final L listener;
    final Executor executor;

    @GuardedBy("this")
    final Queue<ListenerCallQueue.Event<L>> waitQueue = Queues.newArrayDeque();

    @GuardedBy("this")
    final Queue<Object> labelQueue = Queues.newArrayDeque();

    @GuardedBy("this")
    boolean isThreadScheduled;

    PerListenerQueue(L listener, Executor executor) {
      this.listener = checkNotNull(listener);