HTTP POST request using Swift Combine - swift

Question

I'm fairly new to Combine declarative API. I'm trying to implement a generic network layer for a SwiftUI application. For all requests that receive data I understand how to structure the data flow.
My problem is that I have some HTTP POST requests that returns no data. Only a HTTP 200 on success. I can't figure out how to create a publisher that will handle a decoding that can fail since there could be not data in the body of the response. Here's what I tried:
func postResource<Resource: Codable>(_ resource: Resource, to endpoint: Endpoint) -> AnyPublisher<Resource?, NetworkError> {
return Just(resource)
.subscribe(on: queue)
.encode(encoder: JSONEncoder())
.mapError { error -> NetworkError in
return NetworkError.encoding(error)
}
.map { data -> URLRequest in
return endpoint.makeRequest(with: data)
}
.tryMap { request -> Resource? in
self.session.dataTaskPublisher(for: request)
.tryMap { data, response -> Data in
guard let httpUrlResponse = response as? HTTPURLResponse else { throw NetworkError.unknown }
guard (200 ... 299).contains(httpUrlResponse.statusCode) else { throw NetworkError.error(for: httpUrlResponse.statusCode) }
return data
}
.tryMap { data -> Resource? in
return try? JSONDecoder().decode(Resource.self, from: data)
}
}
.mapError({ error -> NetworkError in
switch error {
case is Swift.DecodingError:
return NetworkError.decoding(error)
case let urlError as URLError:
return .urlError(urlError)
case let error as NetworkError:
return error
default:
return .unknown
}
})
.eraseToAnyPublisher()
}
The compiler is complaining with the following error on tryMap row:
Declared closure result 'Publishers.TryMap<URLSession.DataTaskPublisher, Resource?>' is incompatible with contextual type 'Resource?'
Anyone has an idea?
Thanks!

Answer 1

enum NetworkError: Error {
case encoding(Error)
case error(for: Int)
case decoding(Error)
case urlError(URLError)
case unknown
}
func postResource<Resource: Codable>(_ resource: Resource, to endpoint: Endpoint) -> AnyPublisher<Resource?, NetworkError> {
Just(resource)
.subscribe(on: queue)
.encode(encoder: JSONEncoder())
.mapError { error -> NetworkError in
NetworkError.encoding(error)
}
.map { data -> URLRequest in
endpoint.makeRequest(with: data)
}
.flatMap { request in // the key thing is here you should you use flatMap instead of map
URLSession.shared.dataTaskPublisher(for: request)
.tryMap { data, response -> Data in
guard let httpUrlResponse = response as? HTTPURLResponse else { throw NetworkError.unknown }
guard 200 ... 299 ~= httpUrlResponse.statusCode else { throw NetworkError.error(for: httpUrlResponse.statusCode) }
return data
}
.tryMap { data -> Resource? in
try? JSONDecoder().decode(Resource.self, from: data)
}
}
.mapError({ error -> NetworkError in
switch error {
case is Swift.DecodingError:
return NetworkError.decoding(error)
case let urlError as URLError:
return .urlError(urlError)
case let error as NetworkError:
return error
default:
return .unknown
}
})
.receive(on: DispatchQueue.main)
.eraseToAnyPublisher()
}