EventBus实现组件通信的原理

• 在理清原理之前，先要明白EventBus的使用场景和主要方法的调用，可以参考EventBus 3.0的使用。
• Publish/Subscribe模式，就是发布订阅模式。点击在线查看EventBus.java源码。EventBus的核心思想就是把event和订阅者，对应存在一个HashMap中，post方法调用后，再根据入参event，从HashMap中取出对应的订阅者对象，调用订阅方法，完成通信。
• 下面逐步分析EventBus的几个重要方法。
• getDefault()使用单例模式新建唯一单例EventBus实例。

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

• 在register()方法中，传入订阅者对象，获取该对象的类信息，在类信息中寻找包含注解@Subscribe的方法，产生一个方法列表subscriberMethods,

public void register(Object subscriber) {
	//获取订阅者的类信息
    Class subscriberClass = subscriber.getClass();
    //获取订阅者中所有的监听方法
    List subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
    synchronized (this) {
        for (SubscriberMethod subscriberMethod : subscriberMethods) {
            subscribe(subscriber, subscriberMethod);
        }
    }
}

• 如何根据@Subscribe注解找到对应方法？具体查找逻辑在SubscribeMethodFinder.java中，代码如下：

    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;
        }
        for (Method method : methods) {
            int modifiers = method.getModifiers();
            if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
                Class[] parameterTypes = method.getParameterTypes();
                if (parameterTypes.length == 1) {
                //这里是核心
                    Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);
                    if (subscribeAnnotation != null) {
                        Class eventType = parameterTypes[0];
                        if (findState.checkAdd(method, eventType)) {
                            ThreadMode threadMode = subscribeAnnotation.threadMode();
                            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");
            }
        }
    }

• 再具体看subscirbe方法，到底在哪里添加的key-value键值对？

 // Must be called in synchronized block
    private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
        Class eventType = subscriberMethod.eventType;
        Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
        CopyOnWriteArrayList subscriptions = subscriptionsByEventType.get(eventType);
        if (subscriptions == null) {
            subscriptions = new CopyOnWriteArrayList<>();
            //在这里，hashMap添加元素
            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;
            }
        }
        ...
    }

• 存储元素后，post方法是怎么找到订阅者对象的？
  答案是根据key值，取出List，遍历list，逐个发送event。订阅者对象存储在subscriptionsByEventType中，subscriptionsByEventType是一个HashMap，key的类型是Class,value的类型是CopyOnWriteArrayList

    private final Map, CopyOnWriteArrayList> subscriptionsByEventType;

post方法源码：

    public void post(Object event) {
        PostingThreadState postingState = currentPostingThreadState.get();
        List