原文:乐在其中设计模式(C#) – 访问者模式(Visitor Pattern)
乐在其中设计模式(C#) – 访问者模式(Visitor Pattern)
作者:webabcd
介绍
表示一个作用于某对象结构中的各元素的操作。它使你可以在不改变各元素的类的前提下定义作用于这些元素的新操作。
示例
有一个Message实体类,某些对象对它的操作有Insert()和Get()方法,现在要针对其中某一方法进行操作。
MessageModel
using System.Collections.Generic;
/// <param name=”msg”>Message内容</param>
/// <param name=”pt”>Message发布时间</param>
public MessageModel(string msg, DateTime pt)
private DateTime _publishTime;
AbstractElement
using System.Collections.Generic;
public abstract class AbstractElement
/// 执行抽象访问者的Visit()方法(从而执行抽象元素的方法)
/// <param name=”abstractVisitor”>抽象访问者</param>
public abstract string Accept(AbstractVisitor abstractVisitor);
Message
using System.Collections.Generic;
public abstract class Message : AbstractElement
private MessageModel _messageModel;
/// <param name=”mm”>Message实体对象</param>
public Message(MessageModel mm)
public MessageModel MessageModel
set { _messageModel = value; }
/// 执行抽象访问者的Visit()方法(从而执行抽象元素的方法)
/// <param name=”abstractVisitor”>抽象访问者</param>
public override string Accept(AbstractVisitor abstractVisitor)
return abstractVisitor.Visit(this);
public abstract List<MessageModel> Get();
public abstract bool Insert();
SqlMessage
using System.Collections.Generic;
/// Sql方式操作Message(ConcreteElement)
public class SqlMessage : Message
/// <param name=”mm”>Message实体对象</param>
public SqlMessage(MessageModel mm)
public override List<MessageModel> Get()
List<MessageModel> l = new List<MessageModel>();
l.Add(new MessageModel(“SQL方式获取Message”, DateTime.Now));
XmlMessage
using System.Collections.Generic;
/// Xml方式操作Message(ConcreteElement)
public class XmlMessage : Message
/// <param name=”mm”>Message实体对象</param>
public XmlMessage(MessageModel mm)
public override List<MessageModel> Get()
List<MessageModel> l = new List<MessageModel>();
l.Add(new MessageModel(“XML方式获取Message”, DateTime.Now));
AbstractVisitor
using System.Collections.Generic;
public abstract class AbstractVisitor
/// <param name=”abstractElement”>抽象元素</param>
public abstract string Visit(AbstractElement abstractElement);
InsertVisitor
using System.Collections.Generic;
/// Insert访问者(ConcreteVisitor)
public class InsertVisitor : AbstractVisitor
/// <param name=”abstractElement”>抽象元素</param>
public override string Visit(AbstractElement abstractElement)
Message m = abstractElement as Message;
return m.Insert().ToString() + “<br />”;
GetVisitor
using System.Collections.Generic;
public class GetVisitor : AbstractVisitor
/// <param name=”abstractElement”>抽象元素</param>
public override string Visit(AbstractElement abstractElement)
Message m = abstractElement as Message;
].PublishTime.ToString() + “<br />”;
Messages
using System.Collections.Generic;
/// Message集合(ObjectStructure)
private List<Message> _list = new List<Message>();
/// <param name=”message”>Message对象</param>
public void Attach(Message message)
/// <param name=”message”>Message对象</param>
public void Detach(Message message)
/// 执行集合内所有Message对象的Accept()方法(执行抽象访问者的Visit()方法(从而执行抽象元素的方法))
/// <param name=”abstractVisitor”>抽象访问者</param>
public string Accept(AbstractVisitor abstractVisitor)
s += m.Accept(abstractVisitor);
Test
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
public partial class Visitor : System.Web.UI.Page
protected void Page_Load(object sender, EventArgs e)
Pattern.Visitor.Messages m = new Pattern.Visitor.Messages();
m.Attach(new SqlMessage(new MessageModel(“插入”, DateTime.Now)));
m.Attach(new XmlMessage(new MessageModel(“插入”, DateTime.Now)));
Response.Write(m.Accept(new InsertVisitor()));
Response.Write(m.Accept(new GetVisitor()));
运行结果
True
True
SQL方式获取Message 2007-5-27 15:01:53
XML方式获取Message 2007-5-27 15:01:53
参考
http://www.dofactory.com/Patterns/PatternVisitor.aspx
OK