Class LegacyAsyncDispatcher

output/java_guava/1.4.18/Dispatcher.java:147–194

Implementation of a #legacyAsync() dispatcher.

   */

  private static final class LegacyAsyncDispatcher extends Dispatcher {

    // This dispatcher matches the original dispatch behavior of AsyncEventBus.
    //
    // We can't really make any guarantees about the overall dispatch order for this dispatcher in
    // a multithreaded environment for a couple reasons:
    //
    // 1. Subscribers to events posted on different threads can be interleaved with each other
    //    freely. (A event on one thread, B event on another could yield any of
    //    [a1, a2, a3, b1, b2], [a1, b2, a2, a3, b2], [a1, b2, b3, a2, a3], etc.)
    // 2. It's possible for subscribers to actually be dispatched to in a different order than they
    //    were added to the queue. It's easily possible for one thread to take the head of the
    //    queue, immediately followed by another thread taking the next element in the queue. That
    //    second thread can then dispatch to the subscriber it took before the first thread does.
    //
    // All this makes me really wonder if there's any value in queueing here at all. A dispatcher
    // that simply loops through the subscribers and dispatches the event to each would actually
    // probably provide a stronger order guarantee, though that order would obviously be different
    // in some cases.

    /**
     * Global event queue.
     */
    private final ConcurrentLinkedQueue<EventWithSubscriber> queue = Queues.newConcurrentLinkedQueue();

    @Override
    void dispatch(Object event, Iterator<Subscriber> subscribers) {
      checkNotNull(event);
      while (subscribers.hasNext()) {
        queue.add(new EventWithSubscriber(event, subscribers.next()));
      }
      EventWithSubscriber e;
      while ((e = queue.poll()) != null) {
        e.subscriber.dispatchEvent(e.event);
      }
    }

    private static final class EventWithSubscriber {
      private final Object event;

      private final Subscriber subscriber;

      private EventWithSubscriber(Object event, Subscriber subscriber) {
        this.event = event;
        this.subscriber = subscriber;
      }
    }
  }

  /**
   * Implementation of {@link #immediate()}.