이런 느낌입니다

코드 및 간략한 설명

    func showTextFieldAlert(targetView:UIViewController){

        var productNameTextField:UITextField? //Alertに表示したTextField.Textを入れて使います
        var janCodeTextField:UITextField?
        var deadlineDayTextField:UITextField?

        let alert = UIAlertController(title: "データを追加", message:"データを追加する場合は、値を入力して下さい", preferredStyle: .alert)

        alert.addTextField { (alertProductNameTextField:UITextField) in

            alertProductNameTextField.placeholder = "プレスホルダー"
            productNameTextField = alertProductNameTextField) //

        }

        alert.addTextField { (alertJANCodeTextField:UITextField) in

            alertJANCodeTextField.placeholder = "プレスホルダー"
            janCodeTextField = alertJANCodeTextField

        }

        alert.addTextField { (alertDeadlineDayTextField:UITextField) in

            alertDeadlineDayTextField.placeholder = "プレスホルダー"
            deadlineDayTextField = alertDeadlineDayTextField

        }

        alert.addAction(UIAlertAction(title: "キャンセル", style: .default, handler: nil))

        alert.addAction(UIAlertAction(title: "追加", style: .default, handler: { _ in

            //追加が押された時の処理

         }))

        targetView.present(alert, animated: true, completion: nil)

    }

func calendar(_ calendar: FSCalendar, didSelect date: Date, at monthPosition: FSCalendarMonthPosition) {

           //カレンダーの日付がタップされた時に働く処理

     }

실제로 앱으로 작성한 코드

Alert

extension Alert{   

    func showTextFieldAlert(targetView:UIViewController){

        let realmCRUDModel = RealmCRUDModel()

        var productNameTextField:UITextField?
        var janCodeTextField:UITextField?
        var deadlineDayTextField:UITextField?

        let alert = UIAlertController(title: "データを追加", message:"データを追加する場合は、値を入力して下さい", preferredStyle: .alert)

        alert.addTextField { (alertProductNameTextField:UITextField) in

            alertProductNameTextField.placeholder = "商品名"
            productNameTextField = alertProductNameTextField

        }

        alert.addTextField { (alertJANCodeTextField:UITextField) in

            alertJANCodeTextField.placeholder = "JANコード"
            janCodeTextField = alertJANCodeTextField

        }

        alert.addTextField { (alertDeadlineDayTextField:UITextField) in

            alertDeadlineDayTextField.placeholder = "期限 例:2021年01月01日"
            deadlineDayTextField = alertDeadlineDayTextField

        }

        alert.addAction(UIAlertAction(title: "キャンセル", style: .default, handler: nil))

        alert.addAction(UIAlertAction(title: "追加", style: .default, handler: { _ in

            realmCRUDModel.createRealm(createProductName: (productNameTextField?.text)!, createJANCode: (janCodeTextField?.text)!, createDeadlineDay: (deadlineDayTextField?.text)!)

         }))

        targetView.present(alert, animated: true, completion: nil)

    }

}

FSCalendar

    func calendar(_ calendar: FSCalendar, didSelect date: Date, at monthPosition: FSCalendarMonthPosition) {

        alert.showTextFieldAlert(targetView: self)

     }

끝