• 1 概述
• 2 原理
• 3 使用
• 4 源码
• 5 总结

https://greenrobot.org/eventbus/documentation/
https://github.com/greenrobot/EventBus

1 概述

EventBus Publisher 将 Event 交给 EventBus
EventBus 转交给 Subscriber
Subscriber 在其回调中消费处理 Event

提供能力：

• ThreadMode
• StickyEvent (ClassType <-> Event)
• subscribe priority & Event Cancle

2 原理

• register 将会在该类内部进行解析，带有 subscribe 注解的方法会被提取为观察者接受回调

• post 发送 event

• threadMode 处理线程切换，利用队列来保存Event 进行线程调度

• StickyEvent @Subscribe(sticky = true) 则会在 subscriber register 时候将所有的 stickyEvent 回调所有 注解sticky = true 的 subscribe method

• Priority： 解析 @Subscribe(priority = 1) 将在插入 subscription 的时候将该订阅 位置靠前 default priority = 0

• Cancel 在 for 中会分发 Event 到 每个 Subscription Cancel --> break

• index 索引 解析形成 SubscribeMethod 采用注解解析生成器做处理，不使用默认的反射提高效率

        List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);

        ...

        if (ignoreGeneratedIndex) {
            subscriberMethods = findUsingReflection(subscriberClass);
        } else {
            subscriberMethods = findUsingInfo(subscriberClass);
        }

3 使用

官方教程
https://greenrobot.org/eventbus/documentation/how-to-get-started/

代码：

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

        textView.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                EventBus.getDefault().post(new MessageEvent(System.currentTimeMillis()+""));
            }
        });
    }

    @Subscribe(threadMode = ThreadMode.MAIN)
    public void homeText(MessageEvent event) {
        textView.setText(event.content);
    }

    @Override
    protected void onStart() {
        EventBus.getDefault().register(this);
        super.onStart();
    }

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

4 源码

• EventBus
  • eventTypesCache
  • DEFAULT_BUILDER
  • defaultInstance
  • eventTypesCache
  • subscriptionsByEventType
  • typesBySubscriber
  • stickyEvents
  • mainThreadSupport
  • mainThreadPoster
  • backgroundPoster
  • asyncPoster
  • subscriberMethodFinder
  • executorService

• Poster
  • enqueue

• Subscription

  • subscriber
  • subscriberMethod

• SubscriberMethod

  • method
  • threadMode
  • eventType
  • priority
  • sticky

5 总结

• 注解
• 观察者模式
• 线程调度

后台线程 和 主线程 切换：

   private void postToSubscription(Subscription subscription, Object event, boolean isMainThread) {
        switch (subscription.subscriberMethod.threadMode) {
            case POSTING:
                invokeSubscriber(subscription, event);
                break;
            case MAIN:
                if (isMainThread) {
                    invokeSubscriber(subscription, event);
                } else {
                    mainThreadPoster.enqueue(subscription, event);
                }
                break;
            case MAIN_ORDERED:
                if (mainThreadPoster != null) {
                    mainThreadPoster.enqueue(subscription, event);
                } else {
                    // temporary: technically not correct as poster not decoupled from subscriber
                    invokeSubscriber(subscription, event);
                }
                break;
            case BACKGROUND:
                if (isMainThread) {
                    backgroundPoster.enqueue(subscription, event);
                } else {
                    invokeSubscriber(subscription, event);
                }
                break;
            case ASYNC:
                asyncPoster.enqueue(subscription, event);
                break;
            default:
                throw new IllegalStateException("Unknown thread mode: " + subscription.subscriberMethod.threadMode);
        }
    }

class AsyncPoster implements Runnable, Poster {

    private final PendingPostQueue queue;
    private final EventBus eventBus;

    AsyncPoster(EventBus eventBus) {
        this.eventBus = eventBus;
        queue = new PendingPostQueue();
    }

    public void enqueue(Subscription subscription, Object event) {
        PendingPost pendingPost = PendingPost.obtainPendingPost(subscription, event);
        queue.enqueue(pendingPost);
        eventBus.getExecutorService().execute(this);
    }

    @Override
    public void run() {
        PendingPost pendingPost = queue.poll();
        if(pendingPost == null) {
            throw new IllegalStateException("No pending post available");
        }
        eventBus.invokeSubscriber(pendingPost);
    }

}

五种模式的特征各不相同