C#设计模式——命令模式(Command Pattern)

一、概述
通常来说，“活动哀求者”与“活动实现者”是紧耦合的。但在某些场合，比如要对活动举行“记载、取消/重做、事务”等处置惩罚，这种无法反抗厘革的紧耦合是不符合的。在这些环境下，将“活动哀求者”与“活动实现者”解耦，实现二者之间的松耦合就至关重要。下令模式是管理这类标题的一个比力好的方法。
二、下令模式
下令模式将一个哀求封装为一个对象，从而使你可用差别的哀求对客户举行参数化；对哀求列队或记载哀求日志，以及支持可取消的操作。
下令模式的结构图如下

Command界说了下令的接口
ConcreteCommand实现Command接口，界说了具体的下令
Client用于创建具体的下令并设定吸收者
Invoker要求Command实行相应的哀求
Receiver实行与实行一个哀求，任何一个类都大概作为Receiver。
三、示例
假定要实现一个画图体系，要求支持取消功能，下面就用下令模式来实现这一需求。
起首界说一个抽象的下令接口

1. public interface IGraphCommand
2. {
3. void Draw();
4. void Undo();
5. }

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接着实现具体的画图下令

1. public class Line : IGraphCommand
2. {
3. private Point startPoint;
4. private Point endPoint;
5. public Line(Point start, Point end)
6. {
7. startPoint = start;
8. endPoint = end;
9. }
11. public void Draw()
12. {
13. Console.WriteLine("Draw Line:{0} To {1}", startPoint.ToString(), endPoint.ToString());
14. }
16. public void Undo()
17. {
18. Console.WriteLine("Erase Line:{0} To {1}", startPoint.ToString(), endPoint.ToString());
19. }
20. }
22. public class Rectangle : IGraphCommand
23. {
24. private Point topLeftPoint;
25. private Point bottomRightPoint;
26. public Rectangle(Point topLeft, Point bottomRight)
27. {
28. topLeftPoint = topLeft;
29. bottomRightPoint = bottomRight;
30. }
32. public void Draw()
33. {
34. Console.WriteLine("Draw Rectangle: Top Left Point {0}, Bottom Right Point {1}", topLeftPoint.ToString(), bottomRightPoint.ToString());
35. }
37. public void Undo()
38. {
39. Console.WriteLine("Erase Rectangle: Top Left Point {0}, Bottom Right Point {1}", topLeftPoint.ToString(), bottomRightPoint.ToString());
40. }
41. }
43. public class Circle : IGraphCommand
44. {
45. private Point centerPoint;
46. private int radius;
47. public Circle(Point center, int radius)
48. {
49. centerPoint = center;
50. this.radius = radius;
51. }
53. public void Draw()
54. {
55. Console.WriteLine("Draw Circle: Center Point {0}, Radius {1}", centerPoint.ToString(), radius.ToString());
56. }
58. publi cvoid Undo()
59. {
60. Console.WriteLine("Erase Circle: Center Point {0}, Radius {1}", centerPoint.ToString(), radius.ToString());
61. }
62. }

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然后再界说画图类作为下令吸收者

1. public class Graphics
2. {
3. Stack<IGraphCommand> commands =new Stack<IGraphCommand>();
5. public void Draw(IGraphCommand command)
6. {
7. command.Draw();
8. commands.Push(command);
9. }
11. public void Undo()
12. {
13. IGraphCommand command = commands.Pop();
14. command.Undo();
15. }
16. }

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末了看一下怎样调用

1. static void Main(string[] args)
2. {
3. Line line =new Line(new Point(10, 10), new Point(100, 10));
4. Rectangle rectangle =new Rectangle(new Point(20, 20), new Point(50, 30));
5. Circle circle =new Circle(new Point(500, 500), 200);
7. Graphics graphics =new Graphics();
8. graphics.Draw(line);
9. graphics.Draw(rectangle);
10. graphics.Undo();
11. graphics.Draw(circle);
13. Console.ReadLine();
14. }

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