在asp.net中实现观察者模式？难道asp.net中的观察者模式有什么特别么？嗯，基于Http协议的Application难免有些健忘，我是这样实现的，不知道有没有更好的办法？

先谈谈需求吧，以免陷入空谈

最近一个Case， 这样的需求：很多客户端不断的向Web Application提交数据，管理员进入Web的管理页面可以即时的看到这些数据，有多个管理员可以同时浏览，且管理员浏览的数据从管理员开始监视那个时刻起，不能显示以前的数据。从这个场景一看，明显的观察者模式，管理员开始监视时，订阅数据，数据到达的时候向所有订阅了数据的管理员广播数据。

需求如下图：

有了发布者还需要订阅者，我们实现管理员类，来订阅数据

public class Admin
   {
     /**//// <summary>
     /// 用这个保存所有收到的数据
     /// </summary>
     public IList<string> MessageList
     { get; set; }
     public Admin(Monitor monitor)
     {
         MessageList = new List<string>();
         monitor.DataIn += new EventHandler< DataEventArgs>(ReciveMessage);
     }
[img]http://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif[/img]
[img]http://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif[/img]
     private void ReciveMessage(object sender, DataEventArgs e)
[img]http://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif[/img]
[img]http://www.cnblogs.com/Images/OutliningIndicators/ContractedSubBlock.gif[/img]
[img]http://www.cnblogs.com/Images/dot.gif[/img]{
[img]http://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif[/img]        
MessageList.Add(e.Message);
[img]http://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockEnd.gif[/img]
     }
[img]http://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockEnd.gif[/img]
   }
[img]http://www.cnblogs.com/Images/OutliningIndicators/None.gif[/img]

Ok，需要具备的元素我们都写好了，但是如何让它们工作起来？如果使Winform程序，那将毫无悬念。

分析：我们碰到的问题

第一个问题：当客户端发送一个数据包，我们是实例化一个新的Monitor么？如果是，哪么每次实例化一个全新的Monitor，所有在它上面订阅的事件将全部消失了，如果不是那这个Monitor将如何存在呢？总不能真空吧，两个http请求之间如何保存数据呢？不过再把需求一读，好像整个应用程序中就只需要也只能有一个这样的Monitor呢，该是单件模式上场的时候了。

在上面的Monitor的实现中添加下面的代码：

[img]http://www.cnblogs.com/Images/OutliningIndicators/None.gif[/img]
private static Monitor _instance = null;
[img]http://www.cnblogs.com/Images/OutliningIndicators/None.gif[/img]public static
Monitor Current
[img]http://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif[/img]
[img]http://www.cnblogs.com/Images/OutliningIndicators/ContractedBlock.gif[/img]
[img]http://www.cnblogs.com/Images/dot.gif[/img]{
[img]http://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif[/img]
   get
[img]http://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif[/img]
[img]http://www.cnblogs.com/Images/OutliningIndicators/ContractedSubBlock.gif[/img]  
[img]http://www.cnblogs.com/Images/dot.gif[/img]{
[img]http://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif[/img]
     if (_instance == null)
[img]http://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif[/img]
       _instance = new Monitor();
[img]http://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif[/img]
     return _instance;
[img]http://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockEnd.gif[/img]
   }
[img]http://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockEnd.gif[/img]}

但是本系统存在多个客户端，所以为了避免多线程造成问题，还是来Double Check一下吧，修改上面的代码如下：

[img]http://www.cnblogs.com/Images/OutliningIndicators/None.gif[/img]
public static Monitor Current
[img]http://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif[/img]
[img]http://www.cnblogs.com/Images/OutliningIndicators/ContractedBlock.gif[/img]    
[img]http://www.cnblogs.com/Images/dot.gif[/img]{
[img]http://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif[/img]
         get
[img]http://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif[/img]
[img]http://www.cnblogs.com/Images/OutliningIndicators/ContractedSubBlock.gif[/img]
         [img]http://www.cnblogs.com/Images/dot.gif[/img]{
[img]http://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif[/img]          
object o = new object();
[img]http://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif[/img]
           if (_instance == null)
[img]http://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif[/img]
[img]http://www.cnblogs.com/Images/OutliningIndicators/ContractedSubBlock.gif[/img]
           [img]http://www.cnblogs.com/Images/dot.gif[/img]{
[img]http://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif[/img]            
lock (o)
[img]http://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif[/img]
[img]http://www.cnblogs.com/Images/OutliningIndicators/ContractedSubBlock.gif[/img]
             [img]http://www.cnblogs.com/Images/dot.gif[/img]{
[img]http://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif[/img]                
if (_instance == null)
[img]http://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif[/img]
                   _instance = new Monitor();
[img]http://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockEnd.gif[/img]
             }
[img]http://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockEnd.gif[/img]
           }
[img]http://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif[/img]          
return _instance;
[img]http://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockEnd.gif[/img]
         }
[img]http://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockEnd.gif[/img]
     }
[img]http://www.cnblogs.com/Images/OutliningIndicators/None.gif[/img]

(PS:为什么使用单件就可以跨请求保存实例了呢？因为这里使用了一个static member保存Monitor的引用，static member在.net的GC里面是被作为Root的，详细内容请参见框架程序设计那本书)