设计模式之工厂模式读后感

作者(大话设计模式)以生动的比喻，从简单工厂模式到工厂模式，最后到抽象工厂模式，极其美妙的描述了工厂模式。

简单工厂模式(以一个简单的加减乘除计算器为例)

初学者代码：

Class Operation{
     Double a;
     Double b;
     public Double Add(){
          return a+b;
     }
     public Double Sub(){
          return a-b;
     }
     public Double Mul(){
          return a*b;
     }
}

这样的代码对完成当前的功能没问题，关键是在复用和扩展方面很差，如果要新增一个运算功能，只能去修改类Operation，这是违背修改封闭原则的。

简单工厂实现：

        Public Class Operation
            Private _numA As Double
            Private _numB As Double
            Public Property NumA As Double
                Get
                    Return _numA
                End Get
                Set(ByVal value As Double)
                    _numA = value
                End Set
            End Property
            Public Property NumB As Double
                Get
                    Return _numB
                End Get
                Set(ByVal value As Double)
                    _numB = value
                End Set
            End Property
            Public Overridable Function GetResult()
                Dim result = 0
                Return result
            End Function

        End Class

        Public Class AddOperation
            Inherits Operation

            Public Overrides Function GetResult() As Object
                Return Me.NumA + Me.NumB
            End Function
        End Class

        Public Class SubOperation
            Inherits Operation

            Public Overrides Function GetResult() As Object
                Return Me.NumA - Me.NumB
            End Function
        End Class

        Public Class MulOperation
            Inherits Operation

            Public Overrides Function GetResult() As Object
                Return Me.NumA * Me.NumB
            End Function
        End Class

        Public Class OperationFactory
            Public Shared Function CreateOperation(ByVal op As String) As Operation
                Dim operation As Operation = Nothing
                Select Case op
                    Case "+"
                        operation = New AddOperation
                    Case "-"
                        operation = New SubOperation
                    Case "*"
                        operation = New MulOperation
                    Case Else
                        Throw New Exception("操作参数不对")
                End Select
                Return operation
            End Function
        End Class

这样一来，要扩展新的功能，只需要添加Operation的子类即可，而不必更改已有代码。惟独不舒服的是，要在工厂类中添加Case，这产生了新的模式

简单工厂模式＋反射＋配置文件，通过配置文件读取操作类型字符串，通过反射生成相应的操作子类实例。现在代码就相当的完美的。