self.mainTextField.delegate = self

  self.mainTextField(self, action: #selector(self.textFieldDidChange(textField:)), for: .editingChanged)

func textFieldDidChange(textField: UITextField) {
        if textField == self.mainTextField {

            // Some locales use different punctuations.
            var textFormatted = textField.text?.replacingOccurrences(of: ",", with: "")
            textFormatted = textFormatted?.replacingOccurrences(of: ".", with: "")

            let numberFormatter = NumberFormatter()
            numberFormatter.numberStyle = .decimal
            if let text = textFormatted, let textAsInt = Int(text) {
                textField.text = numberFormatter.string(from: NSNumber(value: textAsInt))
            }
        }
    }