群“模”乱舞之策略模式

2019-04-13 17:29发布生成海报

站内文章 / 模拟电子

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在上篇博客中我们提到了简单工厂模式，然而在GoF总结的23个设计模式中并没有她的影子，严格意义上来讲，简单工厂模式并不是一种设计模式，因为她根本不符合设计模式的开放—封闭原则，即软件实体如类、模块等应该可以扩展，但是不可修改。对于设计模式的设计原则，将在下一篇博文中介绍，这里我们先来看一个例子：周所周知，超市收银系统都具有计费功能，那么要你做一个能够根据输入的商品单价和数量，以及计费方式进行计算的模块，你如何做？计费方式也就是打不打折，有没有返利优惠等等。那么我们先用简单工厂模式去实现一下。简单工厂实现

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace 商场收银软件
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        double total = 0.0d;

        private void textBox1_TextChanged(object sender, EventArgs e)
        {

        }

        private void label5_Click(object sender, EventArgs e)
        {

        }

        private void Form1_Load(object sender, EventArgs e)
        {
            cbxType.Items.AddRange(new object[] { "正常收费", "满300返100", "打8折" });
            cbxType .SelectedIndex =0;
        }

        private void buttonOK_Click(object sender, EventArgs e)
        {
            CashSuper csuper = CashFactory.createCashAccept(cbxType.SelectedItem.ToString());
            double totalPrices = 0d;
            totalPrices = csuper.acceptCash(Convert.ToDouble(txtPrice.Text) * Convert.ToDouble(txtNum.Text));
            total = total + totalPrices;
            lbxList.Items.Add("单价：" + txtPrice.Text + "数量：" + txtNum.Text +"计费方式："+ cbxType.SelectedItem + "合计：" + totalPrices.ToString());
            lblResult.Text = total.ToString();
        }

        private void button2_Click(object sender, EventArgs e)
        {
            txtNum.Text = "";
            txtPrice.Text = "";
            lbxList.Items.Clear();
            lblResult.Text = "0.00";

        }
    }
    //现金收费抽象类
    abstract class CashSuper
    {
        public abstract double acceptCash(double money);
    }
    //正常收费子类
    class CashNormal : CashSuper
    {
        public override double acceptCash(double money)
        {
            return money;
        }
    }
    //打折收费子类
    class CashRebate : CashSuper
    {
        private double moneyRebate = 1d;
        public CashRebate(string moneyRebate)
        {
            this.moneyRebate = double.Parse(moneyRebate);
        }
        public override double acceptCash(double money)
        {
            return money * moneyRebate;
        }
    }
    //返利收费子类
    class CashReturn : CashSuper
    {
        private double moneyCondition = 0.0d;
        private double moneyReturn = 0.0d;
        public CashReturn(string moneyCondition, string moneyReturn)
        {
            this.moneyCondition = double.Parse(moneyCondition);
            this.moneyReturn = double.Parse(moneyReturn);
        }
        public override double acceptCash(double money)
        {
            double result = money;
            if (money >= moneyCondition)
                result = money - Math.Floor(money / moneyCondition) * moneyReturn;
            return result;
        }
    }
    //现金收费工厂类
    class CashFactory
    {
        public static CashSuper createCashAccept(string type)
        {
            CashSuper cs = null;
            switch (type)
            {
                case "正常收费":
                    cs = new CashNormal();
                    break;
                case "满300返100":
                    CashReturn cr1 = new CashReturn("300", "100");
                    cs = cr1;
                    break;
                case "打8折":
                    CashRebate cr2 = new CashRebate("0.8");
                    cs = cr2;
                    break;
            }
            return cs;
        }
    }
}

我们看到，这种方案的特点就是将计费方式的实例化放到了工厂里，从而在需要添加新的计费方式的时候，只需在工厂里加一下条件，完了再设计一个相应的类即可。但是这样做同样也破坏了开放—封闭原则，每一次的扩展都要大动干戈，实在是不好，不好。面对算法的时常变动，如何应对？下面我们用另一种方式试试：策略模式实现

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace 商场收银软件
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        double total = 0.0d;

        private void textBox1_TextChanged(object sender, EventArgs e)
        {

        }

        private void label5_Click(object sender, EventArgs e)
        {

        }

        private void Form1_Load(object sender, EventArgs e)
        {
            cbxType.Items.AddRange(new object[] { "正常收费", "满300返100", "打8折" });
            cbxType .SelectedIndex =0;
        }

        private void buttonOK_Click(object sender, EventArgs e)
        {
           //简单工厂模式的客户端代码
            //CashSuper csuper = CashFactory.createCashAccept(cbxType.SelectedItem.ToString());
            //double totalPrices = 0d;
            //totalPrices = csuper.acceptCash(Convert.ToDouble(txtPrice.Text) * Convert.ToDouble(txtNum.Text));
            //total = total + totalPrices;
            //lbxList.Items.Add("单价：" + txtPrice.Text + "数量：" + txtNum.Text +"计费方式："+ cbxType.SelectedItem + "合计：" + totalPrices.ToString());
            //lblResult.Text = total.ToString();

            //策略模式的客户端代码
            CashContext cc = null;
            switch (cbxType.SelectedItem.ToString())
            {
                case "正常收费":
                    cc = new CashContext(new CashNormal());
                    break;
                case "满300返100":
                    cc = new CashContext(new CashReturn("300", "100"));
                    break;
                case "打8折":
                    cc = new CashContext(new CashRebate("0.8"));
                    break;
            }
            double totalPrices = 0d;
            totalPrices = cc.GetResult(Convert.ToDouble(txtPrice.Text) * Convert.ToDouble(txtNum.Text));
            total = total + totalPrices;
            lbxList.Items.Add("单价：" + txtPrice.Text + "数量：" + txtNum.Text + "计费方式：" + cbxType.SelectedItem + "合计：" + totalPrices.ToString());
            lblResult.Text = total.ToString();

        }

        private void button2_Click(object sender, EventArgs e)
        {
            txtNum.Text = "";
            txtPrice.Text = "";
            lbxList.Items.Clear();
            lblResult.Text = "0.00";

        }
    }
    //现金收费抽象类
    abstract class CashSuper
    {
        public abstract double acceptCash(double money);
    }
    //正常收费子类
    class CashNormal : CashSuper
    {
        public override double acceptCash(double money)
        {
            return money;
        }
    }
    //打折收费子类
    class CashRebate : CashSuper
    {
        private double moneyRebate = 1d;
        public CashRebate(string moneyRebate)
        {
            this.moneyRebate = double.Parse(moneyRebate);
        }
        public override double acceptCash(double money)
        {
            return money * moneyRebate;
        }
    }
    //返利收费子类
    class CashReturn : CashSuper
    {
        private double moneyCondition = 0.0d;
        private double moneyReturn = 0.0d;
        public CashReturn(string moneyCondition, string moneyReturn)
        {
            this.moneyCondition = double.Parse(moneyCondition);
            this.moneyReturn = double.Parse(moneyReturn);
        }
        public override double acceptCash(double money)
        {
            double result = money;
            if (money >= moneyCondition)
                result = money - Math.Floor(money / moneyCondition) * moneyReturn;
            return result;
        }
    }
    //现金收费工厂类
    //class CashFactory
    //{
    //    public static CashSuper createCashAccept(string type)
    //    {
    //        CashSuper cs = null;
    //        switch (type)
    //        {
    //            case "正常收费":
    //                cs = new CashNormal();
    //                break;
    //            case "满300返100":
    //                CashReturn cr1 = new CashReturn("300", "100");
    //                cs = cr1;
    //                break;
    //            case "打8折":
    //                CashRebate cr2 = new CashRebate("0.8");
    //                cs = cr2;
    //                break;
    //        }
    //        return cs;
    //    }
    //}
    //CashContext类
    class CashContext
    {
        private CashSuper cs;
        public CashContext(CashSuper csuper)
        {
            this.cs = csuper;
        }
        public double GetResult(double money)
        {
            return cs.acceptCash(money);
        }
    }
}

对比两个方案，我们可以看到不同之处就是策略模式将工厂类撤了，取而代之的是CashContext类，然而又将实现哪一个算法的判断交给了客户端，这样客户端承担的责任又大了，不好，不好…… 怎么办？要不将简单工厂和策略模式混搭？试试看。对于他们的结合，只需将CashContext类改造一下即可，如下：

 //改造后的CashContext类
    class CashContext
    {
        CashSuper cs = null ;
        public CashContext(string type)
        {
            switch (type)
            {
                case "正常收费":
                    CashNormal cs0 = new CashNormal();
                    cs = cs0;
                    break;
                case "满300返100":
                    CashReturn cr1 = new CashReturn("300", "100");
                    cs = cr1;
                    break;
                case "打8折":
                    CashRebate cr2 = new CashRebate("0.8");
                    cs = cr2;
                    break;
            }
            
        }
        public double GetResult(double money)
        {
            return cs.acceptCash(money);
        }
    }

这样一来，客户端的代码就要简单的多了，简单工厂模式需要让客户端和两个类CashSuper和CashFactory打交道，而策略模式与简单工厂结合后的用法，客户端就只需和CashContext打交道就可完成相应的功能，降低了耦合性。我们再回过头去看策略模式的定义：策略模式：定义了算法家族，分别封装起来，让它们之间可以相互替代，此模式让算法的变化，不会影响到使用此算法的客户。

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