最近去维护另外一个项目，各种库使用的都比较老，eventbus使用的是2.x版本，于是来个升级，顺便读下eventbus的源码,在此做个笔记:

EventBus2.x升级3.x

2.x与3.x之间的对应关系：

onEvent--注解ThreadMode.POSTING;onEventMainThread--注解ThreadMode.MAINonEventAsync--注解ThreadMode.BACKGROUNDonEventBackgroundThread--注解ThreadMode.ASYNC

1.包名替换

Eventbus升级之后包名变了，可以ctrl+shift+r全局替换，也可以给studio设置自动导入包名将
import de.greenrobot.event.EventBus;替换成 import org.greenrobot.eventbus.EventBus; 全部替换.

2.方法需要使用注解方式

ctrl+shift+f 全局搜索, 依次添加上面提到的4个注解方法.
例如对onEvent方法,搜索"public void onEvent (" 添加"@Subscribe(threadMode = ThreadMode.POSTING)"
3.EventBus 3.0版本去掉了registerSticky, 换成到每个方法的注解中配置sticky = true. 搜索".registerSticky(", registerSticky替换回register, 再在该类下的方法注解上添加sticky.

EventBus源码解读：

1.EventBus的构造方法

/** Convenience singleton for apps using a process-wide EventBus instance. */public static EventBus getDefault() {    if (defaultInstance == null) {        synchronized (EventBus.class) {            if (defaultInstance == null) {                defaultInstance = new EventBus();            }        }    }    return defaultInstance;}

这里的单利模式用了双重检查模式，接下来看下EventBus的构造方法都做了什么：

...private static final EventBusBuilder DEFAULT_BUILDER = new EventBusBuilder();...public EventBus() {    this(DEFAULT_BUILDER);}

this调用了eventbus的另一个构造方法：

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;}

这里采用建造者模式，通过构造一个EventbusBuilder来对Eventbus进行配置

2.订阅注册

获取到Eventbus后，就可以将订阅者注册到Eventbus中，注册方法如下：

public void register(Object subscriber) {    Class<?> subscriberClass = subscriber.getClass();    //查找订阅者（传进来的subscriber）的所有订阅方法的集合    List subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);    synchronized (this) {        //遍历订阅者的方法，完成注册        for (SubscriberMethod subscriberMethod : subscriberMethods) {            subscribe(subscriber, subscriberMethod);        }    }}

1）查找订阅者的订阅方法

可以看到，注册方法做了两件事：①查找订阅者中所有订阅方法，②订阅者的注册。SubscriberMethod类主要用来保存订阅方法Method对象、线程模式（threadMode）、事件类型（eventType）、优先级（priority）、是否粘性事件（sticky）等。findSubscriberMethods如下：

List findSubscriberMethods(Class<?> subscriberClass) {    //检查缓存中是否有    List subscriberMethods = METHOD_CACHE.get(subscriberClass);    if (subscriberMethods != null) {        return subscriberMethods;    }    ```    //判断是否忽略注解器生成的MyEventBusIndex（参考http://greenrobot.org/eventbus/documentation/subscriber-index/）    //默认false，可以通过EventbusBuilder来设置。因此我们使用通常使用的是findUsingInfo方法来获取订阅方法集合    ```    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 {        //将订阅方法集合放入缓存，下次直接取用，避免重复查找        METHOD_CACHE.put(subscriberClass, subscriberMethods);        return subscriberMethods;    }}

接下来看下findUsingInfo方法：

private List findUsingInfo(Class<?> subscriberClass) {    FindState findState = prepareFindState();    findState.initForSubscriber(subscriberClass);    while (findState.clazz != null) {        //获取订阅者信息        findState.subscriberInfo = getSubscriberInfo(findState);        //如果我们通过EventBuilder配置了MyEventBusIndex,便会获取到subscriberInfo        if (findState.subscriberInfo != null) {            SubscriberMethod[] array = findState.subscriberInfo.getSubscriberMethods();            for (SubscriberMethod subscriberMethod : array) {                if (findState.checkAdd(subscriberMethod.method, subscriberMethod.eventType)) {                    findState.subscriberMethods.add(subscriberMethod);                }            }            //没有通过EventBuilder配置MyEventBusIndex，调用此方法将订阅信息保存到findState，默认没有配置，走此方法        } else {            findUsingReflectionInSingleClass(findState);        }        findState.moveToSuperclass();    }    //对findState做回收处理，并返回订阅方法的集合    return getMethodsAndRelease(findState);}

more没有配置MyEventBusIndex，下面看下findUsingReflectionInSingleClass方法：

private void findUsingReflectionInSingleClass(FindState findState) {    Method[] methods;    try {        //通过反射获取订阅者中的所有方法，这个方法比getMethods()快，特别是代码比较多的类如Activity        methods = findState.clazz.getDeclaredMethods();    } catch (Throwable th) {        methods = findState.clazz.getMethods();        findState.skipSuperClasses = true;    }    for (Method method : methods) {        //获取方法的修饰符，eg. public        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();                        //将订阅方法相关信息保存到findstate中                        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，查找订阅者的订阅方法结束。

2）订阅者的注册过程

查找完订阅者的订阅方法之后，就开始对再所有订阅者方法进行注册，回到register方法中，看那里调用subscribe方法：

// Must be called in synchronized blockprivate void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {    //订阅者订阅的方法的类型    Class<?> eventType = subscriberMethod.eventType;    Subscription newSubscription = new Subscription(subscriber, subscriberMethod);    //根据eventType获取订阅对象集合，如果为空则重新创建    CopyOnWriteArrayList 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> 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).            Set, Object>> entries = stickyEvents.entrySet();            for (Map.Entry, 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);        }    }}

subscribe主要做了两件事，①将Subscription根据evntType封装到subscriptionsByEventType中，将subscribedEvents根据subscriber封装到typeBySubscriber中；②对粘性事件的处理。

3.事件发送

在获取到Eventbus对象之后，通过post方法，完成对事件的发送：

/** Posts the given event to the event bus. */public void post(Object event) {    //PostingThreadState 保存着事件队列（eventQueue）和线程状态信息（isPosting，isMainThread，subscription，event，canceled）    PostingThreadState postingState = currentPostingThreadState.get();    //获取事件队列，并将当前事件插入事件队列    List