Una breve classe da cui prendere spunto:

// MARK: - WebServiceError

public enum WebServiceError: Error {
    case badURL
    case error
    case nodata
    case jsonError
    // ...
}

// MARK: - WebService

public class WebService {
    let baseUrl: String
    public enum HttpMethod: String {
        case GET
        case POST
        case PUT
        case TRACE
        case DELETE
    }

    public init?(baseUrl: String) {
        guard !baseUrl.isEmpty else {
            return nil
        }
        if baseUrl.last! == "/" {
            self.baseUrl = baseUrl
        } else {
            self.baseUrl = baseUrl+"/"
        }
    }

    public func request<T>(with httpMethod: HttpMethod = .GET,
                    of _: T.Type,
                    on path: String, _ completion: @escaping (Result<T, WebServiceError>) -> Void) -> Void where T: Codable
    {
        guard let url = URL(string: baseUrl + path) else {
            completion(.failure(.badURL))
            return
        }
        var request = URLRequest(url: url)
        request.httpMethod = httpMethod.rawValue
        request.setValue("application/json; charset=utf-8", forHTTPHeaderField: "Content-Type")
        request.setValue("application/json; charset=utf-8", forHTTPHeaderField: "Accept")

        let task = URLSession.shared.dataTask(with: request) { data, response, error in

            var result: Result<T, WebServiceError>

            defer {
                completion(result)
            }

            guard error == nil else {
                result = .failure(.error)
                return
            }

            guard let data = data else {
                result = .failure(.nodata)
                return
            }

            let decoder = JSONDecoder()
            decoder.dateDecodingStrategy = .iso8601

            guard let response = try? decoder.decode(T.self, from: data) else {
                result = .failure(.jsonError)
                return
            }
            result = .success(response)
        }

        task.resume()
    }
}