在asp.net中实现观察者模式,或有更好的办法?
2008-05-02 14:30 横刀天笑 阅读(3154) 评论(21) 编辑 收藏 举报先谈谈需求吧,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:
在.net里面我们有事件(event),那就无需使用传统的观察者模式的模型了(如果不了解观察者模式请点击这里)
那么我首先实现一个Monitor类,这个类用来接收客户端传递来的数据并将数据广播出去
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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(关于单件模式请点击这里)
在上面的Monitor的实现中添加下面的代码:
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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:
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(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)
第二个问题: 当管理员页面的ajax请求的时候,每两个请求如何保存数据?呵呵,上面那个问题不是说了么,用单件,但是单件是全局存在的,我们的管理员是多个,每个管理员可以决定是否订阅数据,以及什么时候订阅。想起来没?除了全局数据外我们还有Session
在管理页面上我放置一个“开始监视”的按钮,这个按钮使用ajax请求服务器端的一个HttpHandler,在Handler的ProcessRequest方法里这样来做:
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注意,由于这个Handler需要访问Session,所以你需要让这个Handler继承IRequiresSessionState接口(为什么使用继承而不用实现这个术语?实际上这个接口是一个标记接口,没有任何需要实现的成员,只是标记这个Handler可以访问Session,我不知道为什么MS不使用Attribute,是不是更合理些)
在管理页面还有个一个SetInterval不断的调用一个含有ajax的方法,去请求另外一个Handler,这个Handler将Admin收到的数据返回到web页面,让我们来看看这个Handler的部分实现:
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OK,一个在asp.net环境中实现的观察者模式基本上就算完成了,不过上面只有怎样订阅,那什么时候取消订阅了,可以在Session_End事件里面取消订阅
还查看了一些关于长连接的文章,发现这个不错,准备改进一下。
完整的代码稍后提供,希望这块转头能引来一些玉