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。

using System;

    abstract class State 
    {
        protected string strStatename;        

        abstract public void Pour();
        // do something state-specific here
    }

    class OpenedState : State 
    {        
        public OpenedState ()
        {
            strStatename = "Opened";
        }
        override public void Pour()
        {
            Console.WriteLine("...pouring...");
            Console.WriteLine("...pouring...");
            Console.WriteLine("...pouring...");
        }
    }
    
    class ClosedState : State 
    {        
        public ClosedState()
        {
            strStatename = "Closed";
        }
        override public void Pour()
        {
            Console.WriteLine("ERROR - bottle is closed - cannot pour");
        }
    }

    class ContextColaBottle 
    {
        public enum BottleStateSetting {
            Closed,
            Opened
        };

        // If teh state classes had large amounts of instance data,
        // we could dynamically create them as needed - if this demo
        // they are tiny, so we just  create them as data members
        OpenedState openedState = new OpenedState();
        ClosedState closedState = new ClosedState();

        public ContextColaBottle ()
        {
            // Initialize to closed
            CurrentState = closedState;
        }

        private State CurrentState;
        
        public void SetState(BottleStateSetting newState)
        {
            if (newState == BottleStateSetting.Closed)
            {
                CurrentState = closedState;
            }
            else 
            {
                CurrentState = openedState;
            }
        }

        public void Pour()
        {
            CurrentState.Pour();
        }    
    }

      /// <summary>
    ///    Summary description for Client.
    /// </summary>
    public class Client
    {
        public static int Main(string[] args)
        {
            ContextColaBottle contextColaBottle = new ContextColaBottle();

            Console.WriteLine("initial state is closed");

            Console.WriteLine("Now trying to pour");
              contextColaBottle.Pour();

            Console.WriteLine("Open bottle");
            contextColaBottle.SetState(ContextColaBottle.BottleStateSetting.Opened);

            Console.WriteLine("Try to pour again");
            contextColaBottle.Pour();

            return 0;
        }
    }