XIAOSUO 记录个人学习的足迹

Switch to Strategy：策略模式转换switch分支

switch 语句块很大，并且会随时引入新的判断条件。这时，最好使用策略模式将每个条件封装到单独的类中。

实现策略模式的方式是很多的。我在这里介绍的策略重构使用的是字典策略，这么做的好处是调用者不必修改原来的代码。

public class ClientCode
{
	public decimal CalculateShipping()
	{
		ShippingInfo shippingInfo = new ShippingInfo();
		return shippingInfo.CalculateShippingAmount(State.Alaska);
	}
}
public enum State
{
	Alaska,
	NewYork,
	Florida
}
public class ShippingInfo
{
	public decimal CalculateShippingAmount(State shipToState)
	{
		switch (shipToState)
		{
			case State.Alaska:
				return GetAlaskaShippingAmount();
			case State.NewYork:
				return GetNewYorkShippingAmount();
			case State.Florida:
				return GetFloridaShippingAmount();
			default:
				return 0m;
		}
	}
	private decimal GetAlaskaShippingAmount()
	{
		return 15m;
	}
	private decimal GetNewYorkShippingAmount()
	{
		return 10m;
	}
	private decimal GetFloridaShippingAmount()
	{
		return 3m;
	}
}

要应用该重构，需将每个测试条件至于单独的类中，这些类实现了一个共同的接口。然后将枚举作为字典的键，这样就可以获取正确的实现，并执行其代码了。以后如果希望添加新的条件，只需添加新的实现类，并将其添加至ShippingCalculations字典中。正如前面说过的，这不是实现策略模式的唯一方式。你也可以用觉得好用的方法。我用这种方式实现重构的好处是，不用修改客户端代码。所有的修改都在ShippingInfo类内部。Jayme Davis指出这种重构由于仍然需要在构造函数中进行绑定，所以只不过是增加了一些类而已，但如果绑定IShippingCalculation的策略可以置于IoC中，带来的好处还是很多的，它可以使你更灵活地捆绑策略。

public class ClientCode
{
	public decimal CalculateShipping()
	{
		ShippingInfo shippingInfo = new ShippingInfo();
		return shippingInfo.CalculateShippingAmount(State.Alaska);
	}
}
public enum State
{
	Alaska,
	NewYork,
	Florida
}
public class ShippingInfo
{
	private IDictionary<State, IShippingCalculation> ShippingCalculations
	{ get; set; }
	public ShippingInfo()
	{
		ShippingCalculations = new Dictionary<State, IShippingCalculation>
　　　　   {
　　　　 　　{ State.Alaska, new AlaskShippingCalculation() },
　　　　　　 { State.NewYork, new NewYorkShippingCalculation() },
　　　　 　　{ State.Florida, new FloridaShippingCalculation() }
　　　　 };
	}
	public decimal CalculateShippingAmount(State shipToState)
	{
		return ShippingCalculations[shipToState].Calculate();
	}
}
public interface IShippingCalculation
{
	decimal Calculate();
}
public class AlaskShippingCalculation : IShippingCalculation
{
	public decimal Calculate()
	{
		return 15m;
	}
}
public class NewYorkShippingCalculation : IShippingCalculation
{
	public decimal Calculate()
	{
		return 10m;
	}
}
public class FloridaShippingCalculation : IShippingCalculation
{
	public decimal Calculate()
	{
		return 3m;
	}
}

为了使这个示例圆满，我们来看看在ShippingInfo构造函数中使用Ninject为IoC容器时如何进行绑定。需要更改的地方很多，主要是将state的枚举放在策略内部，以及Ninject向构造函数传递一个IShippingInfo的IEnumerable泛型。接下来我们使用策略类中的state属性创建字典，其余部分保持不变。

public interface IShippingInfo
{
	decimal CalculateShippingAmount(State state);
}
public class ClientCode
{
	[Inject]
	public IShippingInfo ShippingInfo { get; set; }
	public decimal CalculateShipping()
	{
		return ShippingInfo.CalculateShippingAmount(State.Alaska);
	}
}
public enum State
{
	Alaska,
	NewYork,
	Florida
}
public class ShippingInfo : IShippingInfo
{
	private IDictionary<State, IShippingCalculation> ShippingCalculations
	{ get; set; }
	public ShippingInfo(IEnumerable<IShippingCalculation> shippingCalculations)
	{
		ShippingCalculations = shippingCalculations.ToDictionary(
		calc => calc.State);
	}
	public decimal CalculateShippingAmount(State shipToState)
	{
		return ShippingCalculations[shipToState].Calculate();
	}
}
public interface IShippingCalculation
{
	State State { get; }
	decimal Calculate();
}
public class AlaskShippingCalculation : IShippingCalculation
{
	public State State { get { return State.Alaska; } }
	public decimal Calculate()
	{
		return 15m;
	}
}
public class NewYorkShippingCalculation : IShippingCalculation
{
	public State State { get { return State.NewYork; } }
	public decimal Calculate()
	{
		return 10m;
	}
}
public class FloridaShippingCalculation : IShippingCalculation
{
	public State State { get { return State.Florida; } }
	public decimal Calculate()
	{
		return 3m;
	}
}