重构11-Switch to Strategy（Switch到策略模式）

重构没有固定的形式，多年来我使用过不同的版本，并且我敢打赌不同的人也会有不同的版本。 该重构适用于这样的场景：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 double CalculateShipping() {
        ShippingInfo shippingInfo = new ShippingInfo();
        return shippingInfo.CalculateShippingAmount(State.Alaska);
}
}
public enum State {
    Alaska, NewYork, Florida
}
public class ShippingInfo {
    public double CalculateShippingAmount(State shipToState) {
        switch (shipToState) {
            case State.Alaska:
                return GetAlaskaShippingAmount();
            case State.NewYork:
                return GetNewYorkShippingAmount();
            case State.Florida:
                return GetFloridaShippingAmount();
           default:
                return 0d;
}
    }
    
    private double GetAlaskaShippingAmount() {
        return 15d;
}
    private double GetNewYorkShippingAmount() {
        return 10d;
}
    private double GetFloridaShippingAmount() {
        return 3d;
}
}

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。我在这里将字体加粗显示，是因为肯定会有人在评论里指出这点：）用你觉得好用的方法。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 ClientTest {
    public  Double CalculateShipping() {
        ShippingInfo shippingInfo = new ShippingInfo();
        return shippingInfo.CalculateShippingAmount(State.Alaska);
}
}
public enum State {
    Alaska, NewYork, Florida
}
public class ShippingInfo {
    private Dictionary<State, IShippingCalculation> ShippingCalculations;

    public ShippingInfo() {
        ShippingCalculations =new Hashtable<State, IShippingCalculation>();
ShippingCalculations.put(State.Alaska, new AlaskShippingCalculation());
ShippingCalculations.put(State.NewYork, new NewYorkShippingCalculation());
ShippingCalculations.put(State.Florida, new FloridaShippingCalculation());
}

    public  Double CalculateShippingAmount(State shipToState) {
        return ShippingCalculations.get(shipToState).Calculate();
}
}
public interface IShippingCalculation {
         Double Calculate();
}
public class AlaskShippingCalculation implements IShippingCalculation {
    public  Double Calculate () {
        return 15d;
}
}
public class NewYorkShippingCalculation implements IShippingCalculation {
    public  Double Calculate () {
        return 10d;
}
}
    public class FloridaShippingCalculation implements IShippingCalculation {
    public  Double Calculate () {
        return 3d;
}
}

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。（感谢Nate Kohari和Jayme Davis）

public interface IShippingInfo {
    Double CalculateShippingAmount(State state);
}

public class ClientCode {
    public IShippingInfo ShippingInfo;
    public Double CalculateShipping() {
        return ShippingInfo.CalculateShippingAmount(State.Alaska);
}
}
public enum State{
    Alaska, NewYork, Florida;
}

public class ShippingInfo implements IShippingInfo{
    private Dictionary<State, IShippingCalculation> ShippingCalculations=new Hashtable<State, IShippingCalculation>();
    public ShippingInfo(IShippingCalculation<State> shippingCalculations) {
        for(State state:State.values()) {
            ShippingCalculations.put(state, shippingCalculations);
}
    }
    public Double CalculateShippingAmount(State shipToState) {
        return ShippingCalculations.get(shipToState).Calculate();
}
}

public interface IShippingCalculation<State> {
    State getState();
Double Calculate();
}

public class AlaskShippingCalculation implements IShippingCalculation {
    @Override
public State getState() {
        return State.Alaska;
}
    @Override
public Double Calculate() {
        return 15d;
}
}

public class NewYorkShippingCalculation implements IShippingCalculation {
    @Override
public State getState() {
        return State.NewYork;
}
    @Override
public Double Calculate() {
        return 10d;
}
}

public class FloridaShippingCalculation implements IShippingCalculation {
    @Override
public State getState() {
        return State.Florida;
}
    @Override
public Double Calculate() {
        return 3d;
}
}

 

 

 

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