Using flatMap in RestManager with generic function not working - swift

Question

I am new to Combine, so I wanted to create class RestManager for networking with generic
fetchData function. Function is returning AnyPublisher<Result<T, ErrorType>, Never> where ErrorType is enum with .noInternetConnection, .empty and .general cases.
I tried to use URLSession with dataTaskPublisher and flatMap
func fetchData<T: Decodable>(url: URL) -> AnyPublisher<Result<T, ErrorType>, Never> {
URLSession
.shared
.dataTaskPublisher(for: url)
.flatMap { (data, response) -> AnyPublisher<Result<T, ErrorType>, Never> in
switch response.result {
case .success(let data):
if let data = try? JSONDecoder().decode(T.self, from: data){
return Just(data).eraseToAnyPublisher()
}
case .failure(let error):
if let error = error as? URLError {
switch error.code {
case .notConnectedToInternet, .networkConnectionLost, .timedOut:
return Fail(ErrorType.noInternetConnection).eraseToAnyPublisher()
case .cannotDecodeRawData, .cannotDecodeContentData:
return Fail(ErrorType.empty).eraseToAnyPublisher()
default:
return Fail(ErrorType.general).eraseToAnyPublisher()
}
}
}
}
.eraseToAnyPublisher()
}
But I am getting
Cannot convert return expression of type 'AnyPublisher<AnyPublisher<Result<T, ErrorType>, Never>.Output, URLSession.DataTaskPublisher.Failure>' (aka 'AnyPublisher<AnyPublisher<Result<T, ErrorType>, Never>.Output, URLError>') to return type 'AnyPublisher<Result<T, ErrorType>, Never>' error.

Answer 1

There are several major flows in your implementation.
Firstly, you shouldn't be using Result as the Output type of the Publisher and Never as its Failure type. You should be using T as the Output and ErrorType as Failure.
Second, you need tryMap and mapError, not flatMap.
Lastly, you are handling the result of dataTaskPublisher completely wrong. When dataTaskPublisher fails, it emits an error, so you need to handle that in mapError. When it succeeds, it emits its result as data, so you need to be decoding that, not response.
func fetchData<T: Decodable>(url: URL) -> AnyPublisher<T, ErrorType> {
URLSession
.shared
.dataTaskPublisher(for: url)
.tryMap { data, _ in
return try JSONDecoder().decode(T.self, from: data)
}
.mapError { error -> ErrorType in
switch error {
case let urlError as URLError:
switch urlError.code {
case .notConnectedToInternet, .networkConnectionLost, .timedOut:
return .noInternetConnection
case .cannotDecodeRawData, .cannotDecodeContentData:
return .empty
default:
return .general
}
default:
return .general
}
}
.eraseToAnyPublisher()
}