状态模式(State Pattern)

介绍
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。

示例
有一个Message实体类，对它的操作有Insert()和Get()方法，持久化数据在SqlServer数据库中或Xml文件里。在对象内部可以根据用户的不同选择不同的数据持久化方案。

　　MessageModel

using System;
using System.Collections.Generic;
using System.Text;

namespace Pattern.State
{
    /// 

    /// Message实体类
    /// 
    public class MessageModel
    {
        /// 

        /// 构造函数
        /// 
        /// Message内容
        /// Message发布时间
        public MessageModel(string msg, DateTime pt)
        {
            this._message = msg;
            this._publishTime = pt;
        }

        private string _message;
        /// 

        /// Message内容
        /// 
        public string Message
        {
            get { return _message; }
            set { _message = value; }
        }

        private DateTime _publishTime;
        /// 

        /// Message发布时间
        /// 
        public DateTime PublishTime
        {
            get { return _publishTime; }
            set { _publishTime = value; }
        }
    }
}

　　AbstractMessageState

using System;
using System.Collections.Generic;
using System.Text;

namespace Pattern.State
{
    /// 

    /// 状态模式抽象类
    /// 
    public abstract class AbstractMessageState
    {
        /// 

        /// Context类型
        /// 
        protected Message _message;

        /// 

        /// Context类型属性
        /// 
        public Message Message
        {
            get { return _message; }
            set { _message = value; }
        }

        /// 

        /// 改变状态
        /// 
        public abstract void StateChangeCheck();

        /// 

        /// 获取Message
        /// 
        /// 
        public abstract List Get();

        /// 

        /// 插入Message
        /// 
        /// Message实体对象
        /// 
        public abstract bool Insert(MessageModel mm);
    }
}

　　XmlMessage

using System;
using System.Collections.Generic;
using System.Text;

namespace Pattern.State
{
    /// 

    /// Xml方式操作Message
    /// 
    public class XmlMessage : AbstractMessageState
    {
        /// 

        /// 构造函数
        /// 
        /// Context类型参数
        public XmlMessage(Message message)
        {
            this._message = message;
        }

        /// 

        /// 改变状态
        /// 
        public override void StateChangeCheck()
        {
            // 如果UserId是admin则变换状态为SqlMessage
            if (this._message.UserId == "admin")
            {
                this._message.AbstractMessageState = new SqlMessage(this._message);
            }
        }

        /// 

        /// 获取Message
        /// 
        /// 
        public override List Get()
        {
            List l = new List();
            l.Add(new MessageModel("XML方式获取Message", DateTime.Now));

            return l;
        }

        /// 

        /// 插入Message
        /// 
        /// Message实体对象
        /// 
        public override bool Insert(MessageModel mm)
        {
            // 代码略
            return true;
        }
    }
}

　　SqlMessage

using System;
using System.Collections.Generic;
using System.Text;

namespace Pattern.State
{
    /// 

    /// Sql方式操作Message
    /// 
    public class SqlMessage : AbstractMessageState
    {
        /// 

        /// 构造函数
        /// 
        /// Context类型参数
        public SqlMessage(Message message)
        {
            this._message = message;
        }

        /// 

        /// 改变状态
        /// 
        public override void StateChangeCheck()
        {

        }

        /// 

        /// 获取Message
        /// 
        /// 
        public override List Get()
        {
            List l = new List();
            l.Add(new MessageModel("SQL方式获取Message", DateTime.Now));

            return l;
        }

        /// 

        /// 插入Message
        /// 
        /// Message实体对象
        /// 
        public override bool Insert(MessageModel mm)
        {
            // 代码略
            return true;
        }
    }
}

　　Message

using System;
using System.Collections.Generic;
using System.Text;

namespace Pattern.State
{
    /// 

    /// Context类
    /// 
    public class Message
    {
        /// 

        /// 类型为状态模式抽象类的私有变量
        /// 
        private AbstractMessageState _abstractMessageState;

        /// 

        /// 类型为状态模式抽象类的属性
        /// 
        public AbstractMessageState AbstractMessageState
        {
            get { return _abstractMessageState; }
            set { _abstractMessageState = value; }
        }       

        /// 

        /// UserId私有变量
        /// 
        private string _userId;

        /// 

        /// UserId
        /// 
        public string UserId
        {
            get { return _userId; }
            set { _userId = value; }
        }

        /// 

        /// 构造函数
        /// 
        /// UserId
        public Message(string userId)
        {
            this._userId = userId;

            _abstractMessageState = new XmlMessage(this);
        }

        /// 

        /// 获取Message
        /// 
        /// 
        public List Get()
        {
            // 修改状态
            _abstractMessageState.StateChangeCheck();

            return _abstractMessageState.Get();
        }

        /// 

        /// 插入Message
        /// 
        /// Message实体对象
        /// 
        public bool Insert(MessageModel mm)
        {
            // 修改状态
            _abstractMessageState.StateChangeCheck();

            return _abstractMessageState.Insert(mm);
        }
    }
}

　　Client

using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;

using Pattern.State;

public partial class State : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        Message m = new Message("admin");
        Response.Write(m.Insert(new MessageModel("插入", DateTime.Now)));
        Response.Write("
");
        Response.Write(m.Get()[0].Message + " " + m.Get()[0].PublishTime.ToString());
        Response.Write("
");

        m = new Message("user");
        Response.Write(m.Insert(new MessageModel("插入", DateTime.Now)));
        Response.Write("
");
        Response.Write(m.Get()[0].Message + " " + m.Get()[0].PublishTime.ToString());
        Response.Write("
");
    }
}

　　运行结果
　　True
　　SQL方式获取Message 2007-3-6 22:58:17
　　True
　　XML方式获取Message 2007-3-6 22:58:17