ReactiveX emit null or sentinel value after timeout

Question

Looking for a clean way to transform a source Observable to emit a single null (or sentinel value) after not emitting an item for some duration.

Answer 1

You can achieve this with a somewhat complicated publish-amb-timer setup:

PublishSubject ps = PublishSubject.create();
TestScheduler s = Schedulers.test();
TestSubscriber ts = new TestSubscriber<>();

ps.publish(o -> 
    o.take(1).ambWith(Observable.timer(10, TimeUnit.SECONDS, s).map(v -> (Integer)null))
    .repeat().takeUntil(o.ignoreElements())
).subscribe(ts);

ps.onNext(1);
ps.onNext(2);
ps.onNext(3);

s.advanceTimeBy(15, TimeUnit.SECONDS);

ps.onNext(4);
ps.onNext(5);
ps.onNext(6);
ps.onCompleted();

ts.assertValues(1, 2, 3, null, 4, 5, 6);

What happens is that the source is published so you can take items one by one from it or a timer event, make sure the fastest one wins and repeat it with the next value, all without resubscribing to the original source all the time.

Edit fixed the case when the upstream completes the repeat() goes into an infinite loop.