PromisifiedAlert.swift

// Data structure to hold the title of the UIAlertAction and the value that will resolve the promise
struct AlertCommand<T> {
  let title: String
  let returnValue: T
}

// Promisified implementation of the userTapDialog function
func userTapDialog() {
  _ = presentAlertVC(
    title: "Question!",
    message: "What's your favourite color?",
    commands: [
      AlertCommand(title: "Red", returnValue: "Red"),
      AlertCommand(title: "Blue", returnValue: "Blue"),
  ]).then(in: .main) { dialogResult in
    self.localState.color = dialogResult
  }
}

// Helper that allows to transform an alert control with callbacks into a promise
func presentAlertVC<T>(title: String, message: String, commands: [AlertCommand<T>]) -> Promise<T> {
  // Create the promise in the main thread because we need to present it. 
  // Being UI code, it has to run in the main thread.
  return Promise<T>(in: .main) { resolve, _, _ in
    let alertControl = UIAlertController(
      title: title,
      message: message,
      preferredStyle: .alert
    )

    // add commands
    commands.forEach { command in
      alertControl.addAction(UIAlertAction(title: command.title, style: .default, handler: { _ in
        resolve(command.returnValue)
      }))
    }

    // present alert
    self.present(alertControl, animated: true, completion: nil)
  }
}