享元模式

享元模式结构：

 

using Newtonsoft.Json;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace DesignPattern.StructuralPattern
{
    #region 享元模式
    //定义：不在每个对象中保存所有数据，而是通过共享对象所共有的相同状态，减少内存压力
    //优点：降低频繁创建对象造成的内存压力
    //缺点：系统变得更复杂
    //使用场景：对象
    //实现方式：
    /*
     * 
        1.将需要改写为享元的类成员变量拆分为两个部分：
            内在状态： 包含不变的，可在许多对象中重复使用的数据的成员变量。
            外在状态： 包含每个对象各自不同的状态数据的成员变量。
        2.保留类中表示内在状态的成员变量，这些变量仅可在构造函数中获得初始数值,初始化后不能修改。
        3.外在状态所需数据由客户端传入，调用享元类中方法，作为参数传入。
        4.创建工厂类来管理享元缓存池，负责在新建享元时检查已有的享元。
          客户端通过享元的内在状态作为参数key，传递给工厂来请求目标享元。
        5.客户端必须存储和计算外在状态的数值， 因为只有这样才能调用享元对象的方法。
          也可将外在状态和引用享元的成员变量移动到单独的情景类中。
  
     */

    #endregion


    public abstract class FlyweightPattern
    {
        protected Car _sharedData = new Car();
        public FlyweightPattern(Car car)
        {
            _sharedData.Company = car.Company;
            _sharedData.Model = car.Model;
            _sharedData.Color = car.Color;
        }

        public abstract void PassUniqueState(Car uniqueData);
    }

    public class Flyweight : FlyweightPattern
    {
        public Flyweight(Car car) : base(car)
        {

        }

        //use uniqueData to do something
        public override void PassUniqueState(Car uniqueData)
        {
            string shared = JsonConvert.SerializeObject(this._sharedData);
            string unique = JsonConvert.SerializeObject(uniqueData);

            Console.WriteLine($"shared data = {shared},unique data = {uniqueData.Owner} + {uniqueData.Number}");
        }
    }

    public class Car
    {
        //外部状态 作为参数传入
        public string Owner { get; set; }

        public string Number { get; set; }


        //共享数据作为内部状态
        public string Company { get; set; }

        public string Model { get; set; }

        public string Color { get; set; }
    }


    public class FlyweightFactory
    {
        private Dictionary<string, FlyweightPattern> cache = new Dictionary<string, FlyweightPattern>();
        public FlyweightFactory(params Car[] sharedData)
        {
            foreach (var item in sharedData)
            {
                GetFlyweight(item);
            }
        }

        public FlyweightPattern GetFlyweight(Car car)
        {
            string key = GetKey(car);
            if (cache.ContainsKey(key))
            {
                return cache[key];
            }
            else
            {
                FlyweightPattern flyweight = new Flyweight(car);
                cache.Add(key, flyweight);
                return flyweight;
            }
        }

        private string GetKey(Car car)
        {
            return string.Join("_", car.Company, car.Model, car.Color);
        }

        public void ListFlyweight()
        {
            Console.WriteLine($"total count: {cache.Count}");
            foreach (var item in cache)
            {
                Console.WriteLine($"{item.Key}");
            }
        }
    }

    public class FlyweightClient
    {
        public static void Test()
        {
            // 添加需要缓存的数据

            var factory = new FlyweightFactory(
                new Car { Company = "Chevrolet", Model = "Camaro2018", Color = "pink" },
                new Car { Company = "Mercedes Benz", Model = "C300", Color = "black" },
                new Car { Company = "Mercedes Benz", Model = "C500", Color = "red" },
                new Car { Company = "BMW", Model = "M5", Color = "red" },
                new Car { Company = "BMW", Model = "X6", Color = "white" }
            );

            Car car = new Car
            {
                Company = "Chevrolet",
                Model = "Camaro2018",
                Color = "pink",
                Owner = "Howie",
                Number = "H666"
            };

            addCarToPoliceDatabase(factory, car);

            car.Owner = "Spike";
            car.Number = "S888";

            //实现复用
            addCarToPoliceDatabase(factory, car);
        }

        public static void addCarToPoliceDatabase(FlyweightFactory factory, Car car)
        {
            Console.WriteLine("\nClient: Adding a car to database.");

            var flyweight = factory.GetFlyweight(car);

            flyweight.PassUniqueState(car);
            factory.ListFlyweight();
        }
    }
}