可用于应用内的消息事件传递，方便快捷，耦合性低

1.基本用法

public class EventBusMain extends AppCompatActivity {

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.content_main);


        EventBus.getDefault().register(this);

    }

  - 订阅的事件 onEvent1
    @Subscribe
    public void onEvent1(RemindBean bean){

    }
- 订阅的事件 onEvent2
    @Subscribe
    public void onEvent2(UserInfo bean){

    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        EventBus.getDefault().unregister(this);
    }
}

需要发送消息传递的时候：

EventBus.getDefault().post(new RemindBean())

2.源码解读

放上官网的一张原理图，感觉挺清晰的：

发布消息的一方(Publisher)，只需要 post 一个 event 之后就不用管了，EventBus 内部会将event逐一分发给订阅此 event 的订阅者（Subscriber）. 不错就是这样一个东西。

还记得以往我要实现两个不同的activity 之间要传递一些数据的时候，我都是通过定义一个interface的形式完成，时间一长，定义的接口一堆，在回顾查看代码也确实不够美观。好了话不多说，看下大家都在用的Eventbus.

3.首先

 EventBus.getDefault().register(this);

getDefault():

EventBus 是一个单例模式，懒汉式，双重判断
 /** 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;
    }

register 是什么意思呢，就是就跟你订阅报纸一样，报社需要确定几个重要的问题：

订阅者是谁（Subscriber）？
订阅的什么报纸(Event) ？

就是我认为比较重要的，那么register 这一步就是Subscriber 告诉报社，订阅的event

public void register(Object subscriber) {
        - 1.先拿到这个订阅者(subscriber)类的字节码
        Class<?> subscriberClass = subscriber.getClass();

        - 2. 通过这个类的字节码，拿到所有的订阅的 event,存放在List中
        List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);

        synchronized (this) {
          - 3. 循环遍历所有的订阅的方法，完成subscriber 和 subscriberMethod 的关联
            for (SubscriberMethod subscriberMethod : subscriberMethods) {
                subscribe(subscriber, subscriberMethod);
            }
        }
    }

我们看下这个如何根据subscriberClass 找到这个订阅的 method的，findSubscriberMethods:

 List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {
        - 1.先从缓存中取
        List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);

        - 2. 第一次肯定 null
        if (subscriberMethods != null) {
            return subscriberMethods;
        }

        - 3. 查找默认也是false，感兴趣的可以看下
        if (ignoreGeneratedIndex) {
            subscriberMethods = findUsingReflection(subscriberClass);
        } else {

          - 4. 所以是走这里
            subscriberMethods = findUsingInfo(subscriberClass);
        }
        if (subscriberMethods.isEmpty()) {
            throw new EventBusException("Subscriber " + subscriberClass
                    + " and its super classes have no public methods with the @Subscribe annotation");
        } else {
            - 5. 找到之后添加到缓存中，key是 subscriber ;value 是：methods
            METHOD_CACHE.put(subscriberClass, subscriberMethods);
            return subscriberMethods;
        }
    }

看下：findUsingInfo(subscriberClass)

private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
        - 1. 我认为就是准备一个查找结果得存储对象
        FindState findState = prepareFindState();

        - 2. 将订阅者的subscriberClass 存储起来，保存在一个FindState 类中的subscriberClass 
        同时赋值给clazz变量中,以下代码能够看出
//      void initForSubscriber(Class<?> subscriberClass) {
 //      this.subscriberClass = clazz = subscriberClass;
//}
        findState.initForSubscriber(subscriberClass);

        while (findState.clazz != null) {进入循环中
          //获取subscriberInfo 信息，返回null
            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 {
                  - 3. 进入到这里了
                findUsingReflectionInSingleClass(findState);
            }
            - 4. 查找父类中的方法
            findState.moveToSuperclass();
        }
        return getMethodsAndRelease(findState);
    }

findUsingReflectionInSingleClass 如下：

 private void findUsingReflectionInSingleClass(FindState findState) {
        Method[] methods;
        try {
            // This is faster than getMethods, especially when subscribers are fat classes like Activities
          - 1. 通过订阅者的字节码查找当前类中所有生命的方法
            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;
        }
          - 2. 循环遍历所有的方法
        for (Method method : methods) {
            - 3. 获取方法的修饰符
            int modifiers = method.getModifiers();
      
            - 4.判断修饰符，订阅方法的修饰符不能是private，static
            if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
                - 5. 获取方法的所有的参数
                Class<?>[] parameterTypes = method.getParameterTypes();

                - 6.判断参数的个数，只能有1个参数，订阅方法中
                if (parameterTypes.length == 1) {
                - 7.获取方法上具有subscribe 注解
                    Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);

                    - 8.含有subscribe注解的方法，就是该类订阅的方法，其它不符合的可能就是普通的方法
                    if (subscribeAnnotation != null) {

                      - 9. 获取第一个参数eventType
                        Class<?> eventType = parameterTypes[0];
                          
                        if (findState.checkAdd(method, eventType)) {
                          - 10. 获取注解的mode，就是我们在注解上标识的，
                      有mainThread,Posting,background,async
                            ThreadMode threadMode = subscribeAnnotation.threadMode();

                          - 11. 将订阅方法的一系列信息（方法名称，threadMode,优先级，是否是粘性等）添加到集合subscriberMethods中去
                            findState.subscriberMethods.add(new SubscriberMethod(method, eventType, threadMode,
                                    subscribeAnnotation.priority(), subscribeAnnotation.sticky()));
                        }
                    }
                } else if (strictMethodVerification && method.isAnnotationPresent(Subscribe.class)) {
              - 12. 参数是多个的时候抛出异常
                    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)) {
            - 13. 方法的修饰符不是public的，抛出异常                

              String methodName = method.getDeclaringClass().getName() + "." + method.getName();
                throw new EventBusException(methodName +
                        " is a illegal @Subscribe method: must be public, non-static, and non-abstract");
            }
        }
    }

这样我们将所有信息都保存到findState 类中去了。再回头看我们原先那个方法,到第三步了：

private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {
        - 1. 我认为就是准备一个查找结果得存储对象
        FindState findState = prepareFindState();

        - 2. 将订阅者的subscriberClass 存储起来，保存在一个FindState 类中的subscriberClass 
        同时赋值给clazz变量中,以下代码能够看出
//      void initForSubscriber(Class<?> subscriberClass) {
 //      this.subscriberClass = clazz = subscriberClass;
//}
        findState.initForSubscriber(subscriberClass);

        while (findState.clazz != null) {进入循环中
          //获取subscriberInfo 信息，返回null
            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 {
                  - 3. 进入到这里了，上面已经分析所有信息保存到findState中
                findUsingReflectionInSingleClass(findState);
            }
            - 4. 查找父类中的方法
            findState.moveToSuperclass();
        }
        return getMethodsAndRelease(findState);
    }

在这个getMethodsAndRelease(findState):

private List<SubscriberMethod> getMethodsAndRelease(FindState findState) {
        - 1. 取出里面的subscriberMethods
        List<SubscriberMethod> subscriberMethods = new ArrayList<>(findState.subscriberMethods);
        findState.recycle();
        synchronized (FIND_STATE_POOL) {
            for (int i = 0; i < POOL_SIZE; i++) {
                if (FIND_STATE_POOL[i] == null) {
                    FIND_STATE_POOL[i] = findState;
                    break;
                }
            }
        }
      - 2. 返回集合
        return subscriberMethods;
    }

至此，我们知道了根据订阅者（subscriber）的clazz 找到了所有订阅的方法事件 methods

回到最初的第一步register:

public void register(Object subscriber) {
        Class<?> subscriberClass = subscriber.getClass();
        - 2. 完成
        List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
        synchronized (this) {
            for (SubscriberMethod subscriberMethod : subscriberMethods) {
            - 3.循环遍历所有的订阅方法和订阅者之间建立关联
                subscribe(subscriber, subscriberMethod);
            }
        }
    }

subscribe(subscriber, subscriberMethod) 方法：

// Must be called in synchronized block
    private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
        - 1. 订阅方法的eventType的字节码
        Class<?> eventType = subscriberMethod.eventType;

        - 2. 订阅者和订阅方法封装成一个Subscription 对象
        Subscription newSubscription = new Subscription(subscriber, subscriberMethod);

        - 3. subscriptionsByEventType 第一次也是null ,根据eventType
        CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);

        - 4. 第一次肯定为null
        if (subscriptions == null) {
            subscriptions = new CopyOnWriteArrayList<>();

            - 5. key 为 eventType, value 是subscriptions对象
            subscriptionsByEventType.put(eventType, subscriptions);
        } else {
          - 抛出异常
            if (subscriptions.contains(newSubscription)) {
                throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event "
                        + eventType);
            }
        }

      - 6. 获取所有添加的subscriptions
        int size = subscriptions.size();
        for (int i = 0; i <= size; i++) {
          - 7. 会判断每个订阅方法的优先级，添加到这个 subscriptions中，按照优先级
            if (i == size || subscriberMethod.priority > subscriptions.get(i).subscriberMethod.priority) {
                subscriptions.add(i, newSubscription);
                break;
            }
        }
    
      - 8.获取订阅的方法集合
        List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
        if (subscribedEvents == null) {
            subscribedEvents = new ArrayList<>();
          - 9. 为空添加到 typesBySubscriber
            typesBySubscriber.put(subscriber, subscribedEvents);
        }
        - 10. 订阅事件添加到subscribedEvents集合中去
        subscribedEvents.add(eventType);

        - 11. 判断是否是粘性事件的关联
        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<Class>).
                Set<Map.Entry<Class<?>, Object>> entries = stickyEvents.entrySet();
                for (Map.Entry<Class<?>, 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);
            }
        }
    }

到此，如果你跟着我一步步看到这里，应该大概明白一些了，还有一部分没完，就是register 前半部分完成订阅，存储等工作；剩下post(event) 方法就是将event 分发给相应订阅过此事件的订阅者了。