Dispatcher

Dispatcher是guava EventBus的事件分发器.

Dispatcher是抽象类, 抽象方法: abstract void dispatch(Object event, Iterator<Subscriber> subscribers);
定义了3个内部类: ImmediateDispatcher LegacyAsyncDispatcher PerThreadQueuedDispatcher.
这3个类是Dispatcher的实现.

LegacyAsyncDispatcher现在用PerThreadQueuedDispatcher代替

Subscriber包括EventBus, Object target, Method method, Executor executor.

   final void dispatchEvent(final Object event) {
     executor.execute(new Runnable() {
       @Override
       public void run() {
         try {
           invokeSubscriberMethod(event);  // 调用method, method.invoke(target, event)
         } catch (InvocationTargetException e) {
           bus.handleSubscriberException(e.getCause(), context(event));
         }
       }
     });
   }

Subscriber的dispatchEvent()

  @Override
     void dispatch(Object event, Iterator<Subscriber> subscribers) {
       checkNotNull(event);
       while (subscribers.hasNext()) {
         subscribers.next().dispatchEvent(event);
       }
     }

ImmediateDispatcher的dispatch()

  void dispatch(Object event, Iterator<Subscriber> subscribers) {
      checkNotNull(event);
      checkNotNull(subscribers);
      Queue<Event> queueForThread = queue.get();  // Queues.newArrayDeque()
      queueForThread.offer(new Event(event, subscribers));

      if (!dispatching.get()) {
        dispatching.set(true);  // 是线程安全的
        try {
          Event nextEvent;
          while ((nextEvent = queueForThread.poll()) != null) {
            while (nextEvent.subscribers.hasNext()) {
              nextEvent.subscribers.next().dispatchEvent(nextEvent.event);
            }
          }
        } finally {
          // 清除
          dispatching.remove();
          queue.remove();
        }
      }
    }

 PerThreadQueuedDispatcher的dispatch()

 PerThreadQueuedDispatcher包括ThreadLocal<Queue<Event>> queue和ThreadLocal<Boolean> dispatching

 dispatching表示是否正在执行dispatch, queue用于让每个线程有一个独立的事件队列.