberMethods = findUsingReflection(subscriberClass);
    } else {
        subscriberMethods = findUsingInfo(subscriberClass);
    }
	//如果订阅者类中没有被 @Subscribe且public声明的方法就会报异常
    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;
    }
}
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findUsingInfo()

 List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
    FindState findState = prepareFindState();
    findState.initForSubscriber(subscriberClass);
    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 {
            findUsingReflectionInSingleClass(findState);
        }
        findState.moveToSuperclass();
    }
    return getMethodsAndRelease(findState);
}
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其中以线程池的形式获取FindState对象,并初始化Subscriber订阅者对象

FindState findState = prepareFindState();
findState.initForSubscriber(subscriberClass);
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正常情况下第一次使用if (findState.subscriberInfo != null)这个判断会为false,接下来进入findUsingReflectionInSingleClass(findState);流程

另外需要注意的是每一次的循环都会调用findState.moveToSuperclass()检索父类的方法,所以对于一个订阅者来说,子类和父类中的方法都会被检索到,顺序是子类->父类

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) {
        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");
        }
    }
}
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  1. 通过放射获取所有的类中所有的方法数
  2. 过滤方法 1.public修饰 2.非静态,非抽象 3.一个参数 4.函数拥有@Subscribe的注解
  3. findState.checkAdd(method, eventType)校验是否可以加入到list中

findState.checkAdd(method, eventType)这里分两种情况

  1. anyMethodByEventType()中没有的直接返回true
  2. anyMethodByEventType()中有的,做2次校验,

这次根据 方法名>参数名进行完整校验,因为同一个类中同名同参的函数是不存在的,而同名不同参的在前一步已经被过滤了,所以这里确保在一个类中不会重复注册.

但如果子类重写父类的方法的话,就会出现相同的methodKey。这时EventBus会做一次验证,并保留子类的订阅信息。由于扫描是由子类向父类的顺序,故此时应当保留methodClassOld而忽略methodClass。如果代码上的注释 Revert the put

if (methodClassOld == null || methodClassOld.isAssignableFrom(methodClass)) {
    // Only add if not already found in a sub class
    return true;
} else {
    // Revert the put, old class is further down the class hierarchy
    subscriberClassByMethodKey.put(methodKey, methodClassOld);
    return false;
}
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checkAdd() 之后

ThreadMode threadMode = subscribeAnnotation.threadMode();
findState.subscriberMethods.add(new SubscriberMethod(method, eventType, threadMode,
subscribeAnnotation.priority(), subscribeAnnotation.sticky()));
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添加到findState中,以一个arrayList保存

接下来回到 getMethodsAndRelease()方法中,return 一个方法集合 List

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;
}
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3.回到开头的 register()方法中

for (SubscriberMethod subscriberMethod : subscriberMethods) {
    subscribe(subscriber, subscriberMethod);
}
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相对而言注册流程还是比较简单的,主要是让最开始提到的两个map(subscriptionsByEventType和typesBySubscriber)里面填加数据,留着事件分发时候使用。最多加一些优先级/粘性事件的判断。

 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);
        }
    }
}	
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post流程

同样先介绍几个重要的类

PostingThreadState.class (事件分发状态)

final static class PostingThreadState {
	//事件队列
    final List<Object> eventQueue = new ArrayList<>();
    //是否正在分发,防止并发状态下同一个事件发出多次
	boolean isPosting;
	//是否在主线程
    boolean isMainThread;
    Subscription subscription;
    Object event;
    boolean canceled;
}
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ThreadLocal.class (线程数据的存储类)
在指定线程存储数据,数据存储之后,只有在指定线程才能获取到之前存储过的数据。

post流程

public void post(Object event) {
    PostingThreadState postingState = currentPostingThreadState.get();
    List<Object> eventQueue = postingState.eventQueue;
    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()) {
                postSingleEvent(eventQueue.remove(0), postingState);
            }
        } finally {
            postingState.isPosting = false;
            postingState.isMainThread = false;
        }
    }
}
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核心部分只有下面这句,遍历发送eventQueue队列中的event,并且移除已经发送的event

postSingleEvent(eventQueue.remove(0), postingState);
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postSingleEvent.class

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));
        }
    }
}
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eventInheritance变量默认情况下被赋值为true,其中lookupAllEventTypes函数会遍历eventclass,得到其父类和接口的class类,事件派发的核心部分在postSingleEventForEventType()

postSingleEventForEventType.class

 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;
}
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这个函数的逻辑也是十分简单,根据eventClass从subscriptionsByEventType列表中获取订阅者列表,接着遍历订阅者列表,以此将event回调。读了这里时,发现将eventClass作为key,而不是event作为key,估计是因为class对象能追溯到其父类和接口实现吧。

到此,post流程也结束了,比起注册流程还要简单。

线程调度 postToSubscription()

几种poster类型

1.mainThreadPoster 创建于HandlerPoster

HandlerPoster.class

主要变量
//队列
private final PendingPostQueue queue;
//最大存在秒数 通常为10s,超过则会报错,这就跟广播的onReciver回调10s没处理完成就会报ANR错误有些类似
private final int maxMillisInsideHandleMessage;
private final EventBus eventBus;
//标志是否活动可用的
private boolean handlerActive;
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核心逻辑 handleMessage

@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) {
                    // Check again, this time in synchronized
                    pendingPost = queue.poll();
                    if (pendingPost == null) {
                        handlerActive = false;
                        return;
                    }
                }
            }
            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;
    }
}
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遍历队列,执行eventBus.invokeSubscriber(pendingPost)方法

void invokeSubscriber(Subscription subscription, Object event) {
    try {
		//通过反射的方式,直接调用订阅该事件方法。
        subscription.subscriberMethod.method.invoke(subscription.subscriber, event);
    } catch (InvocationTargetException e) {
        handleSubscriberException(subscription, event, e.getCause());
    } catch (IllegalAccessException e) {
        throw new IllegalStateException("Unexpected exception", e);
    }
}
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**2.BackgroundPoster:**实现 Runnable,会将PendingPostQueue队列中所有的事件分发出去

**3.AsyncPoster:**同样实现 Runnable,只会分发一个PendingPostQueue队列中的事件

postToSubscription.class

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);
    }
}
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逻辑也很好理解,相同线程时直接invokeSubscriber反射回调,不同线程则发到相同线程去回调。

unregister流程

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());
    }
}
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主要工作分两步

  1. 移除订阅者的所有订阅信息
  2. 移除订阅者和其所有Event的关联

另外看过的一篇博文上写了有关于EventBus的优化操作,记录下来

  1. eventInheritance默认为true,会遍历class对应的父类和接口对象,如果程序中没有使用到事件的继承关系,可以将该值设为false
  2. ignoreGeneratedIndex表示是否使用生成的APT代码去优化查找事件的过程,如果项目中没有接入EventBus的APT,也可以将其设置为false

补充

hashMap.put()

hashMap.put() 会根据key返回对应的value值
如果put的时候没有对应的key值,则添加到map中,如果有 先返回后添加

map.put("222", "222");
String value2=map.put("222","333");
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此时的value2为"222",而map中 key="222"的value为"333"

instanceof, isinstance,isAssignableFrom的区别

instanceof运算符 只被用于对象引用变量,检查左边的被测试对象 是不是 右边类或接口的 实例化。如果被测对象是null值,则测试结果总是false。 形象地:自身实例或子类实例 instanceof 自身类 返回true

  String s=new String("javaisland"); 
  System.out.println(s instanceof String); //true 
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Class类的isInstance(Object obj)方法,obj是被测试的对象,如果obj是调用这个方法的class或接口 的实例,则返回true。这个方法是instanceof运算符的动态等价。 形象地:自身类.class.isInstance(自身实例或子类实例) 返回true

  String s=new String("javaisland"); 
  System.out.println(String.class.isInstance(s)); //true 
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Class类的isAssignableFrom(Class cls)方法,如果调用这个方法的class或接口 与 参数cls表示的类或接口相同,或者是参数cls表示的类或接口的父类,则返回true。 形象地:自身类.class.isAssignableFrom(自身类或子类.class) 返回true 例:

System.out.println(ArrayList.class.isAssignableFrom(Object.class));  //false 
System.out.println(Object.class.isAssignableFrom(ArrayList.class));  //true
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参考文章

EventBus源码分析 EventBus

另外

个人的github
闲暇之余写的故事

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