async-await-sample.swift

func getMoviePackage(for movieID: String, completion: MoviePackageFetchCompletion? = nil) {
  var movieURL: URL?
  var subtitleURL: URL?

  var movieFile: File?
  var subtitleFile: File?

  /// The `DispatchGroup` for the parallel URL fetch operations
  let urlFetchGroup = DispatchGroup()

  urlFetchGroup.enter()
  service.getURL(for: movieID, type: .movie) { result, error in
    guard error == nil else {
      urlFetchGroup.leave()
      return
    }

    guard let url = result.url else {
      urlFetchgroup.leave()
      return
    }

    movieURL = url
    urlFetchGroup.leave()
  }

  urlFetchGroup.enter()
  service.getURL(for: movieID, type: .subtitle) { result, error in
    guard error == nil else {
      urlFetchGroup.leave()
      return
    }

    guard let url = result.url else {
      urlFetchgroup.leave()
      return
    }

    subtitleURL = url
    urlFetchGroup.leave()
  }

  /// Commence downloading when URL fetch has finished for both
  /// types.
  urlFetchGroup.notify(queue: .main) {
    /// Exit early and trigger callback with error if either of
    /// the URLs are nil.
    guard let theMovieURL = movieURL,
          let theSubtitleURL = subtitleURL else {
            completion(nil, .serviceError)
            return
    }

    /// If both URLs are not nil, continue onto the download phase
    let downloadGroup = DispatchGroup()
    
    /// ... repeat with downloading files ...
    
    downloadGroup.notify(queue: .main) {
      guard let movie = movieFile,
            let subtitle = subtitleFile
        else {
          completion(nil, .serviceError)
          return
      }

      completion(MoviePackage(movie: movie, subtitle: subtitle),
                              nil)
    }
  }
}