How to use combineLatest for two public subjects in RXSwfit - ios

Question

I'm new to RXSwift and I try to use combineLatest to combine the latest results from two public subjects
What I tried to do:
let sub1 = PublicSubject<Type1>()
let sub2 = PublicSubject<Type2>()
NetworkService1.fetch { sub1Value in
sub1.onNext(sub1Value)
}
NetworkService2.fetch { sub21Value in
sub2.onNext(sub2Value)
}
Observable.combineLatest(sub1.asObservable(), sub2.asObservable()) {
val1, val2 in
// do something with val1 and val2
// It seems it never hits this block
}
Not sure I'm doing the right thing.

Answer 1

It's better, if your NetworkService returns Observables. Then you don't have to create PublicSubjects and it's more beautiful.
I would do it like this:
let result1 = NetworkService1.shared.fetch()
let result2 = NetworkService2.shared.fetch()
Observable.combineLatest(result1, result2) { r1, r2 in
// Do stuff with r1, r2 with are APIResult<[YourModel]> and return result
}.subscribe(onNext: { result in
// You need to subscribe to run fetch methods
// Do stuff with result of combine latest
}).disposed(by: disposeBag)
This is example of a fetch method using Alamofire that returns Observable:
func fetch() -> Observable<APIResult<[YourModel]>> {
return Observable<APIResult<[YourModel]>>.create { (observer) -> Disposable in
Alamofire.request(yourURLString,
method: .post,
parameters: nil,
headers: APIManager.headers())
.responseJSON(completionHandler: { dataResponse in
switch dataResponse.result {
case .success(let value):
// parse value to someArray here
observer.onNext(APIResult.success(someArray))
case .failure(_):
guard let code = dataResponse.response?.statusCode else {
observer.onNext(APIResult.failure(APIError.unknownError))
break
}
observer.onNext(APIResult.failure(APIError.networkError(code:code)))
}
observer.onCompleted()
})
return Disposables.create()
}
}
APIResult allows you to pass errors too:
enum APIResult<Value> {
case success(Value)
case failure(APIError)
}

Answer 2

Just a couple of observations first:
There is no PublicSubject, it's PublishSubject (probably just a typo ;)
You don't need to call asObservable() in order to use PublishSubjects as arguments for combineLatest
You have to subscribe to your Observable (in this case Observable.combineLatest), otherwise nothing will happen.
Even if you subscribe correctly to Observable.combineLatest, you won't get the values, that have been emitted before the subscription, so those fetch calls have to be triggered after the subscription.
Since a piece of code is worth a thousand words:
let disposeBag = DisposeBag()
let sub1 = PublishSubject<String>()
let sub2 = PublishSubject<String>()
Observable.combineLatest(sub1, sub2) {
val1, val2 in
// do something with val1 and val2
// IT SHOULD WORK NOW
}.subscribe().disposed(by: disposeBag)
NetworkService1.fetch { sub1Value in
sub1.onNext(sub1Value)
}
NetworkService2.fetch { sub2Value in
sub2.onNext(sub2Value)
}