ios – 如何在警报控制器的文本框中设置数字键盘[swift]

我试图呼叫一个具有文本字段的警报控制器.但是我想立即显示数字键盘,因为用户只能输入数字.
我尝试了以下,但它不工作.

let alert = UIAlertController(title: "New rating",message: "Rate this!",preferredStyle: UIAlertControllerStyle.Alert)

  let cancelAction = UIAlertAction(title: "Cancel",style: .Default,handler: { (action: UIAlertAction!) in })

  let saveAction = UIAlertAction(title: "Save",handler: { (action: UIAlertAction!) in

    let textField = alert.textFields![0] as UITextField
    textField.keyboardType = UIKeyboardType.numberPad

    self.updaterating(textField.text)
  })

  alert.addTextFieldWithConfigurationHandler { (textField: UITextField!) in }

  alert.addAction(cancelAction)
  alert.addAction(saveAction)

  self.presentViewController(alert,animated: true,completion: nil)

alert.addTextField(configurationHandler: { textField in
    textField.keyboardType = .numberPad
})