Timeout with per item fallback (no completing after fallback) - reactive-programming

Question

If I have items coming from a source periodically, how can I put timeout on each item to be able to substitute a missing item with a fallback (which is a function of previous item), and then keep streaming from the original source? Note, that if an item is not coming even after fallback, the same timeout policy should apply (that is timeout interval restarts from the latest fallback)
Existing operator timeout(timeoutSelector, other) is not suitable, as the sequence terminates after the fallback (other).
Trying to split the source into window(1) and then applying timeout() on each window does not work either, as there is no previous item available to to feed the timeoutSelector.
Is there any graceful way to do this?

Answer 1

You can achieve this via publish(Func1) trick:
TestScheduler s = Schedulers.test();
Scheduler.Worker w = s.createWorker();
PublishSubject<Long> source = PublishSubject.<Long>create();
PublishSubject<Long> other = PublishSubject.create();
source
.publish(o -> {
AtomicReference<Long> last = new AtomicReference<>();
return o
.doOnNext(last::set)
.doOnCompleted(() -> other.onCompleted())
.timeout(75, TimeUnit.MILLISECONDS, s)
.doOnError(e -> {
if (last.get() != null) {
other.onNext(- last.get());
}
})
.retry();
}
).mergeWith(other)
.forEach(System.out::println);
w.schedule(() -> source.onNext(1L), 0, TimeUnit.MILLISECONDS);
w.schedule(() -> source.onNext(2L), 50, TimeUnit.MILLISECONDS);
w.schedule(() -> source.onNext(3L), 150, TimeUnit.MILLISECONDS);
w.schedule(() -> source.onNext(4L), 200, TimeUnit.MILLISECONDS);
w.schedule(() -> source.onNext(5L), 500, TimeUnit.MILLISECONDS);
w.schedule(() -> source.onNext(6L), 550, TimeUnit.MILLISECONDS);
s.advanceTimeBy(1, TimeUnit.SECONDS);
You may need to apply onBackpressureXXX before publish and within mergeWith.