PromisifiedDelegate.swift

// Data structure for a Movie
struct Movie: Codable {
  let id: String
  let title: String
  let description: String
  let director: String
  let producer: String
  let releaseDate: String
  let rtScore: String
  let url: URL
}

// Function that starts the download and updates the UI afterward
func downloadMovies() {
  self.promisifyDownload(url: URL(string: "https://ghibliapi.herokuapp.com/films")!)
    .then(in: .main) { [weak self] movies in
    // download finished. Update the state.
    self?.localState.movies = movies.count
  }
  .catch(in: .main) { [weak self] error in
    // download errored, present the alert with the error.
    let vc = UIAlertController(title: "Error", message: "Error obtained: \(error.localizedDescription)", preferredStyle: .alert)
    vc.addAction(UIAlertAction(title: "Ok", style: .default, handler: nil))
    self.present(vc, animated: true, completion: nil)
  }
}

// Function that transform the download in a promise.
func promisifyDownload(url: URL) -> Promise<[Movie]> {
  return Promise<[Movie]>(in: .background) { [weak self] resolve, reject, _ in
    // data delegate that will resolve or reject the promise.
    let dataDelegate = DataDelegate(resolve: resolve, reject: reject)
                                            
    let session = URLSession(configuration: .default, delegate: dataDelegate, delegateQueue: nil)
    let task = session.dataTask(with: url)
    // start the download.                                            
    task.resume()
  }
}

// Delegate for the download of the movies.
class DataDelegate: NSObject, URLSessionDataDelegate {

  let resolve: ([Movie]) -> ()
  let reject: (Error) -> ()

  init(resolve: @escaping ([Movie]) -> (), reject: @escaping (Error) -> ()) {
    self.resolve = resolve
    self.reject = reject
  }

  func urlSession(_ session: URLSession, dataTask: URLSessionDataTask, didReceive data: Data) {
    do {
      // decode the received data
      let decoder = JSONDecoder()
      decoder.keyDecodingStrategy = .convertFromSnakeCase
      let movies = try decoder.decode([Movie].self, from: data)
      // resolve the promise so that the ui can be updated.
      self.resolve(movies)
    } catch {
      self.reject(error)
    }
  }
}