Android事件总线：EventBus

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.MAIN
onEventAsync--注解ThreadMode.BACKGROUND
onEventBackgroundThread--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 block
private 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