Combine 3 single results - one stream

Question

I'm trying to do calculating one by one, asynchronously.

fun method1(): Int{
return 2+2
}

fun method2(value: Int): Int{
return value * 2
}

fun method3(value: Int): Int{
return value * 3
}

Now I want method 2 work after method 1 and take result from her. Also method 3 work after method 2 and take result from her. Is it possible to do that with rxkotlin and make it in one stream ?

Answer 1

If you want to chain that methods invocation you can do as presented below:

   fun chain(): Single<Int> {
        return Single.just(method1())
                .map { result1 -> method2(result1) }
                .map { result2 -> method3(result2) }
   }

However, if operations in method1, method2 or method3 are asynchronous I would suggest to wrap them in Single.

For reference map vs flatMap take a look on:

https://medium.com/mindorks/rxjava-operator-map-vs-flatmap-427c09678784

Answer 2

This can be done using flatMap as follows :

fun performAsyncCalculation(): Observable<Int> {
        return method1()
            .flatMap { result1 -> method2(result1) }
            .flatMap { result2 -> method3(result2) }
}

where the methods are :

fun method1(): Observable<Int> {
    return Observable.create { emitter ->
        // perform calculation to produce requiredInt1
        emitter.onNext(requiredInt1)
        emitter.onComplete()
}

fun method2(requiredInt1: Int): Observable<Int> {
    return Observable.create { emitter ->
        // perform calculation to produce requiredInt2
        emitter.onNext(requiredInt2)
        emitter.onComplete()
}

fun method3(requiredInt2: Int): Observable<Int> {
    return Observable.create { emitter ->
        // perform calculation to produce requiredInt3
        emitter.onNext(requiredInt3)
        emitter.onComplete()
}