行为型设计模式之状态模式(State)

结构
意图	允许一个对象在其内部状态改变时改变它的行为。对象看起来似乎修改了它的类。
适用性	一个对象的行为取决于它的状态, 并且它必须在运行时刻根据状态改变它的行为。 一个操作中含有庞大的多分支的条件语句，且这些分支依赖于该对象的状态。这个状态通常用一个或多个枚举常量表示。通常, 有多个操作包含这一相同的条件结构。S t a t e模式将每一个条件分支放入一个独立的类中。这使得你可以根据对象自身的情况将对象的状态作为一个对象，这一对象可以不依赖于其他对象而独立变化。

 1 using System;
 2
 3     abstract class State
 4     {
 5         protected string strStatename;
 6
 7         abstract public void Pour();
 8         // do something state-specific here
 9     }
10
11     class OpenedState : State
12     {
13         public OpenedState ()
14         {
15             strStatename = "Opened";
16         }
17         override public void Pour()
18         {
19             Console.WriteLine("...pouring...");
20             Console.WriteLine("...pouring...");
21             Console.WriteLine("...pouring...");
22         }
23     }
24
25     class ClosedState : State
26     {
27         public ClosedState()
28         {
29             strStatename = "Closed";
30         }
31         override public void Pour()
32         {
33             Console.WriteLine("ERROR - bottle is closed - cannot pour");
34         }
35     }
36
37     class ContextColaBottle
38     {
39         public enum BottleStateSetting {
40             Closed,
41             Opened
42         };
43
44         // If teh state classes had large amounts of instance data,
45         // we could dynamically create them as needed - if this demo
46         // they are tiny, so we just  create them as data members
47         OpenedState openedState = new OpenedState();
48         ClosedState closedState = new ClosedState();
49
50         public ContextColaBottle ()
51         {
52             // Initialize to closed
53             CurrentState = closedState;
54         }
55
56         private State CurrentState;
57
58         public void SetState(BottleStateSetting newState)
59         {
60             if (newState == BottleStateSetting.Closed)
61             {
62                 CurrentState = closedState;
63             }
64             else
65             {
66                 CurrentState = openedState;
67             }
68         }
69
70         public void Pour()
71         {
72             CurrentState.Pour();
73         }
74     }
75
76       /// <summary>
77     ///    Summary description for Client.
78     /// </summary>
79     public class Client
80     {
81         public static int Main(string[] args)
82         {
83             ContextColaBottle contextColaBottle = new ContextColaBottle();
84
85             Console.WriteLine("initial state is closed");
86
87             Console.WriteLine("Now trying to pour");
88               contextColaBottle.Pour();
89
90             Console.WriteLine("Open bottle");
91             contextColaBottle.SetState(ContextColaBottle.BottleStateSetting.Opened);
92
93             Console.WriteLine("Try to pour again");
94             contextColaBottle.Pour();
95
96             return 0;
97         }
98     }

状态模式