解讀asp.net中的觀察者模式_.Net教程

2024-07-10 12:51:19

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在asp.net中實現觀察者模式？難道asp.net中的觀察者模式有什么特別么？嗯，基于Http協議的Application難免有些健忘，我是這樣實現的，不知道有沒有更好的辦法？

先談談需求吧，以免陷入空談

最近一個Case，這樣的需求：很多客戶端不斷的向Web Application提交數據，管理員進入Web的管理頁面可以即時的看到這些數據，有多個管理員可以同時瀏覽，且管理員瀏覽的數據從管理員開始監視那個時刻起，不能顯示以前的數據。從這個場景一看，明顯的觀察者模式，管理員開始監視時，訂閱數據，數據到達的時候向所有訂閱了數據的管理員廣播數據。

需求如下圖：

有了發布者還需要訂閱者，我們實現管理員類，來訂閱數據

以下為引用的內容：
public class Admin
{
/**//// <summary>
/// 用這個保存所有收到的數據
/// </summary>
public IList<string> MessageList
{ get; set; }
public Admin(Monitor monitor)
{
MessageList = new List<string>();
monitor.DataIn = new EventHandler< DataEventArgs>(ReciveMessage);
}
[img]http://m.survivalescaperooms.com/Images/OutliningIndicators/InBlock.gif[/img]
[img]http://m.survivalescaperooms.com/Images/OutliningIndicators/InBlock.gif[/img]
private void ReciveMessage(object sender, DataEventArgs e)
[img]http://m.survivalescaperooms.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif[/img]
[img]http://m.survivalescaperooms.com/Images/OutliningIndicators/ContractedSubBlock.gif[/img]
[img]http://m.survivalescaperooms.com/Images/dot.gif[/img]{
[img]http://m.survivalescaperooms.com/Images/OutliningIndicators/InBlock.gif[/img]
MessageList.Add(e.Message);
[img]http://m.survivalescaperooms.com/Images/OutliningIndicators/ExpandedSubBlockEnd.gif[/img]
}
[img]http://m.survivalescaperooms.com/Images/OutliningIndicators/ExpandedBlockEnd.gif[/img]
}
[img]http://m.survivalescaperooms.com/Images/OutliningIndicators/None.gif[/img]

Ok，需要具備的元素我們都寫好了，但是如何讓它們工作起來？如果使Winform程序，那將毫無懸念。

分析：我們碰到的問題

第一個問題：當客戶端發送一個數據包，我們是實例化一個新的Monitor么？如果是，哪么每次實例化一個全新的Monitor，所有在它上面訂閱的事件將全部消失了，如果不是那這個Monitor將如何存在呢？總不能真空吧，兩個http請求之間如何保存數據呢？不過再把需求一讀，好像整個應用程序中就只需要也只能有一個這樣的Monitor呢，該是單件模式上場的時候了。

在上面的Monitor的實現中添加下面的代碼：

以下為引用的內容：
[img]http://m.survivalescaperooms.com/Images/OutliningIndicators/None.gif[/img]
private static Monitor _instance = null;
[img]http://m.survivalescaperooms.com/Images/OutliningIndicators/None.gif[/img]public static
Monitor Current
[img]http://m.survivalescaperooms.com/Images/OutliningIndicators/ExpandedBlockStart.gif[/img]
[img]http://m.survivalescaperooms.com/Images/OutliningIndicators/ContractedBlock.gif[/img]
[img]http://m.survivalescaperooms.com/Images/dot.gif[/img]{
[img]http://m.survivalescaperooms.com/Images/OutliningIndicators/InBlock.gif[/img]
get
[img]http://m.survivalescaperooms.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif[/img]
[img]http://m.survivalescaperooms.com/Images/OutliningIndicators/ContractedSubBlock.gif[/img]
[img]http://m.survivalescaperooms.com/Images/dot.gif[/img]{
[img]http://m.survivalescaperooms.com/Images/OutliningIndicators/InBlock.gif[/img]
if (_instance == null)
[img]http://m.survivalescaperooms.com/Images/OutliningIndicators/InBlock.gif[/img]
_instance = new Monitor();
[img]http://m.survivalescaperooms.com/Images/OutliningIndicators/InBlock.gif[/img]
return _instance;
[img]http://m.survivalescaperooms.com/Images/OutliningIndicators/ExpandedSubBlockEnd.gif[/img]
}
[img]http://m.survivalescaperooms.com/Images/OutliningIndicators/ExpandedBlockEnd.gif[/img]}

但是本系統存在多個客戶端，所以為了避免多線程造成問題，還是來Double Check一下吧，修改上面的代碼如下：

以下為引用的內容：
[img]http://m.survivalescaperooms.com/Images/OutliningIndicators/None.gif[/img]
public static Monitor Current
[img]http://m.survivalescaperooms.com/Images/OutliningIndicators/ExpandedBlockStart.gif[/img]
[img]http://m.survivalescaperooms.com/Images/OutliningIndicators/ContractedBlock.gif[/img]
[img]http://m.survivalescaperooms.com/Images/dot.gif[/img]{
[img]http://m.survivalescaperooms.com/Images/OutliningIndicators/InBlock.gif[/img]
get
[img]http://m.survivalescaperooms.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif[/img]
[img]http://m.survivalescaperooms.com/Images/OutliningIndicators/ContractedSubBlock.gif[/img]
[img]http://m.survivalescaperooms.com/Images/dot.gif[/img]{
[img]http://m.survivalescaperooms.com/Images/OutliningIndicators/InBlock.gif[/img]
object o = new object();
[img]http://m.survivalescaperooms.com/Images/OutliningIndicators/InBlock.gif[/img]
if (_instance == null)
[img]http://m.survivalescaperooms.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif[/img]
[img]http://m.survivalescaperooms.com/Images/OutliningIndicators/ContractedSubBlock.gif[/img]
[img]http://m.survivalescaperooms.com/Images/dot.gif[/img]{
[img]http://m.survivalescaperooms.com/Images/OutliningIndicators/InBlock.gif[/img]
lock (o)
[img]http://m.survivalescaperooms.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif[/img]
[img]http://m.survivalescaperooms.com/Images/OutliningIndicators/ContractedSubBlock.gif[/img]
[img]http://m.survivalescaperooms.com/Images/dot.gif[/img]{
[img]http://m.survivalescaperooms.com/Images/OutliningIndicators/InBlock.gif[/img]
if (_instance == null)
[img]http://m.survivalescaperooms.com/Images/OutliningIndicators/InBlock.gif[/img]
_instance = new Monitor();
[img]http://m.survivalescaperooms.com/Images/OutliningIndicators/ExpandedSubBlockEnd.gif[/img]
}
[img]http://m.survivalescaperooms.com/Images/OutliningIndicators/ExpandedSubBlockEnd.gif[/img]
}
[img]http://m.survivalescaperooms.com/Images/OutliningIndicators/InBlock.gif[/img]
return _instance;
[img]http://m.survivalescaperooms.com/Images/OutliningIndicators/ExpandedSubBlockEnd.gif[/img]
}
[img]http://m.survivalescaperooms.com/Images/OutliningIndicators/ExpandedBlockEnd.gif[/img]
}
[img]http://m.survivalescaperooms.com/Images/OutliningIndicators/None.gif[/img]

(PS:為什么使用單件就可以跨請求保存實例了呢？因為這里使用了一個static member保存Monitor的引用，static member在.net的GC里面是被作為Root的，詳細內容請參見框架程序設計那本書)

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