状态模式(State Pattern)

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所属的类。

  using System;
   using System.Collections.Generic;
 
    // State Pattern               Judith Bishop  Oct 2007
    // Simple game where the context changes the state based on user input
    // Has four states, each with 6 operations 
    
   interface IState {
     int MoveUp(Context context);
     int MoveDown(Context context);
   }
 
   // State 1
   class NormalState : IState {
     public  int MoveUp(Context context) {
       context.Counter+=2;
       return context.Counter;
     }
 
     public int MoveDown(Context context) {
         if (context.Counter < Context.limit) {
           context.State = new FastState();
           Console.Write("|| ");
         }
         context.Counter-=2;
         return context.Counter;
     }
   }
 
   // State 2
   class FastState : IState {
     public int MoveUp(Context context) {
       context.Counter+=5;
       return context.Counter;
     }
 
     public int MoveDown(Context context) {
        if (context.Counter < Context.limit) {
         context.State = new NormalState();
         Console.Write("||");
       }
       context.Counter-=5;
       return context.Counter;
     }
   }
 
   // Context
   class Context {
     public const int limit = 10;
     public IState State {get; set; }
     public int Counter = limit;
       
     public int Request(int n) {
       if (n==2)
         return State.MoveUp(this);
       else
         return State.MoveDown(this);
     }
   }
    
   static class Program {
      // The user interface
     static void Main () {
       Context context = new Context();
       context.State = new NormalState();
       Random r = new Random(37);
       for (int i = 5; i<=25; i++) {
         int command = r.Next(3);
         Console.Write(context.Request(command)+" ");
       }
       Console.WriteLine();
       Console.ReadKey();
     }
   }
    /* Output
    8 10 8 || 6 11 16 11 6 ||1 3 || 1 ||-4 || -6 -1 4 ||-1 || -3 2 7 ||2 4 
    */