二十四种设计模式：观察者模式(Observer Pattern)

观察者模式(Observer Pattern)

介绍
定义对象间的一种一对多的依赖关系,以便当一个对象的状态发生改变时,所有依赖于它的对象都得到通知并自动刷新。

示例
有一个Message实体类，某些对象对它的操作有Insert()方法，当发生某些改变的时候，通知所有这些对象并执行Insert()方法。

　　MessageModel

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

namespace Pattern.Observer
{
    /// <summary>
    /// Message实体类
    /// </summary>
    public class MessageModel
    {
        /// <summary>
        /// 构造函数
        /// </summary>
        /// <param name="msg">Message内容</param>
        /// <param name="pt">Message发布时间</param>
        public MessageModel(string msg, DateTime pt)
        {
            this._message = msg;
            this._publishTime = pt;
        }

        private string _message;
        /// <summary>
        /// Message内容
        /// </summary>
        public string Message
        {
            get { return _message; }
            set { _message = value; }
        }

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

　　IMessage

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

namespace Pattern.Observer
{
    /// <summary>
    /// 操作Message的接口（Observer）
    /// </summary>
    public interface IMessage
    {
        /// <summary>
        /// 插入Message
        /// </summary>
        /// <param name="mm">Message实体对象</param>
        /// <returns></returns>
        string Insert(MessageModel mm);
    }
}

　　SqlMessage

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

namespace Pattern.Observer
{
    /// <summary>
    /// Sql方式操作Message（ConcreteObserver）
    /// </summary>
    public class SqlMessage : IMessage
    {
        /// <summary>
        /// 插入Message
        /// </summary>
        /// <param name="mm">Message实体对象</param>
        /// <returns></returns>
        public string Insert(MessageModel mm)
        {
            return "sql方式：" + mm.Message + " " + mm.PublishTime.ToString();
        } 
    }
}

　　XmlMessage

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

namespace Pattern.Observer
{
    /// <summary>
    /// Xml方式操作Message（ConcreteObserver）
    /// </summary>
    public class XmlMessage : IMessage
    {
        /// <summary>
        /// 插入Message
        /// </summary>
        /// <param name="mm">Message实体对象</param>
        /// <returns></returns>
        public string Insert(MessageModel mm)
        {
            return "xml方式：" + mm.Message + " " + mm.PublishTime.ToString();
        } 
    }
}

　　AbstractMessageSubject

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

namespace Pattern.Observer
{
    /// <summary>
    /// 抽象Message主题（Subject）
    /// </summary>
    public abstract class AbstractMessageSubject
    {
        private MessageModel _messageModel;
        private List<IMessage> list = new List<IMessage>();

        /// <summary>
        /// 构造函数
        /// </summary>
        /// <param name="mm">Message实体对象</param>
        public AbstractMessageSubject(MessageModel mm)
        {
            this._messageModel = mm;
        }

        /// <summary>
        /// 添加一个实现IMessage接口的对象
        /// </summary>
        /// <param name="m">实现IMessage接口的对象</param>
        public void Attach(IMessage m)
        {
            list.Add(m);
        }

        /// <summary>
        /// 移除一个实现IMessage接口的对象
        /// </summary>
        /// <param name="m">实现IMessage接口的对象</param>
        public void Detach(IMessage m)
        {
            list.Remove(m);
        }

        /// <summary>
        /// 通知所有观察者
        /// </summary>
        /// <returns></returns>
        public string Notify()
        {
            string s = "";
            foreach (IMessage i in list)
            {
                s += i.Insert(_messageModel);
                s += "<br />";
            }

            return s;
        }

        /// <summary>
        /// Message发布时间
        /// </summary>
        public DateTime PublishTime
        {
            get { return _messageModel.PublishTime; }
            set
            {
                _messageModel.PublishTime = value;
                Notify();
            }
        }
    }
}

　　MessageSubject

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

namespace Pattern.Observer
{
    /// <summary>
    /// Message主题（ConcreteSubject）
    /// </summary>
    public class MessageSubject : AbstractMessageSubject
    {
        /// <summary>
        /// 构造函数
        /// </summary>
        /// <param name="mm">Message实体对象</param>
        public MessageSubject(MessageModel mm)
            : base(mm)
        {
 
        }
    }
}

　　Test

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.Observer;

public partial class Observer : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        MessageSubject m = new MessageSubject(new MessageModel("插入Message", DateTime.Now));

        SqlMessage sqlMessage = new SqlMessage();
        XmlMessage xmlMessage = new XmlMessage();

        m.Attach(sqlMessage);
        m.Attach(xmlMessage);

        // System.Threading.Thread.Sleep(1000);
        // 修改了PublishTime就会通知所有观察者
        // m.PublishTime = DateTime.Now;
        
        Response.Write(m.Notify());
    }
}

　　运行结果
　　sql方式：插入Message 2007-5-26 9:38:18
　　xml方式：插入Message 2007-5-26 9:38:18