How do I sequentially and nonparallel loop through an array in RxSwift? - rx-swift

Question

I have a list of objects i need to send to a server and i would like to do this one after the other (not in parallel). After all objects have been sent and there was no error i want to run additional Observables which do different things.
let objects = [1, 2, 3]
let _ = Observable.from(objects).flatMap { object -> Observable<Void> in
return Observable.create { observer in
print("Starting request \(object)")
DispatchQueue.main.asyncAfter(deadline: .now() + 2) { // one request takes ~2sec
print("Request \(object) finished")
observer.onNext(Void())
observer.onCompleted()
}
return Disposables.create()
}
}.flatMap { result -> Observable<Void> in
print("Do something else (but only once)")
return Observable.just(Void())
}.subscribe(
onNext: {
print("Next")
},
onCompleted: {
print("Done")
}
)
What i get is
Starting request 1
Starting request 2
Starting request 3
Request 1 finished
Do something else (but only once)
Next
Request 2 finished
Do something else (but only once)
Next
Request 3 finished
Do something else (but only once)
Next
Done
The whole process ends after 2 sec. What i want is
Starting request 1
Request 1 finished
Starting request 2
Request 2 finished
Starting request 3
Request 3 finished
Do something else (but only once)
Next
Done
The whole sequence should end after 6 seconds (because it's not executed parallel).
I got this to work with a recursive function. But with lots of requests this ends in a deep recursion stack which i would like to avoid.

Answer 1

Use concatMap instead of flatMap in order to send them one at a time instead of all at once. Learn more here:
RxSwift’s Many Faces of FlatMap
Then to do something just once afterwards, use toArray(). Here is a complete example:
let objects = [1, 2, 3]
_ = Observable.from(objects)
.concatMap { object -> Observable<Void> in
return Observable.just(())
.debug("Starting Request \(object)")
.delay(.seconds(2), scheduler: MainScheduler.instance)
.debug("Request \(object) finished")
}
.toArray()
.flatMap { results -> Single<Void> in
print("Do something else (but only once)")
return Single.just(())
}
.subscribe(
onSuccess: { print("done") },
onError: { print("error", $0) }
)