Using data from another observable

Question

I'm failing to get into the ReactiveX mindset or the codebase I am working in is just poorly written.

Suppose I have some Observable A() and I need data from another Observable B() in order to do validation on data coming through A, how do I accomplish this in RxJava (would prefer RxKotlin implementation). Note that both A and B return a Single of a List of objects.

fun B(): Single<List<Bar>> {
  ...
}

fun A() : Single<List<Foo>> {
  Single.just(readRecords()).map { record ->
    // val bar = B.getAll()??? This seems like an anti-pattern and I'm not sure if it would necessarily be right to .subscribe()???

    if (bar.contains(record)) {
      // ... some validation
    }
  }
}

Update 1: Should emphasize that the validation requires multiple sources so you could have B, C, D, etc.

Answer 1

So you can use .map {} operator:

fun A() : Single<List<Foo>> {
    return B.getAll().map { allFromB ->  
        val record = readRecords()
        if (allFromB.contains(record)) {
           // ... some validation
        }
        ...
    }
}

UPDATE if you have few of such Observables you need to use Observables.combineLatest() or Observable.combineLatest() (depends on RX version you use):

fun B(): Single<List<BarB>> {
   ...
}

fun C(): Single<List<BarC>> {
   ...
}

fun D(): Single<List<BarD>> {
   ...
}

fun A() : Single<List<Foo>> {
    return Observable.combineLatest(B.getAll(), C.getAll(), D.getAll) { barB, barC, barD -> 
            val record = readRecords()
            //Do your staff here with all this vals and return List<Foo>
        }
}