Android EventBus基础入门及源码分析

—— 迷茫是什么？迷茫是大事干不了，小事不想干。能力配不上欲望，才华配不上梦想。

前言

时隔多年，那些曾经学过且用过的知识早已记忆模糊。如果不反复研究学习，使用起来也会很生涩，如新知识一样。本编为巩固EventBus所写。一个人为什么要努力，因为喜欢的东西很贵想去的地方都很远，想爱的人很完美。

一、简介

官方文档：https://greenrobot.org/eventbus/documentation/

Github：https://github.com/greenrobot/EventBus

（1）是什么：是一个事件发布/订阅的轻量级框架。基于观察者模式，实现组件间的通讯。代码简洁且解耦。

（2）有什么用：可以替代传统的Intent,Handler,Broadcast或接口函数。

​二、基本使用

（1）添加依赖 （不是最新） 基于以前学过的版本

implementation 'org.greenrobot:eventbus:3.0.0'

（2）定义消息事件（可以配置传递参数）

public static class MessageEvent { /* Additional fields if needed */ }

（3）定义接收事件的线程方法（发送的事件，将在该方法中收到）

@Subscribe(threadMode = ThreadMode.MAIN)  
public void onMessageEvent(MessageEvent event) {/* Do something */};

（4）EventBus初始化 （与广播相似，需要订阅与取消）

 @Override
 public void onStart() {
     super.onStart();
     EventBus.getDefault().register(this);
 }

 @Override
 public void onStop() {
     super.onStop();
     EventBus.getDefault().unregister(this);
 }

（5）发送事件

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

（6）添加混淆

-keepattributes *Annotation*
-keepclassmembers class * {
    @org.greenrobot.eventbus.Subscribe <methods>;
}
-keep enum org.greenrobot.eventbus.ThreadMode { *; }

# Only required if you use AsyncExecutor
-keepclassmembers class * extends org.greenrobot.eventbus.util.ThrowableFailureEvent {
    <init>(java.lang.Throwable);
}

简单的整理了一下 。正确姿势参考官方文档。

三、源码分析

（1）EventBus.getDefault()

* 单例模式 双重效验锁  线程安全 懒加载
public static EventBus getDefault() {
   if (defaultInstance == null) {
       synchronized (EventBus.class) {
          if (defaultInstance == null) {
              defaultInstance = new EventBus();
          }
       }
    }
    return defaultInstance;
}

* 使用构建者配置EventBus 属性
    private static final EventBusBuilder DEFAULT_BUILDER = new EventBusBuilder();
    public EventBus() {
        this(DEFAULT_BUILDER);
    }

    * 属性简介
   EventBus(EventBusBuilder builder) {
        * 保存Event集合
        subscriptionsByEventType = new HashMap<>();
        typesBySubscriber = new HashMap<>();
        stickyEvents = new ConcurrentHashMap<>();
        * 线程调度
        mainThreadPoster = new HandlerPoster(this, Looper.getMainLooper(), 10);
        backgroundPoster = new BackgroundPoster(this);
        asyncPoster = new AsyncPoster(this);
        * 索引
        indexCount = builder.subscriberInfoIndexes != null ? builder.subscriberInfoIndexes.size() : 0;
        * EventBus订阅方法
        subscriberMethodFinder = new SubscriberMethodFinder(builder.subscriberInfoIndexes,
                builder.strictMethodVerification, builder.ignoreGeneratedIndex);
        * EventBus日志
        logSubscriberExceptions = builder.logSubscriberExceptions;
        logNoSubscriberMessages = builder.logNoSubscriberMessages;
        sendSubscriberExceptionEvent = builder.sendSubscriberExceptionEvent;
        * 无消息发送
        sendNoSubscriberEvent = builder.sendNoSubscriberEvent;
        * 异常事件
        throwSubscriberException = builder.throwSubscriberException;
        * EventBus继承关系
        eventInheritance = builder.eventInheritance;
        * 线程池
        executorService = builder.executorService;
    }

（2）EventBus.getDefault().register(this)

* 注册给定的订阅方以接收事件
public void register(Object subscriber) {
    * 利用反射获取订阅的类
    Class subscriberClass = subscriber.getClass();
    * 根据订阅的类找到 该类下的订阅方法   -> 1
    List subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
    synchronized (this) {
        * 便利所有的订阅方法
        for (SubscriberMethod subscriberMethod : subscriberMethods) {
                        * --> 2
            subscribe(subscriber, subscriberMethod);
        }
    }
}

1.subscriberMethodFinder.findSubscriberMethods(subscriberClass)

* 获取所有的订阅方法
List findSubscriberMethods(Class subscriberClass) {
    * 判断是否已缓存
    List subscriberMethods = METHOD_CACHE.get(subscriberClass);
    if (subscriberMethods != null) {
        return subscriberMethods;
    }
    * ignoreGeneratedIndex 忽略生成的索引 默认为false
    if (ignoreGeneratedIndex) {
        subscriberMethods = findUsingReflection(subscriberClass);
    } else {
        * 通过反射获取到订阅方法列表 -> 1.1
        subscriberMethods = findUsingInfo(subscriberClass);
    }
    * 订阅方法列表为空时 抛出异常
    if (subscriberMethods.isEmpty()) {
        throw new EventBusException("Subscriber" + subscriberClass
                + " and its super classes have no public methods with the @Subscribe annotation");
    } else {
        * 缓存该订阅类的所有订阅方法
        METHOD_CACHE.put(subscriberClass, subscriberMethods);
        return subscriberMethods;
    }
}

1.1.findUsingInfo(Class subscriberClass)

* 通过反射获取到订阅方法列表
private List findUsingInfo(Class subscriberClass) {
    * 创建FindState 并初始化
    FindState findState = prepareFindState();
    findState.initForSubscriber(subscriberClass);
    while (findState.clazz != null) {
        * 判断findState是否已经有缓存订阅信息
        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 {
            * 利用反射机制 将订阅方法信息 存储在findState 中
            findUsingReflectionInSingleClass(findState);
        }
        * 移除订阅类
        findState.moveToSuperclass();
    }
    * 回收FindState对象，获取订阅方法列表
    return getMethodsAndRelease(findState);
}

2.subscribe(Object subscriber, SubscriberMethod subscriberMethod)

* 判断是否已经注册/订阅过该事件
* 按照优先级缓存订阅事件
* 判断是否已经缓存在typesBySubscriber中 
* 判断是否是粘性事件  并分发粘性事件

private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
    * 订阅方法类型
    Class< eventType = subscriberMethod.eventType;
    * 创建 订阅事件
    Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
    * 判断是否缓存过 订阅事件 列表
    CopyOnWriteArrayList subscriptions = subscriptionsByEventType.get(eventType);
    if (subscriptions == null) {
        subscriptions = new CopyOnWriteArrayList<>();
        subscriptionsByEventType.put(eventType, subscriptions);
    } else {
        * 判断是否已经订阅过  订阅过则抛出异常
        if (subscriptions.contains(newSubscription)) {
            throw new EventBusException("Subscriber" + subscriber.getClass() + " already registered to event "+ eventType);
        }
    }
    * 按照优先级缓存订阅事件  subscriptionsByEventType
    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;
        }
    }
    * 判断是否已经缓存在typesBySubscriber中  
    List> subscribedEvents = typesBySubscriber.get(subscriber);
    if (subscribedEvents == null) {
        subscribedEvents = new ArrayList<>();
        typesBySubscriber.put(subscriber, subscribedEvents);
    }
    subscribedEvents.add(eventType);
    * 判断是否是粘性事件  并分发粘性事件
    if (subscriberMethod.sticky) {
        if (eventInheritance) {
            Set, Object>> entries = stickyEvents.entrySet();
            for (Map.Entry, Object> entry : entries) {
                Class candidateEventType = entry.getKey();
                if (eventType.isAssignableFrom(candidateEventType)) {
                    Object stickyEvent = entry.getValue();
                    * 分发事件
                    checkPostStickyEventToSubscription(newSubscription, stickyEvent);
                }
            }
        } else {
            Object stickyEvent = stickyEvents.get(eventType);
            * 分发事件  -->2.1
            checkPostStickyEventToSubscription(newSubscription, stickyEvent);
        }
    }
}

2.1checkPostStickyEventToSubscription(Subscription newSubscription, Object stickyEvent)

 * 判断粘性事件是否为空  
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, Looper.getMainLooper() == Looper.myLooper());
    }
}
* 根据线程模式进行事件分发
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);
    }
}

* 利用反射 执行订阅方法
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);
    }
}

*  将事件添加到 PendingPostQueue 队列中  执行handler 从队列冲取出消息进行处理 并利用反射 执行订阅方法 
void enqueue(Subscription subscription, Object event) {
    PendingPost pendingPost = PendingPost.obtainPendingPost(subscription, event);
    synchronized (this) {
        queue.enqueue(pendingPost);
        if (!handlerActive) {
            handlerActive = true;
            if (!sendMessage(obtainMessage())) {
                throw new EventBusException("Could not send handler message");
            }
        }
    }
}

总结:

1.利用反射获取到订阅类的所有订阅方法

2.判断是否已经注册/订阅过该事件

3.按照优先级缓存订阅事件

4.判断是否是粘性事件 并分发粘性事件 (1)同一个线程 利用反射 执行订阅方法 (2)不同线程 将事件添加到 PendingPostQueue 队列中 执行handler 从队列冲取出消息进行处理 并利用反射 执行订阅方法

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

* 发送事件
public void post(Object event) {
    * 获取当前线程的信息
    PostingThreadState postingState = currentPostingThreadState.get();
    * 将事件添加到当前线程的队列中
    List