ResultWithDelegate.swift

protocol ThingsServiceDelegate {
  func service(_ ThingsService, didFinishWithResult: Result<[Thing]>)
}

class ThingsService {
  private var delegate: ThingsServiceDelegate
  init(with delegate: ThingsServiceDelegate) {
    self.delegate = delegate
  }

  func getThings() {
    sendHttpRequest {
      var result: Result<[Thing]> = ...
      ...
      delegate.service(self, didFinishWithResult: result)
    }
  }
}