ISEbd-21.Fedotov.I.A.Diner/Diner/DinerShopImplement/DataFileSingleton.cs

using DinerFileImplement.Models;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Xml.Linq;

namespace DinerFileImplement
{
    internal class DataFileSingleton
    {
        private static DataFileSingleton? instance;

        private readonly string FoodFileName = "Food.xml";

        private readonly string OrderFileName = "Order.xml";

        private readonly string SnackFileName = "Snack.xml";

        public List<Food> Foods { get; set; }
        public List<Order> Orders { get; private set; }
        public List<Snack> Snacks { get; private set; }

        private DataFileSingleton() {
            Foods = LoadData(FoodFileName, "Food", x => Food.Create(x)!)!;
            Snacks = LoadData(SnackFileName, "Snack", x => Snack.Create(x)!)!;
            Orders = LoadData(OrderFileName, "Order", x => Order.Create(x)!)!;
        }
        public static DataFileSingleton GetInstance() { 
            if (instance == null) instance = new DataFileSingleton();
            return instance;
        }
        public void SaveFoods() => SaveData(Foods, FoodFileName, "Foods",
                                                x => x.GetXElement);
        public void SaveSnacks() => SaveData(Snacks, SnackFileName, "Snacks",
                                                x => x.GetXElement);
        public void SaveOrder() => SaveData(Orders, OrderFileName, "Orders",
                                                x => x.GetXElement);
        private static void SaveData<T>(List<T> data, string filename, string xmlNodeName,
                                        Func<T, XElement> selectFunction)
        {
            if (data != null) { 
                new XDocument(new XElement(xmlNodeName, data.Select(selectFunction).ToArray()))
                    .Save(filename);
            }
        }
        private static List<T>? LoadData<T>(string filename, string xmlNodeName, 
                                            Func<XElement, T> selectFunction)
        {
            if (File.Exists(filename))
            {
                return XDocument.Load(filename)?.Root?.Elements(xmlNodeName)?
                                    .Select(selectFunction)?.ToList();
            }
            return new List<T>();
        }
    }
}