rxkotlin parallel execute and collect to single

Demo.kt

import io.reactivex.Flowable
import io.reactivex.schedulers.Schedulers
import java.util.*

class Demo {

    data class Result(val a: String, val b: String, val c: String)
    fun start() {
        val a = Flowable.just("A")
        val b = Flowable.just("B")
        val c = Flowable.just("C")

        fun slowApiCall(a: String): String {
            Thread.sleep(300)
            return a
        }

        fun asyncApiCall(a: String): Flowable<String> {
            return Flowable.fromCallable {
                slowApiCall(a)
            }
        }
        
        //async sequential collect to single
        val data = Flowable.concat(a, b, c)
                .concatMap ({
                    asyncApiCall(it).subscribeOn(Schedulers.io())
                    //maxConcurrency
                }, 3)
                .collect({ mutableListOf<String>() }, { acc, value ->
                    println(value) // A, B, C
                    acc.add(value)
                })
                .map { Result(it[0], it[1], it[2]) }
                .blockingGet()
        println("async collect ${Date().toInstant()}: $data")
    }
}

fun main(args: Array<String>) {
    Demo().start()
}