【设计模式】简单工厂模式——以一个简单的计算器为例
本文内容参考自《大话设计模式》（程杰 著）
注：以下代码为C#实现
1.拙劣的代码
class Program
{
static void Main(string[] args)
{
Console.Write("请输入数字A：");
string A = Console.ReadLine();
Console.Write("请选择运算符号(+、-、*、/)：");
string B = Console.ReadLine();
Console.Write("请输入数字B：");
string C = Console.ReadLine();
string D = "";
if (B == "+")
D = Convert.ToString(Convert.ToDouble(A) + Convert.ToDouble(C));
if (B == "-")
D = Convert.ToString(Convert.ToDouble(A) - Convert.ToDouble(C));
if (B == "*")
D = Convert.ToString(Convert.ToDouble(A) * Convert.ToDouble(C));
if (O == "/")
D = Convert.ToString(Convert.ToDouble(A) / Convert.ToDouble(C));
Console.WriteLine("结果是：" + D);
}
}
毛病：命名不规范，判断分支做多次无用功，除数为0时出错，输入不是数字……
2.规范化
class Program
{
static void Main(string[] args)
{
try
{
Console.Write("请输入数字A：");
string strNumberA = Console.ReadLine();
Console.Write("请选择运算符号(+、-、*、/)：");
string strOperate = Console.ReadLine();
Console.Write("请输入数字B：");
string strNumberB = Console.ReadLine();
string strResult = "";
switch (strOperate)
{
case "+":
strResult = Convert.ToString(Convert.ToDouble(strNumberA)
+ Convert.ToDouble(strNumberB));
break;
case "-":
strResult = Convert.ToString(Convert.ToDouble(strNumberA)
- Convert.ToDouble(strNumberB));
break;
case "*":
strResult = Convert.ToString(Convert.ToDouble(strNumberA)
* Convert.ToDouble(strNumberB));
break;
case "/":
if (strNumberB != "0")
strResult = Convert.ToString(Convert.ToDouble(strNumberA)
/ Convert.ToDouble(strNumberB));
else
strResult = "除数不能为0";
break;
}
Console.WriteLine("结果是：" + strResult);
Console.ReadLine();
}
catch (Exception ex)
{
Console.WriteLine("您的输入有错：" + ex.Message);
}
}
}以上代码规范了，但是可重用性差！
3.业务的封装
Operation运算类
public class Operation
{
public static double GetResult(double numberA, double numberB, string operate)
{
double result = 0d;
switch (operate)
{
case "+":
result = numberA + numberB;
break;
case "-":
result = numberA - numberB;
break;
case "*":
result = numberA * numberB;
break;
case "/":
result = numberA / numberB;
break;
}
return result;
}
}客户端代码
static void Main(string[] args)
{
try
{
Console.Write("请输入数字A：");
string strNumberA = Console.ReadLine();
Console.Write("请选择运算符号(+、-、*、/)：");
string strOperate = Console.ReadLine();
Console.Write("请输入数字B：");
string strNumberB = Console.ReadLine();
string strResult = "";
strResult = Convert.ToString(Operation.GetResult(Convert.ToDouble(strNumberA),
Convert.ToDouble(strNumberB), strOperate));
Console.WriteLine("结果是：" + strResult);
Console.ReadLine();
}
catch (Exception ex)
{
Console.WriteLine("您的输入有错：" + ex.Message);
}
}问题：如果我要增加其他运算，或者是想要修改运算的法则，那么其他代码都会受到影响，比如要重新编译，调试等等。
4.松耦合
Operation运算类
public class Operation
{
private double _numberA = 0;
private double _numberB = 0;
public double NumberA
{
get { return _numberA; }
set { _numberA = value; }
}
public double NumberB
{
get { return _numberB; }
set { _numberB = value; }
}
public virtual double GetResult()
{
double result = 0;
return result;
}
}加减乘除类
class OperationAdd : Operation
{
public override double GetResult()
{
double result = 0;
result = NumberA + NumberB;
return result;
}
}
class OperationSub : Operation
{
public override double GetResult()
{
double result = 0;
result = NumberA - NumberB;
return result;
}
}
class OperationMul : Operation
{
public override double GetResult()
{
double result = 0;
result = NumberA * NumberB;
return result;
}
}
class OperationDiv : Operation
{
public override double GetResult()
{
double result = 0;
if (NumberB==0)
throw new Exception("除数不能为0。");
result = NumberA / NumberB;
return result;
}
}在这里，我们定义了一个虚函数，然后其他运算继承操作类并实现该函数，这样子，如果要增加其他运算，只需要增加一个类来继承操作类就可以了，而且单个操作的修改完全不影响其他的运算！
5.简单工厂模式
使用上面代码之后，我们在客户端需要实例化相应的类，比如‘+’操作需要实例化OperationAdd，减操作需要实例化OperationSub，这样子暴露给客户的信息太多了，客户必须知道每个操作它对应的类！
使用简单的工厂模式可以避免这个问题：
简单运算工厂类
public class OperationFactory
{
public static Operation createOperate(string operate)
{
Operation oper = null;
switch (operate)
{
case "+":
oper = new OperationAdd();
break;
case "-":
oper = new OperationSub();
break;
case "*":
oper = new OperationMul();
break;
case "/":
oper = new OperationDiv();
break;
}
return oper;
}
}客户端代码
Operation oper;
oper = OperationFactory.createOperate(“+”);
oper.NumberA = 1;
oper.NumberB = 2;
double result = oper.GetResult();如果一来，客户端就简化了很多，它只需要知道 OperationFactory，并通过createOperate传入相应的操作就可以了！
当我们需要修改加法运算时，只需要修改OperationAdd，当我们需要增加其他运算时，只需要增加相应的运算子类，并在运算类工厂中增加分支即可。
同时，界面显示和我们的业务逻辑也是分离的～
UML图：
