EventBus源码学习

栏目: IOS · Android · 发布时间: 5年前

内容简介：看了别人的源码文章，一开始有点懵，还要复习下反射和注解器的知识。然后自己也做了下笔记，加深印象。参考文章：

看了别人的源码文章，一开始有点懵，还要复习下反射和注解器的知识。然后自己也做了下笔记，加深印象。

参考文章：

blog.csdn.net/u012933743/…

www.jianshu.com/p/c4d106419… .

描述

EventBus是Android和 Java 的发布/订阅事件总线。

EventBus源码学习

优缺点：

  • 简化了组件之间的通信，将事件发布者和订阅者分离
  • 代码简单
  • 包体积小缺点：利用反射，性能可能会差一点。可以使用EventBus annotation processor（EventBus注解处理器），在编译期间创建了订阅者索引。（提升EventBus性能： greenrobot.org/eventbus/do…

三要素：

  • Event：自定义事件类型
  • Subscriber：事件订阅者。在EventBus3.0之前我们必须定义以onEvent开头的那几个方法，分别是onEvent、onEventMainThread、onEventBackgroundThread和onEventAsync，而在3.0之后事件处理的方法名可以随意取，不过需要加上注解@subscribe()，并且指定线程模型，默认是POSTING。
  • Publisher：事件发布者。可以在任意线程里发布事件，一般情况下，使用EventBus.getDefault()就可以得到一个EventBus对象，然后再调用post(Object)方法即可。

4个线程：

  • POSTING:在发布线程的事件执行
  • MAIN：在主线程执行。
  • BACKGROUND：如果发布事件的线程为主线程则新建一个线程执行，否则在发布事件的线程执行。
  • ASYNC：在新的线程执行

利用Handler切换到主线程。BACKGROUND线程，使用到了线程池ExecutorService。

说说EventBus里面用到的知识点

  • 注解处理器：

使用EventBus annotation processor（EventBus注解处理器），在编译期间创建了订阅者索引。（提升EventBus性能： greenrobot.org/eventbus/do… ）。可以看到，会在build包下面生成一个EventBus的索引类。SUBSCRIBER_INDEX是一个Map类型数据，存放着每一个类里面的注解信息

EventBus源码学习 
/** This class is generated by EventBus, do not edit. */
public class MyEventBusIndex implements SubscriberInfoIndex {
    private static final Map<Class<?>, SubscriberInfo> SUBSCRIBER_INDEX;

    static {
        SUBSCRIBER_INDEX = new HashMap<Class<?>, SubscriberInfo>();

        putIndex(new SimpleSubscriberInfo(com.example.camera_learning.eventbus.EventBusActivity.class, true,
                new SubscriberMethodInfo[] {
            new SubscriberMethodInfo("onEvent", com.example.camera_learning.eventbus.TestEvent.class),
            new SubscriberMethodInfo("onEvent", com.example.camera_learning.eventbus.TestEvent2.class),
        }));

    }

    private static void putIndex(SubscriberInfo info) {
        SUBSCRIBER_INDEX.put(info.getSubscriberClass(), info);
    }

    @Override
    public SubscriberInfo getSubscriberInfo(Class<?> subscriberClass) {
        SubscriberInfo info = SUBSCRIBER_INDEX.get(subscriberClass);
        if (info != null) {
            return info;
        } else {
            return null;
        }
    }
}

复制代码
  • 反射

如果索引类中有缓存相关的注解信息的话，那就会直接用索引类里面的注解信息。如果没有的话，就需要利用反射去获取类中的所有方法，然后拿到有@Subscribe的方法。

post方法最后需要利用invoke来调用订阅者对象的订阅方法（具体代码可以看下面）

  • 设计模式：单例模式、建造者模式、观察者模式
  • ThreadLocal：EventBus会通过ThreadLocal为每个线程维护一个PostingThreadState对象。
private final ThreadLocal<PostingThreadState> currentPostingThreadState = new ThreadLocal<PostingThreadState>() {
    @Override
    protected PostingThreadState initialValue() {
        return new PostingThreadState();
    }
};

final static class PostingThreadState {
    //分发队列
    final List<Object> eventQueue = new ArrayList<>();
    //是否正在分发
    boolean isPosting;
    //是否是主线程
    boolean isMainThread;
    //订阅关系
    Subscription subscription;
    //当前正在分发的事件
    Object event;
    //是否取消
    boolean canceled;
}
复制代码

EventBus里面的一些数据结构

比较重要的就是下面两个map集合。

//key为事件类型，value值为Subscription的集合。这样可以根据事件的类型，直接获取订阅该事件的所有类中的所有方法
private final Map<Class<?>, CopyOnWriteArrayList<Subscription>> subscriptionsByEventType;
//key为订阅对象，value为该类中的所有订阅事件类型的集合。
private final Map<Object, List<Class<?>>> typesBySubscriber;
//粘性事件
private final Map<Class<?>, Object> stickyEvents;

//Subscription类，表示一个订阅关系，包含订阅对象和订阅方法
final class Subscription {
    final Object subscriber;
    final SubscriberMethod subscriberMethod;
}
复制代码

阅读源码

getDefault()方法

  • 利用双重校验锁实现单例模式，保证线程安全
  • 使用Builder建造者模式，将一个复杂对象的构建和它的表示分离。建造者模式一般用来创建复杂对象，用户可以不用关心其建造过程和细节。在EventBus中，指的是EventBusBuilder类，可以使用自定义参数创建EventBus实例。
public static EventBus getDefault() {
    if (defaultInstance == null) {
        synchronized (EventBus.class) {
            if (defaultInstance == null) {
                defaultInstance = new EventBus();
            }
        }
    }
    return defaultInstance;
}

public EventBus() {
    this(DEFAULT_BUILDER);
}
EventBus(EventBusBuilder builder) {
    logger = builder.getLogger();
    subscriptionsByEventType = new HashMap<>();
    typesBySubscriber = new HashMap<>();
    stickyEvents = new ConcurrentHashMap<>();
    mainThreadSupport = builder.getMainThreadSupport();
    mainThreadPoster = mainThreadSupport != null ? mainThreadSupport.createPoster(this) : null;
    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;
}

public class EventBusBuilder {
    private final static ExecutorService DEFAULT_EXECUTOR_SERVICE = Executors.newCachedThreadPool();
    boolean logSubscriberExceptions = true;
    boolean logNoSubscriberMessages = true;
    boolean sendSubscriberExceptionEvent = true;
    boolean sendNoSubscriberEvent = true;
    boolean throwSubscriberException;
    boolean eventInheritance = true;
    boolean ignoreGeneratedIndex;
    boolean strictMethodVerification;
    ExecutorService executorService = DEFAULT_EXECUTOR_SERVICE;
    List<Class<?>> skipMethodVerificationForClasses;
    List<SubscriberInfoIndex> subscriberInfoIndexes;
    Logger logger;
    MainThreadSupport mainThreadSupport;
    EventBusBuilder() {
    }

 /** Builds an EventBus based on the current configuration. */
public EventBus build() {
    return new EventBus(this);
}
复制代码

register方法

register的最终目的是，将注解的信息保存到对应的map中。而获取的过程，优先通过注解器生成的注解信息获取，如果没有的话，再利用反射获取注解方法。

public void register(Object subscriber) {
    //拿到class对象
    Class<?> subscriberClass = subscriber.getClass();
    //利用SubscriberMethodFinder类来找出这个类中的所有订阅方法
    List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
    //拿到类中所有的订阅方法后，调用subscribe方法
    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);
        }
    }
}


复制代码

下面看一下SubscriberMethodFinder类。

//这个map对象，key值为订阅者类，value为该类中的所有订阅方法的list集合。已经register过的话，相应的数据已经在map中存在了，这个时候就可以直接从map中读取就行了，不用再次利用反射进行多余的遍历类中的方法。有点空间换时间的思想。
private static final Map<Class<?>, List<SubscriberMethod>> METHOD_CACHE = new ConcurrentHashMap<>();

//通过这个方法，遍历出某个类中的所有订阅方法
List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
    //根据缓存值，判断是否注册过EventBus
    List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);
    if (subscriberMethods != null) {
        return subscriberMethods;
    }
    //默认为false，所以一般是先走findUsingInfo方法，通过自动生成的索引类比如MyEventBusIndex来获取到被注解的办法。这种办法比通过findUsingReflection（）利用反射去获取方法的性能好一点。如果没有获取到索引中存储的订阅者Subscriber信息的话，才去走利用反射的办法。
    if (ignoreGeneratedIndex) {
        subscriberMethods = findUsingReflection(subscriberClass);
    } else {
        subscriberMethods = findUsingInfo(subscriberClass);
    }
    //如果订阅者类和父类中，没有@Subscribe利用监听事件的方法，就会抛这个错误。
    if (subscriberMethods.isEmpty()) {
        throw new EventBusException("Subscriber " + subscriberClass
                + " and its super classes have no public methods with the @Subscribe annotation");
    } else {
        //获取到订阅者类中的所有注解方法后，保存到cache中
        METHOD_CACHE.put(subscriberClass, subscriberMethods);
        return subscriberMethods;
    }
}

//利用注解处理器生成的索引类，来获取类中的订阅方法
private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
    FindState findState = prepareFindState();
    findState.initForSubscriber(subscriberClass);
    while (findState.clazz != null) {
        //在生成的索引类中，获取保存的订阅信息
        findState.subscriberInfo = getSubscriberInfo(findState);
        if (findState.subscriberInfo != null) {
            //获取拿到这个订阅类中的所有的订阅方法
            SubscriberMethod[] array = findState.subscriberInfo.getSubscriberMethods();
            //遍历，检查后，添加到list集合
            for (SubscriberMethod subscriberMethod : array) {
                if (findState.checkAdd(subscriberMethod.method, subscriberMethod.eventType)) {
                    findState.subscriberMethods.add(subscriberMethod);
                }
            }
        } else {
            //如果在索引类中找不到订阅信息，那就通过反射的办法来获取
            findUsingReflectionInSingleClass(findState);
        }
        findState.moveToSuperclass();
    }
    return getMethodsAndRelease(findState);
}

//在生成的索引类中，获取保存的订阅信息
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;
        }
    }
    
    //如果subscriberInfoIndexes为空，也就是没有利用生成索引的办法来提高EventBus性能，直接返回null
    //不为空的话，遍历，拿到这个订阅类中的所有注解方法
    if (subscriberInfoIndexes != null) {
        for (SubscriberInfoIndex index : subscriberInfoIndexes) {
            SubscriberInfo info = index.getSubscriberInfo(findState.clazz);
            if (info != null) {
                return info;
            }
        }
    }
    return null;
}


//使用subscribe方法
private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
    //订阅方法的事件类型
    Class<?> eventType = subscriberMethod.eventType;
    //创建一个Subscription，包含订阅类对象和订阅方法
    Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
    //获取到这种事件类型的list集合
    CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
    
    if (subscriptions == null) {
        //保存到subscriptionsByEventType里面，key为事件类型，value为Subscription
        subscriptions = new CopyOnWriteArrayList<>();
        subscriptionsByEventType.put(eventType, subscriptions);
    } else {
        //处理重复注册的情况
        if (subscriptions.contains(newSubscription)) {
            throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event "
                    + eventType);
        }
    }
    //这部分的作用，是根据事件优先级进行排序
    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;
        }
    }
    //根据订阅对象，获取订阅的所有的事件类型
    List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
    if (subscribedEvents == null) {
        subscribedEvents = new ArrayList<>();
        typesBySubscriber.put(subscriber, subscribedEvents);
    }
    subscribedEvents.add(eventType);
    
    //处理粘性事件
    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();
                    checkPostStickyEventToSubscription(newSubscription, stickyEvent);
                }
            }
        } else {
            Object stickyEvent = stickyEvents.get(eventType);
            checkPostStickyEventToSubscription(newSubscription, stickyEvent);
        }
    }
}




复制代码

post方法

post方法最后会走到postSingleEventForEventType方法,拿到注册这个事件的订阅者信息，利用反射调用订阅方法的执行。

public void post(Object event) {
    //获取当前线程的PostingThreadState对象
    PostingThreadState postingState = currentPostingThreadState.get();
    //获取当前的分发队列
    List<Object> eventQueue = postingState.eventQueue;
    //将事件添加到队列中
    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 {
            //当事件队列不为空的时候，一个一个进行分发
            while (!eventQueue.isEmpty()) {
                postSingleEvent(eventQueue.remove(0), postingState);
            }
        } finally {
            postingState.isPosting = false;
            postingState.isMainThread = false;
        }
    }
}

复制代码
private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass) {
    CopyOnWriteArrayList<Subscription> subscriptions;
    synchronized (this) {
        //去map集合中，根据事件类型，获取到所有的订阅方法
        subscriptions = subscriptionsByEventType.get(eventClass);
    }
    if (subscriptions != null && !subscriptions.isEmpty()) {
        //遍历订阅该事件的list集合
        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;
}

//根据线程进行分发事件
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);
    }
}


//最后是利用反射，调用订阅者对象的订阅方法
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);
    }
}



复制代码

unregister方法

unregister方法就是要把这个类中的注解信息都给移除掉。

public synchronized void unregister(Object subscriber) {
    //获取这个订阅者种的所有订阅事件类型list
    List<Class<?>> subscribedTypes = typesBySubscriber.get(subscriber);
    if (subscribedTypes != null) {
        for (Class<?> eventType : subscribedTypes) {
            //遍历事件类型list，移除subscriptionsByEventType中包含这个订阅者的所有订阅关系
            unsubscribeByEventType(subscriber, eventType);
        }
        //移除这个订阅对象
        typesBySubscriber.remove(subscriber);
    } else {
        logger.log(Level.WARNING, "Subscriber to unregister was not registered before: " + subscriber.getClass());
    }
}

//移除subscriptionsByEventType中包含这个订阅者的所有订阅关系
private void unsubscribeByEventType(Object subscriber, Class<?> eventType) {
    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);
            if (subscription.subscriber == subscriber) {
                subscription.active = false;
                subscriptions.remove(i);
                i--;
                size--;
            }
        }
    }
}

复制代码

以上所述就是小编给大家介绍的《EventBus源码学习》，希望对大家有所帮助，如果大家有任何疑问请给我留言，小编会及时回复大家的。在此也非常感谢大家对 码农网 的支持！

查看所有标签

猜你喜欢:

本站部分资源来源于网络，本站转载出于传递更多信息之目的，版权归原作者或者来源机构所有，如转载稿涉及版权问题，请联系我们。

码农书籍
Linux内核设计的艺术

Linux内核设计的艺术

新设计团队 / 机械工业出版社华章公司 / 2011-6-20 / 79.00元

关于Linux内核的书已经不计其数，但这本书却是独树一帜的，它的内容代表着Linux内核研究成果的世界顶尖级水平，它在世界范围内首次提出并阐述了操作系统设计的核心指导思想——主奴机制，这是所有操作系统研究者的一笔宝贵财富。本书可能也代表着同类图书的顶尖水平，是一本真正能引导我们较为容易地、极为透彻地理解Linux内核的经典之作，也可能是当前唯一能从本质上指引我们去设计和开发拥有自主知识产权的操作系......一起来看看 《Linux内核设计的艺术》 这本书的介绍吧!

码农工具
RGB转16进制工具
RGB转16进制工具

RGB HEX 互转工具

HTML 编码/解码
HTML 编码/解码

HTML 编码/解码

XML、JSON 在线转换
XML、JSON 在线转换

在线XML、JSON转换工具

  • New
  • 文章
  • 话题
  • 教程
    • 关注 码农网 公众号
    版权所有，保留一切权利！© 2018-2024 码农网 粤ICP备17054400号-3