Using Combine operators to transform Future into Publisher - swift

Question

I'm using an API (Firebase) that exposes an async interface for most of its method calls. For every request I make through my own API, I want to add a user's token as a header, if such a token exists. I'm trying to make the entire process part of the same pipeline in Combine.
I have the following code:
struct Response<T> {
let value: T
let response: URLResponse
}
...
func makeRequest<T: Decodable>(_ req: URLRequest, _ decoder: JSONDecoder = JSONDecoder()) -> AnyPublisher<T, Error> {
var request = req
return Future<String?, Error> { promise in
if let currentUser = Auth.auth().currentUser {
currentUser.getIDToken() { (idToken, error) in
if error != nil {
promise(.failure(error!))
} else {
promise(.success(idToken))
}
}
} else {
promise(.success(nil))
}
}
.map { idToken -> URLSession.DataTaskPublisher in
if idToken != nil {
request.addValue("Bearer \(idToken!)", forHTTPHeaderField: "Authorization")
}
return URLSession.shared.dataTaskPublisher(for: request)
}
.tryMap { result -> Response<T> in
let value = try decoder.decode(T.self, from: result.data)
return Response(value: value, response: result.response)
}
.receive(on: DispatchQueue.main)
.map(\.value)
.eraseToAnyPublisher()
}
I get an error inside tryMap operator when trying to JSON decode the response data:
Value of type 'URLSession.DataTaskPublisher' has no member 'data'
I'm still wrapping my head around Combine, but can't understand what I'm doing wrong here. Any help would be greatly appreciated!

Answer 1

You are trying to map to another publisher. Most of the time, this is a sign that you need flatMap. If you use map instead, you'll get a publisher that publishes another publisher, which is almost certainly not what you want.
However, flatMap requires that the upstream publisher (the promise) has the same failure type as the publisher that you are mapping to. However, they aren't the same in this case, so you need to call mapError on the data session publisher to change its error type:
return Future<String?, Error> { promise in
promise(.failure(NSError()))
}
// flatMap and notice the change in return type
.flatMap { idToken -> Publishers.MapError<URLSession.DataTaskPublisher, Error> in
if idToken != nil {
request.addValue("Bearer \(idToken!)", forHTTPHeaderField: "Authorization")
}
return URLSession.shared.dataTaskPublisher(for: request)
// change the error type
.mapError { $0 as Error } // "as Error" isn't technically needed. Just for clarity
}
.tryMap { result -> Response<T> in
let value = try decoder.decode(T.self, from: result.data)
return Response(value: value, response: result.response)
}
.receive(on: DispatchQueue.main)
.map(\.value)
.eraseToAnyPublisher()