Combining multiple flowables

Question

I am migrating my project from Spring to Ktor, and decided to replace the implementation of reactive streams which was initially Reactor to RxJava 2. Although I faced some problem when trying to combine multiple streams into single one at the end of the reactive pipeline. Here it is how it looks like:

internal interface Aggregator {
    fun acquireSomethingFromSomewhere(keyword: String): Flowable<Some>
}

fun acquireSomething(keyword: String) = Flowable
    .fromIterable(aggregators)
    .map { it.acquireSomethingFromSomewhere(keyword) }
    .flatMap { ??? }

The thing is, each call of acquireSomethingFromSomewhere returns Flowable<Some>, is there any operator which could help me combine them in the one stream at the end? In Reactor I just used:

fun acquireSomething(keyword: String) = Flux
    .fromIterable(aggregators)
    .map { it.acquireSomethingFromSomewhere(keyword) }
    .flatMap { Flux.concat(it) }

But in RxJava I can't find any operator which could solve my problem, as each of them takes Publisher as an argument, and Flowable does not implement it.

Does `.flatMap { it.acquireSomethingFromSomewhere(keyword) }` suit your needs? — Caramiriel, Jan 26 '19 at 20:59

score 1 · Answer 1 · answered Jan 27 '19 at 06:01

First of all, if the function you provide in map returns a Flowable, you will end up with nested Flowables (a.k.a. Flowable<Flowable<T>>), which is not what you probably want. This is because the map function only transform the element inside the container (T) -> R (in this case the container is Flowable). In your case, you want to transform the element inside the first container returning a new container (T) -> Flowable<R>, this function is called flatMap. In the case of Rx you have more functions (operators) depending on their behavior, like concatMap and switchMap, but the signatures are the same.

Example

fun acquireSomething(keyword: String) = Flowable
    .fromIterable(aggregators)
    .flatMap { it.acquireSomethingFromSomewhere(keyword) }

PS

If you want to know more about the theory behind, you can follow Arrow-kt documentation of Functor and Monad

Combining multiple flowables

1 Answers1