APIClient startDecodableRequest

APIClient.swift

import Foundation

class APIClient {

  // 1 - Error enum
  enum APIClientError: Error {
    case dataTaskFailed(Error)
    case noHTTPURLResponse
    case badHTTPStatusCode(HTTPURLResponse)
    case noData
    case decodingFailed(Error)
    case unknown(Error)
  }

  let session = URLSession.shared

  func startDecodableRequest<T: Decodable>(
    _ request: URLRequest,
    decoder: JSONDecoder = JSONDecoder(),
    completionQueue: DispatchQueue = DispatchQueue.main,
    completion: @escaping ((Result<T, APIClientError>) -> Void)
  ) {

    let task = session.dataTask(with: request,
                                completionHandler: { data, response, error in
      do {
        // 2 - Check error and status code
        if let error = error {
          throw APIClientError.dataTaskFailed(error)
        }

        guard let urlResponse = response as? HTTPURLResponse else {
          throw APIClientError.noHTTPURLResponse
        }
        let validStatusCodes = 200...399
        guard validStatusCodes.contains(urlResponse.statusCode) else {
          throw APIClientError.badHTTPStatusCode(urlResponse)
        }

        // 3 - Decode data
        guard let data = data else {
          throw APIClientError.noData
        }

        let decoded: T
        do {
          decoded = try decoder.decode(T.self, from: data)
        }
        catch {
          throw APIClientError.decodingFailed(error)
        }

        // 4 - Call completion for success
        completionQueue.async {
          completion(.success(decoded))
        }
      }
      catch {
        // 5 - Call completion for failure
        let finalError = error as? APIClientError ?? .unknown(error)
        completionQueue.async {
          completion(.failure(finalError))
        }
      }
    })

    task.resume()
  }

}