由浅入深了解EventBus:

Posted 2020-09-14 飞蛾扑火

线程模型

  在EventBus3.0框架中执行线程的快速切换，通过ThreadMode来指定线程在哪个线程中执行;

  在EventBus3.0框架线程模型有个PendingPost 类负责数据的传递;

final class PendingPost {
    private final static List<PendingPost> pendingPostPool = new ArrayList<PendingPost>();
    Object event;
    Subscription subscription;
    PendingPost next;

    private PendingPost(Object event, Subscription subscription) {
        this.event = event;
        this.subscription = subscription;
    }
}

     PendingPost 类中维护了3个字段，其中event为事件类的实例，subscription是监听回调信息封装，next 的类型也是一个PendingPost ，通过next可以构建一个列表;

在类的内部也维护着一个静态的PendingPost 对象的对象池，当需要PendingPost 实例时，首先从对象池中获取，当获取不到时在进行对象的new创建;

    ThreadMode.MAIN 当调用线程不是主线程时，需要把事件执行推送到主线程中，在EventBus3.0框架中的EventBus类中维护着一个HandlerPoster对象来进行主线程数据的处理;HandlerPoster类是一个Handler,监听事件函数的执行应该在主线程回调的handleMessage 方法中,在源码中也确实是在handleMessage方法执行：

    eventBus.invokeSubscriber(pendingPost);

   ThreadMode.Background 与ThreadMode.AsyncPoster 执行的方式差不多相同，都是从后台线程池中取出线程进行执行;在EventBus类中初始化了BackgroundPoster与AsyncPoster来对这2种线程模型进行处理，这2个类都继承了Runnable 接口;

final class BackgroundPoster implements Runnable {

    private final PendingPostQueue queue;
    private final EventBus eventBus;

    private volatile boolean executorRunning;

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

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

  在BackgroundPoster 类中PendingPostQueue 是一个存储了PendingPost类型的队列，eventBus对应的就是EventBus类的实例,在BackgroundPoster 类中enqueue方法是在EventBus分发Post方法内部进行调用的;

 eventBus.getExecutorService()获取EventBus类中的ExecutorService,在源码我们可以发现ExecutorService=Executors.newCachedThreadPool();

 当执行eventBus.getExecutorService().execute(this);代码时,就跳转到BackgroundPoster 的run方法中

 @Override
    public void run() {
        try {
            try {
                while (true) {
                    PendingPost pendingPost = queue.poll(1000);
                    if (pendingPost == null) {
                        synchronized (this) {
                            // Check again, this time in synchronized
                            pendingPost = queue.poll();
                            if (pendingPost == null) {
                                executorRunning = false;
                                return;
                            }
                        }
                    }
                    eventBus.invokeSubscriber(pendingPost);
                }
            } catch (InterruptedException e) {
                Log.w("Event", Thread.currentThread().getName() + " was interruppted", e);
            }
        } finally {
            executorRunning = false;
        }
    }

从run方法中可以看出，最终执行的还是eventBus.invokeSubscriber(pendingPost) 方法;