目录

一、添加gradle依赖

二、基础操作

1、EventBus.getDefault();    

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

3、EventBus.getDefault().post(object);  

4、EventBus.getDefault().unregister();  

二、四个onEvent方法

三、EventBus3.0

四、原理简述

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定义

EventBus是一款在 Android 开发中使用的发布、订阅事件总线框架，基于观察者模式，将事件的接收者和发送者分开，可以在Activity、Fragment、Service等组件交互通信，使用简单、效率高、体积小。

可以替代Intent、Broadcast、Thread+Handle异步线程间传递消息。

githup地址：https://github.com/greenrobot/EventBus

一、添加gradle依赖

 

implementation 'org.greenrobot:eventbus:3.1.1'

 

二、基础操作

1、EventBus.getDefault();    

getDefault是一个单例方法，保证返回唯一的EventBus实例对象。

public static EventBus getDefault() {        if (defaultInstance == null) {            synchronized (EventBus.class) {                if (defaultInstance == null) {                    defaultInstance = new EventBus();                }            }        }        return defaultInstance;    }

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

public void register(Object subscriber) {        // 反射获取当前注册类的Class对象        Class<?> subscriberClass = subscriber.getClass();        // 1、查找：根据Class查找当前类中订阅了事件的方法集合        List subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);        synchronized (this) {            // 2、注册：循环遍历订阅了事件的方法集合，以完成注册            for (SubscriberMethod subscriberMethod : subscriberMethods) {                subscribe(subscriber, subscriberMethod);            }        }    }

主要分为查找和注册两部分，首先来看查找的过程，从findSubscriberMethods()开始： 

List findSubscriberMethods(Class<?> subscriberClass) {        // METHOD_CACHE是一个ConcurrentHashMap，直接保存了subscriberClass和对应SubscriberMethod的集合，以提高注册效率，赋值重复查找。        List subscriberMethods = METHOD_CACHE.get(subscriberClass);        if (subscriberMethods != null) {            return subscriberMethods;        }        // 由于使用了默认的EventBusBuilder，则ignoreGeneratedIndex属性默认为false，即是否忽略注解生成器        if (ignoreGeneratedIndex) {            subscriberMethods = findUsingReflection(subscriberClass);        } else {            subscriberMethods = findUsingInfo(subscriberClass);        }        // 如果对应类中没有符合条件的方法，则抛出异常        if (subscriberMethods.isEmpty()) {            throw new EventBusException("Subscriber " + subscriberClass                    + " and its super classes have no public methods with the @Subscribe annotation");        } else {            // 保存查找到的订阅事件的方法            METHOD_CACHE.put(subscriberClass, subscriberMethods);            return subscriberMethods;        }    }

先从缓存中查找，如果找到则直接返回，否则去做下一步的查找过程，然后缓存查找到的集合，根据上边的注释可知findUsingInfo()方法会被调用： 

private List findUsingInfo(Class<?> subscriberClass) {        FindState findState = prepareFindState();        findState.initForSubscriber(subscriberClass);        // 初始状态下findState.clazz就是subscriberClass        while (findState.clazz != null) {            findState.subscriberInfo = getSubscriberInfo(findState);            // 条件不成立            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 {                // 通过反射查找订阅事件的方法                findUsingReflectionInSingleClass(findState);            }            // 修改findState.clazz为subscriberClass的父类Class，即需要遍历父类            findState.moveToSuperclass();        }        // 查找到的方法保存在了FindState实例的subscriberMethods集合中。        // 使用subscriberMethods构建一个新的List        // 释放掉findState        return getMethodsAndRelease(findState);    }

findUsingInfo()方法会在当前要注册的类以及其父类中查找订阅事件的方法，这里出现了一个FindState类，它是SubscriberMethodFinder的内部类，用来辅助查找订阅事件的方法，具体的查找过程在findUsingReflectionInSingleClass()方法，它主要通过反射查找订阅事件的方法： 

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;        }        // 循环遍历当前类的方法，筛选出符合条件的        for (Method method : methods) {            // 获得方法的修饰符            int modifiers = method.getModifiers();            // 如果是public类型，但非abstract、static等            if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {                // 获得当前方法所有参数的类型                Class<?>[] parameterTypes = method.getParameterTypes();                // 如果当前方法只有一个参数                if (parameterTypes.length == 1) {                    Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);                    // 如果当前方法使用了Subscribe注解                    if (subscribeAnnotation != null) {                        // 得到该参数的类型                        Class<?> eventType = parameterTypes[0];                        // checkAdd()方法用来判断FindState的anyMethodByEventType map是否已经添加过以当前eventType为key的键值对，没添加过则返回true                        if (findState.checkAdd(method, eventType)) {                             // 得到Subscribe注解的threadMode属性值，即线程模式                            ThreadMode threadMode = subscribeAnnotation.threadMode();                            // 创建一个SubscriberMethod对象，并添加到subscriberMethods集合                            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");            }        }    }

 

到此register()方法中findSubscriberMethods()流程就分析完了，我们已经找到了当前注册类及其父类中订阅事件的方法的集合。接下来分析具体的注册流程，即register()中的subscribe()方法：

private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {        // 得到当前订阅了事件的方法的参数类型        Class<?> eventType = subscriberMethod.eventType;        // Subscription类保存了要注册的类对象以及当前的subscriberMethod        Subscription newSubscription = new Subscription(subscriber, subscriberMethod);        // subscriptionsByEventType是一个HashMap，保存了以eventType为key,Subscription对象集合为value的键值对        // 先查找subscriptionsByEventType是否存在以当前eventType为key的值        CopyOnWriteArrayList subscriptions = subscriptionsByEventType.get(eventType);        // 如果不存在，则创建一个subscriptions，并保存到subscriptionsByEventType        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);            }        }        // 添加上边创建的newSubscription对象到subscriptions中        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;            }        }        // typesBySubscribere也是一个HashMap，保存了以当前要注册类的对象为key，注册类中订阅事件的方法的参数类型的集合为value的键值对        // 查找是否存在对应的参数类型集合        List> subscribedEvents = typesBySubscriber.get(subscriber);        // 不存在则创建一个subscribedEvents，并保存到typesBySubscriber        if (subscribedEvents == null) {            subscribedEvents = new ArrayList<>();            typesBySubscriber.put(subscriber, subscribedEvents);        }        // 保存当前订阅了事件的方法的参数类型        subscribedEvents.add(eventType);        // 粘性事件相关的，后边具体分析        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);            }        }    }

这就是注册的核心流程，所以subscribe()方法主要是得到了subscriptionsByEventType、typesBySubscriber两个 HashMap。我们在发送事件的时候要用到subscriptionsByEventType，完成事件的处理。当取消 EventBus 注册的时候要用到typesBySubscriber、subscriptionsByEventType，完成相关资源的释放。 

3、EventBus.getDefault().post(object);  

public void post(Object event) {        //事件队列和线程模式等信息        PostingThreadState postingState = currentPostingThreadState.get();        List