example.java

  /**
   * Groups items within a given time interval (unless the max size is reached, then earlier),
   * and picks the single element to signal downstream using the provided `pick` function.
   */
  public <A> Flow<A, A, NotUsed> debounceSelect(FiniteDuration interval, Function<List<A>, A> pick, int max) {
    return Flow.<A>create().groupedWithin(max, interval).map(group -> pick.apply(group));
  }

  class RefreshSignal {
    public RefreshSignal(Object a) {}
  }


final SourceQueueWithComplete<RefreshSignal> queue =
  Source.<RefreshSignal>queue(4, OverflowStrategy.dropHead()) // give it some buffer space, but we can drop signals anyway, since if we have at least one, it means we want to trigger one refresh
    .via(debounceSelect(FiniteDuration.create(1, TimeUnit.SECONDS), it -> it.get(0), 100)) // picking any of the refresh signals
    .to(Sink.actorRef(target, new RefreshSignal("done")))
    .run(materializer); // your Materializer

final CompletionStage<QueueOfferResult> offer1 = queue.offer(new RefreshSignal("refresh"));
final CompletionStage<QueueOfferResult> offer2 = queue.offer(new RefreshSignal("refresh"));
final CompletionStage<QueueOfferResult> offer3 = queue.offer(new RefreshSignal("refresh"));
final CompletionStage<QueueOfferResult> offer4 = queue.offer(new RefreshSignal("refresh"));