[置顶] 重构—改善代码既有设计 之switch/case使用

Switch / Case如下，假设Employee类中有这样两个方法:

PayAmount. 根据员工类型获得员工的薪水:

        public int PayAmount(EmployeeType empType)
        {
            switch (empType)
            {
                case EmployeeType.ENGINEER:
                    return m_basicSalary;
                case EmployeeType.SALESMAN:
                    return m_basicSalary + m_commission;
                case EmployeeType.MANAGER:
                    return 2 * m_basicSalary;
                default:
                    throw new Exception("no such employee type!");
            }
        }

GetDescription. 根据员工类型获得职责描述:

        public string GetDescription(EmployeeType empType)
        {
            switch (empType)
            {
                case EmployeeType.ENGINEER:
                    return "Coding, Debug, Optimization";
                case EmployeeType.SALESMAN:
                    return "Getting contracts";
                case EmployeeType.MANAGER:
                    return "Analysis, Scheduling, Reporting";
                default:
                    throw new Exception("no such employee type!");
            }
        }
这两个方法含有两个几乎一模一样的switch-case语句，如果将来需要增加一种情况，比如“Senior Engineer”那么我们需要去找到这2个方法并分别修改他们。假如一个庞大的系统中有很多这样的方法，改起来就会很麻烦。

现在我们设法将冗余的switch-case语句提炼出来，以期将来需求变化时只需做一次修改即可。

 

 

Solution1 使用简单工厂模式:

为Employee类构造一个简单工厂，能够根据不同的EmployeeType返回相应的子类实例。

        public abstract class Employee
        {
            protected int basicSalary;
            protected int commission;
            public static Employee GetEmployee(EmployeeType empType)
            {
                switch (empType)
                {
                    case EmployeeType.ENGINEER:
                        return new Engineer();
                    case EmployeeType.SALESMAN:
                        return new SalesMan();
                    case EmployeeType.MANAGER:
                        return new Manager();
                    default:
                        throw new Exception("no such employee type!");
                }
            }
            public abstract int PayAmount();
            public abstract string GetDescription();
        }

为switch - case 语句中的不同case分别创建子类，接着将不同case下的逻辑提炼成方法装入相应的子类：

        public class Engineer : Employee
        {
            public override int PayAmount()
            {
                return basicSalary;
            }
            public override string GetDescription()
            {
                return "Coding, Debug, Optimization";
            }
        }
        public class SalesMan : Employee
        {
            public override int PayAmount()
            {
                return basicSalary + commission;
            }
            public override string GetDescription()
            {
                return "Getting contracts";
            }
        }
        public class Manager : Employee
        {
            public override int PayAmount()
            {
                return 2 * basicSalary;
            }
            public override string GetDescription()
            {
                return " Analysis, Scheduling, Reporting ";
            }
        }

 

至此，EmployeeType参数就可以从两个方法中移除了，我们只需在构造Employee对象时指定一次EmployeeType，就能获得想要的Employee行为：

 

        Employee emp = Employee.GetEmployee(EmployeeType.ENGINEER);
        Console.WriteLine(emp.GetDescription());
        Console.WriteLine(emp.PayAmount());

Solution2 使用State模式:

如果一个Employee对象的EmployeeType是可变的（比如一个engineer升职成了manager），那么简单工厂就不适用了。
这种情况下我们可以使用State模式来解决问题，主要做法就是将EmployeType相关的逻辑提炼出来作为独立的一族类，而Employee类将EmployeeType的实例作为自己的一个property，这样，每个Employee的employee type就成为可变的了：

增加一个EmployeeTypeManager类，这个类专门用于描述和EmployeeType相关的行为。

 

    public abstract class EmployeeTypeManager
    {
        public int basicSalary;
        public int commission;
        public abstract int PayAmount();
        public abstract string GetDescription();
    }

我们把Employee子类中和EmployeeType 相关的方法提取到EmployeeTypeManager类的子类中去。省事的做法是直接将之前的Engineer，SalesMan和Manager类声明成EmployeeType的子类 :P

这里，因为这些类只包含一些和EmployeeType相关的行为，而没有自己的状态，所以我们使用了Singleton的实现。此处仅以Engineer类为例说明：

    public class Engineer : EmployeeTypeManager
    {
        private Engineer() { }
        private static Engineer m_Instance;
        public static Engineer Instance
        {
            get
            {
                if (m_Instance == null)
                    m_Instance = new Engineer();
                return m_Instance;
            }
        }
        public override int PayAmount()
        {
            return basicSalary;
        }
        public override string GetDescription()
        {
            return "Coding, Debug, Optimization";
        }
    }

 

接着在Employee类中声明一个EmployeeTypeManager类型的私有成员并为EmployeeType暴露一个property供外界修改:

        private EmployeeTypeManager m_TypeManager;
        public EmployeeType EmpType
        {
            get { return m_type; }
            set { m_type = value; }
        }

将switch-case逻辑从原先的简单工厂挪到EmployeeType的set方法中去：

        public EmployeeType EmpType
        {
            get { return m_type; }
            set
            {
                m_type = value;
                switch (m_type)
                {
                    case EmployeeType.ENGINEER:
                        m_TypeManager = Engineer.Instance;
                        break;
                    case EmployeeType.SALESMAN:
                        m_TypeManager = SalesMan.Instance;
                        break;
                    case EmployeeType.MANAGER:
                        m_TypeManager = Manager.Instance;
                        break;
                    default:
                        throw new Exception("no such employee type!");
                }
            }
        }

最后将对Employee类的两个成员方法的调用委托给EmployeeTypeManager:

        public int PayAmount()
        {
            return m_TypeManager.PayAmount();
        }
        public string GetDescription()
        {
            return m_TypeManager.GetDescription();
        }

Employee类内置的EmployeeTypeManager对象将随着其EmployeeType的改变而改变，从而同一个Employee对象也随之有了不同的行为。

整个过程将各个case的实现逻辑从Employee的子类移到了EmployeeTypeManager及其子类中去，相应的，switch - case判断也被从Employee的简单工厂中移动到了property里。