design-patterns - 策略和享元模式

Question

我读到“策略对象通常是很好的享元”(来自可重用面向对象软件的设计模式元素)，我想知道如何实现这一点。我没有在互联网上找到任何示例。

右下方的代码 (C#) 是否遵循了这个想法？

谢谢!

using System;
using System.Collections.Generic;

namespace StrategyFlyweight
{
    class Program
    {
        static void Main(string[] args)
        {
            Client client = new Client();
            for(int i = 1; i <= 10;i++)
            {
                client.Execute(i);
            }
            Console.ReadKey();
        }
    }

    public interface IStrategy
    {
        void Check(int number);
    }

    public class ConcreteStrategyEven : IStrategy
    {
        public void Check(int number)
        {
            Console.WriteLine("{0} is an even number...", number);
        }
    }

    public class ConcreteStrategyOdd : IStrategy
    {
        public void Check(int number)
        {
            Console.WriteLine("{0} is an odd number...", number);
        }
    }

    public class FlyweightFactory
    {
        private Dictionary<string, IStrategy> _sharedObjects = new Dictionary<string, IStrategy>();

        public IStrategy GetObject(int param)
        {
            string key = (param % 2 == 0) ? "even" : "odd";

            if (_sharedObjects.ContainsKey(key))
                return _sharedObjects[key];
            else
            {
                IStrategy strategy = null;
                switch (key)
                {
                    case "even":
                        strategy = new ConcreteStrategyEven();
                        break;
                    case "odd":
                        strategy = new ConcreteStrategyOdd();
                        break;
                }
                _sharedObjects.Add(key, strategy);
                return strategy;
            }
        }
    }

    public class Client
    {
        private IStrategy _strategy;
        private FlyweightFactory _flyweightFactory = new FlyweightFactory();

        public void Execute(int param)
        {
            ChangeStrategy(param);
            _strategy.Check(param);
        }

        private void ChangeStrategy(int param)
        {
            _strategy = _flyweightFactory.GetObject(param);
        }
    }
}

Answer 1

我认为，为了在这里正确实现享元模式，您的工厂方法应该始终返回特定策略的相同实例(如 ConcreteStrategyEven)，而不是每次都构造一个新实例。

如果我没记错的话，说 Strategy 对象是好的享元的意思是它们通常不封装任何状态(因为它们代表算法而不是实体)并且可以重用。

这是享元工厂示例的链接:http://www.java2s.com/Code/Java/Design-Pattern/FlyweightFactory.htm .请特别注意这部分:

  public synchronized FlyweightIntr getFlyweight(String divisionName) {
    if (lstFlyweight.get(divisionName) == null) {
      FlyweightIntr fw = new Flyweight(divisionName);
      lstFlyweight.put(divisionName, fw);
      return fw;
    } else {
      return (FlyweightIntr) lstFlyweight.get(divisionName);
    }
  }

这里在工厂方法中，一个新的 FlyweightIntr 只有在正确的不可用时才被初始化；否则从 lstFlyweight 中检索。