主讲：李建忠
来源：
1: public abstract class Shape
2: { 3: public abstract void Draw();
4:
5: //预料到将来可能会引入新的操作
6: public abstract void Accept(ShapeVisitor v);
7: }
8:
9: public abstract class ShapeVisitor
10: { 11: public abstract void Visit(Rectangle shape); //#1
12: public abstract void Visit(Circle shape); //#2
13: public abstract void Visit(Line shape); //#3
14: }
15:
16: public class MyVisitor:ShapeVisitor
17: { 18: public override void Visit(Rectangle shape); //#1
19: { 20: //增加对Rectangle的操作
21: }
22:
23: public override void Visit(Circle shape); //#2
24: { 25: //增加对Circle的操作
26: }
27:
28: public override void Visit(Line shape); //#3
29: { 30: //增加对Line的操作
31: }
32: }
33:
34: public class Rectangle:Shape
35: { 36: public override void Draw()
37: { 38: //...
39: }
40:
41: public override void Accept(ShapeVisitor v)
42: { 43: v.Visit(this); //调用 #1
44: }
45: }
46:
47: public class Circle:Shape
48: { 49: public override void Draw()
50: { 51: //...
52: }
53:
54: public override void Accept(ShapeVisitor v)
55: { 56: v.Visit(this); //调用 #2
57: }
58: }
59:
60: public class Line:Shape
61: { 62: public override void Draw()
63: { 64: //...
65: }
66:
67: public override void Accept(ShapeVisitor v)
68: { 69: v.Visit(this); //调用 #3
70: }
71: }
72:
73: class App
74: { 75: ShapeVisitor visitor;
76: public App(ShapeVisitor visitor)
77: { 78: this.visitor=visitor;
79: }
80:
81: public static void Process(Shape shape)
82: { 83: //两处多态：
84: //1.Accept方法的调用对象 shape
85: //2.Accept等方法的参数 visitor
86: shape.Accept(visitor);
87: }
88: }
89:
90: App app=new App(new MyVisitor());
91: app.Process(new Line());