设计模式之美：Visitor（访问者）

  1 namespace VisitorPattern.Implementation1
  2 {
  3   public abstract class Element
  4   {
  5     public abstract void Accept(Visitor visitor);
  6   }
  7 
  8   public abstract class Visitor
  9   {
 10     public abstract void Visit(ConcreteElementA element);
 11     public abstract void Visit(ConcreteElementB element);
 12   }
 13 
 14   public class ObjectStructure
 15   {
 16     private List<Element> _elements = new List<Element>();
 17 
 18     public void Attach(Element element)
 19     {
 20       _elements.Add(element);
 21     }
 22 
 23     public void Detach(Element element)
 24     {
 25       _elements.Remove(element);
 26     }
 27 
 28     public void Accept(Visitor visitor)
 29     {
 30       foreach (var element in _elements)
 31       {
 32         element.Accept(visitor);
 33       }
 34     }
 35   }
 36 
 37   public class ConcreteElementA : Element
 38   {
 39     public string Name { get; set; }
 40 
 41     public override void Accept(Visitor visitor)
 42     {
 43       visitor.Visit(this);
 44     }
 45   }
 46 
 47   public class ConcreteElementB : Element
 48   {
 49     public string ID { get; set; }
 50 
 51     public override void Accept(Visitor visitor)
 52     {
 53       visitor.Visit(this);
 54     }
 55   }
 56 
 57   public class ConcreteVisitorA : Visitor
 58   {
 59     public override void Visit(ConcreteElementA element)
 60     {
 61       Console.WriteLine(
 62         "ConcreteVisitorA visited ConcreteElementA : {0}", 
 63         element.Name);
 64     }
 65 
 66     public override void Visit(ConcreteElementB element)
 67     {
 68       Console.WriteLine(
 69         "ConcreteVisitorA visited ConcreteElementB : {0}", 
 70         element.ID);
 71     }
 72   }
 73 
 74   public class ConcreteVisitorB : Visitor
 75   {
 76     public override void Visit(ConcreteElementA element)
 77     {
 78       Console.WriteLine(
 79         "ConcreteVisitorB visited ConcreteElementA : {0}", 
 80         element.Name);
 81     }
 82 
 83     public override void Visit(ConcreteElementB element)
 84     {
 85       Console.WriteLine(
 86         "ConcreteVisitorB visited ConcreteElementB : {0}", 
 87         element.ID);
 88     }
 89   }
 90 
 91   public class Client
 92   {
 93     public void TestCase1()
 94     {
 95       var objectStructure = new ObjectStructure();
 96 
 97       objectStructure.Attach(new ConcreteElementA());
 98       objectStructure.Attach(new ConcreteElementB());
 99 
100       objectStructure.Accept(new ConcreteVisitorA());
101       objectStructure.Accept(new ConcreteVisitorB());
102     }
103   }
104 }