RxJava - Any way to combine scan and flatMap?

Question

Let's say I have a function that takes a String and a long and returns a Single<String>.

Single<String> stringAddition(String someString, long value) {
  return Single.just(someString + Long.toString(value));
}

Now I have this Observable...

Observable.interval(1, SECONDS)
  .scan("", (cumulativeString, item) -> {
    // Need to return the result of stringAddition(cummulativeString, item)
  });

I'm at a loss on how to do this. Scan needs me to return a String, but II would like to use the method that returns a Single<String>. To me it seems like I need something that can combine the behaviour of both scan and flatMap. Is there any RxJava2 wizardry that can help me?

Answer 1

You can achieve it as follows. This can be somewhat shortened if stringAddition would have returned Observable

Observable<String> scanned = Observable.interval(1, TimeUnit.SECONDS)
            .scan(
                    Observable.just(""),
                    (cumulativeString, item) ->
                        cumulativeString
                          .flatMap(str -> stringAddition(str, item).toObservable())
            )
            .flatMap(it -> it);