Android中模块更新的机制有很多种，EventBus又是其中使用起来较为方便简单的一种，本文就针对EventBus在平常工作中的用法，做一个个人总结。主要分为以下几点来展示：

1. 介绍
2. 源码分析
3. 用法示例

下图也就是本文要讲的主要内容：

1、介绍：
EventBus是针一款对Android的发布/订阅事件总线。它可以让我们很轻松的实现在Android各个组件之间传递消息，并且代码的可读性更好，耦合度更低。我们可以在要进行实时界面更新的交互上使用该模块以替代一些消息机制的处理。EventBus的·优点是用法简单，不必写繁琐的代码，而且耦合度也低。缺点其实个人发现无明显的缺点，可能就是得针对每个事件定义也一个事件类，感觉会造成重复造轮子的感觉。此外，关于事件总线还有的一个框架是Otto，这边就不分析这个框架了。

2、源码分析：
按照惯例，我们在代码里面查看EventBus.java的源码，这里就不贴出整个类的源码了。我们首先看下，该类的构造方法：

/**     * Creates a new EventBus instance; each instance is a separate scope in which events are delivered. To use a     * central bus, consider {@link #getDefault()}.     */    public EventBus() {        this(DEFAULT_BUILDER);    }    EventBus(EventBusBuilder builder) {        subscriptionsByEventType = new HashMap, CopyOnWriteArrayList>();        typesBySubscriber = new HashMap>>();        stickyEvents = new ConcurrentHashMap, Object>();        mainThreadPoster = new HandlerPoster(this, Looper.getMainLooper(), 10);        backgroundPoster = new BackgroundPoster(this);        asyncPoster = new AsyncPoster(this);        subscriberMethodFinder = new SubscriberMethodFinder(builder.skipMethodVerificationForClasses);        logSubscriberExceptions = builder.logSubscriberExceptions;        logNoSubscriberMessages = builder.logNoSubscriberMessages;        sendSubscriberExceptionEvent = builder.sendSubscriberExceptionEvent;        sendNoSubscriberEvent = builder.sendNoSubscriberEvent;        throwSubscriberException = builder.throwSubscriberException;        eventInheritance = builder.eventInheritance;        executorService = builder.executorService;    }

主要是一些要用到类的对象的初始化，用的是Builder模式。
注册的方法：

/**     * Registers the given subscriber to receive events. Subscribers must call {@link #unregister(Object)} once they     * are no longer interested in receiving events.     * 
     * Subscribers have event handling methods that must be annotated by {@link Subscribe}.     * The {@link Subscribe} annotation also allows configuration like {@link     * ThreadMode} and priority.     */    public void register(Object subscriber) {        Class<?> subscriberClass = subscriber.getClass();        List subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);        synchronized (this) {            for (SubscriberMethod subscriberMethod : subscriberMethods) {                subscribe(subscriber, subscriberMethod);            }        }    }

通过SubscriberMethodFinder该类的findSubscriberMethods方法，查询到SubscriberMethod列表，SubscriberMethod类的构造方法：

public SubscriberMethod(Method method, Class<?> eventType, ThreadMode threadMode, int priority, boolean sticky) {        this.method = method;        this.threadMode = threadMode;        this.eventType = eventType;        this.priority = priority;        this.sticky = sticky;    }

可知，五个参数为方法、事件类型、线程模式、优先级、sticky，SubscriberMethod的其他方法：

@Override    public boolean equals(Object other) {        if (other == this) {            return true;        } else if (other instanceof SubscriberMethod) {            checkMethodString();            SubscriberMethod otherSubscriberMethod = (SubscriberMethod)other;            otherSubscriberMethod.checkMethodString();            // Don't use method.equals because of http://code.google.com/p/android/issues/detail?id=7811#c6            return methodString.equals(otherSubscriberMethod.methodString);        } else {            return false;        }    }    private synchronized void checkMethodString() {        if (methodString == null) {            // Method.toString has more overhead, just take relevant parts of the method            StringBuilder builder = new StringBuilder(64);            builder.append(method.getDeclaringClass().getName());            builder.append('#').append(method.getName());            builder.append('(').append(eventType.getName());            methodString = builder.toString();        }    }

现在清楚了，其实就是查询SubscriberMethodFinder类的作用是在Subscriber中找到所有以methodName（即默认的onEvent）开头的方法，具体我们可以看下findSubscriberMethods方法的代码。
EventBus中 findSubscriberMethods方法 2.4源码：

List findSubscriberMethods(Class<?> subscriberClass) {        String key = subscriberClass.getName();        List subscriberMethods;        synchronized (methodCache) {            subscriberMethods = methodCache.get(key);        }        if (subscriberMethods != null) {            return subscriberMethods;        }        subscriberMethods = new ArrayList();        Class<?> clazz = subscriberClass;        HashSet<String> eventTypesFound = new HashSet<String>();        StringBuilder methodKeyBuilder = new StringBuilder();        while (clazz != null) {            String name = clazz.getName();            if (name.startsWith("java.") || name.startsWith("javax.") || name.startsWith("android.")) {                // Skip system classes, this just degrades performance                break;            }            // Starting with EventBus 2.2 we enforced methods to be public (might change with annotations again)            Method[] methods = clazz.getDeclaredMethods();            for (Method method : methods) {                String methodName = method.getName();                if (methodName.startsWith(ON_EVENT_METHOD_NAME)) {                    int modifiers = method.getModifiers();                    if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {                        Class<?>[] parameterTypes = method.getParameterTypes();                        if (parameterTypes.length == 1) {                            String modifierString = methodName.substring(ON_EVENT_METHOD_NAME.length());                            ThreadMode threadMode;                            if (modifierString.length() == 0) {                                threadMode = ThreadMode.PostThread;                            } else if (modifierString.equals("MainThread")) {                                threadMode = ThreadMode.MainThread;                            } else if (modifierString.equals("BackgroundThread")) {                                threadMode = ThreadMode.BackgroundThread;                            } else if (modifierString.equals("Async")) {                                threadMode = ThreadMode.Async;                            } else {                                if (skipMethodVerificationForClasses.containsKey(clazz)) {                                    continue;                                } else {                                    throw new EventBusException("Illegal onEvent method, check for typos: " + method);                                }                            }                            Class<?> eventType = parameterTypes[0];                            methodKeyBuilder.setLength(0);                            methodKeyBuilder.append(methodName);                            methodKeyBuilder.append('>').append(eventType.getName());                            String methodKey = methodKeyBuilder.toString();                            if (eventTypesFound.add(methodKey)) {                                // Only add if not already found in a sub class                                subscriberMethods.add(new SubscriberMethod(method, threadMode, eventType));                            }                        }                    } else if (!skipMethodVerificationForClasses.containsKey(clazz)) {                        Log.d(EventBus.TAG, "Skipping method (not public, static or abstract): " + clazz + "."                                + methodName);                    }                }            }            clazz = clazz.getSuperclass();        }        if (subscriberMethods.isEmpty()) {            throw new EventBusException("Subscriber " + subscriberClass + " has no public methods called "                    + ON_EVENT_METHOD_NAME);        } else {            synchronized (methodCache) {                methodCache.put(key, subscriberMethods);            }            return subscriberMethods;        }    }

现在应该豁然开朗，就是2.4版本，必须使用三种方法接收post过来的消息：MainThread、BackgroundThread、Async。而在3.0中的源码则是这样的：

 List findSubscriberMethods(Class<?> subscriberClass) {        List subscriberMethods = METHOD_CACHE.get(subscriberClass);        if (subscriberMethods != null) {            return subscriberMethods;        }        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 {            METHOD_CACHE.put(subscriberClass, subscriberMethods);            return subscriberMethods;        }    }

subscriberMethods = findUsingReflection(subscriberClass);说的是利用反射来查询方法。
所以我们3.0通过注解，可以使用自定义的方法，不在受限，看完了注册的代码，
接下来就看下post的源码。

/** Posts the given event to the event bus. */    public void post(Object event) {        PostingThreadState postingState = currentPostingThreadState.get();        List