How to convert Observable<Type> to Single<Type> in RxSwift?

Question

I currently have a method that returns an observable. However, it only ever returns one element.

func getMyResponse(queryID: String) -> Observable<Response> {
    return observable
}

I am trying to refactor this code from returning Observable<Response> to Single<Response>.

func getMyResponse(queryID: String) -> Single<Response> {
    return observable.first()
}

The above is not working and gives the following error:

Cannot convert return expression of type 'PrimitiveSequence<SingleTrait, Response?>' to return type 'Single<Response>`.

Anyone know how to fix this issue? Any insight would be much appreciated!

Answer 1

You are telling the compiler that you will return a Single<Response> from the function, but you are actually returning a Single<Response?>. That's an error.

Note that Single<Response?> is a typealias of PrimitiveSequence<SingleTrait, Response?>.

For more information. You could do either of the following:

func option1(queryID: String) -> Single<Response> {
    return observable.asSingle()
}

func option2(queryID: String) -> Single<Response?> {
    return observable.first()
}

For option1... If the observable completes without emitting a value the Single will emit an error RxError.noElements. If the observable emits a second value before completing, the asSingle() will convert that second value into an error RxError.moreThanOneElement

For option2... If the observable completes without emitting a value, it will emit nil and complete. And the observable will be disposed before it gets a chance to emit a second value.