RxSwift - how to chain observables sequentially

Question

Assume I have array of Ints:

var items = [1, 2, 3, 4, 5]

and a function that takes Int argument and basing on it sends network request:

Answer 1

Operator .concatMap() guarantees that order remains intact, as opposed to .flatMap()

i.e.

Observable.from([1,2,3,4,5]).concatMap( /* your request code */ )

UPDATE

let source = Observable.from([1,2,3,4,5])
let trigger = BehaviorSubject<Int>(value: 0)
let feed = Observable.zip(source, trigger) { return $0.0 }

feed.concatMap {
    /* your request code */
    trigger.onNext(0) // possibly in .onComplete of the service call
}