重构工厂而不违反开放式关闭原则

Question

我有两个版本的工厂类基于该文章设计

http://www.oodesign.com/factory-pattern.html

public abstract class Employee
{
    public string Name { get; set; }
    protected string Role { get; set; }
    public abstract string GetRole();
}

public class Manager : Employee
{
    public Manager()
    {
        Role = "MGR";
    }
    public override string GetRole()
    {
        return this.Role;
    }
}

版本1:简单,违反开放式关闭原则

当我添加一个新的具体类时,每次都需要更改SimpleEmployeeFactory

public class SimpleEmployeeFactory
{
    public static Employee GetEmployee(int typeId)
    {
        switch (typeId)
        {
            case 1:
                return new Manager();
            case 2:
                return new TechnicalLead();
            default:
                return null; //if the id doesn't have any 
        }
    }
}

版本2:

在我们使用工厂调用之前,Refactored Factory仍需要创建Concrete类

public abstract class Employee
{
    public string Name { get; set; }
    protected string Role { get; set; }
    public abstract string GetRole();      
    public abstract Employee createEmployee();
} 

public class ChiefTechnologyOfficer : Employee
{
    public ChiefTechnologyOfficer()
    {
        this.Role = "CTO";
    }
    static ChiefTechnologyOfficer()
    {            
        RefactoredFactory.Instance.registerEmployee(5, new ChiefTechnologyOfficer());
    }
    public override string GetRole()
    {            
        return this.Role;
    }        
    public override Employee createEmployee()
    {
        return new ChiefTechnologyOfficer();
    }
}

厂

class RefactoredFactory
{
    private static readonly RefactoredFactory instance = new RefactoredFactory();

    static RefactoredFactory()
    {
    }

    private RefactoredFactory()
    {
    }

    public static RefactoredFactory Instance
    {
        get
        {
            return instance;
        }
    }

    private Dictionary<int, Employee> registeredEmployees = new Dictionary<int, Employee>();

    public void registerEmployee(int typeId, Employee employeeInst)
    {
        registeredEmployees.Add(typeId, employeeInst);
    }

    public Employee createEmployee(int typeId)
    { 
        return ((Employee)registeredEmployees[typeId]).createEmployee();
    }
}

客户

 Employee emp = SimpleEmployeeFactory.GetEmployee(1);
 Activator.CreateInstance(typeof(ChiefTechnologyOfficer)); //Avoid
 Employee empFNoR = RefactoredFactory.Instance.createEmployee(5);

您可以看到Activator.CreateInstance(typeof(ChiefTechnologyOfficer))调用以使具体类在Factory中注册.否则我们无法检索该对象

有没有办法创建一个不违反OCP原则的Factory类,并且创建一个像我在RefactoredFactory类中使用的对象？

Answer 1

看起来像是typeId患有功能羡慕症。相反，定义一个多态类型来捕获类型；例如一个接口：

public interface IEmployeeType
{
    Employee Create()
}

现在您可以定义例如 aManagerType和 aTechnicalLeadType等。示例：

public class ManagerType : IEmployeeType
{
    public Employee Create()
    {
        return new Manager();
    }
}

这本质上是一个抽象工厂，它的优点是当您需要创建新的子类型时，您始终可以创建新的实现。

如果您位于系统的边界，并且必须将整数等原始值转换为多态值，则可以使用角色提示模式之一- 特别是Metadata、Role Interface或（我最喜欢的）Partial Type Name。

客户

给定一个IEmployeeType实例employeeType，客户只需：

Employee emp = employeeType.Create();