[翻译] ASP.NET MVC Tip #1 - 使用扩展方法创建新的HTML Helper

r囧r小猫 2021-12-07 14:15 189阅读 0赞

原文地址：[http://weblogs.asp.net/stephenwalther/archive/2008/06/13/asp-net-mvc-tip-1-creating-new-html-helpers-with-extension-methods.aspx][http_weblogs.asp.net_stephenwalther_archive_2008_06_13_asp-net-mvc-tip-1-creating-new-html-helpers-with-extension-methods.aspx]  
  
摘要：在这个Tip中，Stephen Walther创建了两个新的HTML Helper，你可以直接在ASP.NET MVC视图中使用它们。Stephen Walther展示了如何用扩展方法创建用于显示无序列表和有序列表的HTML Helper。  
  
  
  
在为ASP.NET MVC应用程序创建视图时，你可以使用HTML Helper来呈现标准的HTML标签。例如，对于下面的代码：

![None.gif][] < input  name ="inpSubmit"  type ="submit"  value ="Click Here!"  />

可以写为：

![None.gif][] <% =  Html.SubmitButton( " inpSubmit " ,  " Click Here! " )  %>

后面这种写法可以为你节省大量的时间。但如果没有与你要呈现的标签对应的HTML Helper呢？例如，假设你要在视图中为数据库记录显示一个无序列表。HtmlHelper类中并未包含用于显示无序列表的方法。别灰心，如果HTML Helper中没有你需要的方法，尽管去扩展它吧！  
  
通过建立新的扩展方法，你可以向HtmlHelper类添加新的功能。扩展方法看上去和普通的实例方法类似。然而，与实例方法不同，你可以在完全不同的类中为另外一个类添加扩展方法。  
  
在Visual Basic .NET中，创建扩展方法的方法是创建一个模块，并用一个特殊的特性来声明一个方法。在C\#中，可以在一个静态类中定义扩展方法，并使用关键字this来指出要扩展的类。  
  
下面展示了如何为HtmlHelper类添加扩展方法来为数据库记录显示有序列表和无序列表：  
  
清单1 - ListExtensions.vb (VB.NET)

1 ![None.gif][] Imports  System  
  2 ![None.gif][] Imports  System.Collections  
  3 ![None.gif][] Imports  System.Text  
  4 ![None.gif][] Imports  System.Web  
  5 ![None.gif][] Imports  System.Web.Mvc  
  6 ![None.gif][] Imports  System.Runtime.CompilerServices  
  7 ![None.gif][]  
  8 ![ExpandedBlockStart.gif][]![ContractedBlock.gif][] Namespace HtmlHelpers Namespace HtmlHelpers  
 9![ExpandedSubBlockStart.gif][]![ContractedSubBlock.gif][]PublicModule ListExtensionsModule ListExtensions  
10![InBlock.gif][]<Extension()> \_  
11![ExpandedSubBlockStart.gif][]![ContractedSubBlock.gif][]PublicFunction OrderedList()Function OrderedList(ByVal HtmlHelper As HtmlHelper, ByVal items AsObject) AsString  
12![InBlock.gif][]Return"<ol>"\+ ListExtensions.GetListItems(items) \+"</ol>"  
13![ExpandedSubBlockEnd.gif][]End Function  
14![InBlock.gif][]  
15![InBlock.gif][]<Extension()> \_  
16![ExpandedSubBlockStart.gif][]![ContractedSubBlock.gif][]PublicFunction UnorderedList()Function UnorderedList(ByVal HtmlHelper As HtmlHelper, ByVal items AsObject) AsString  
17![InBlock.gif][]Return"<ul>"\+ ListExtensions.GetListItems(items) \+"</ul>"  
18![ExpandedSubBlockEnd.gif][]End Function  
19![InBlock.gif][]  
20![ExpandedSubBlockStart.gif][]![ContractedSubBlock.gif][]PrivateFunction GetListItems()Function GetListItems(ByVal items AsObject) AsString  
21![InBlock.gif][]If items IsNothingThen  
22![InBlock.gif][]ThrowNew ArgumentNullException("items")  
23![InBlock.gif][]EndIf  
24![InBlock.gif][]IfNotTypeOf items Is IEnumerable Then  
25![InBlock.gif][]ThrowNew InvalidCastException("items must be IEnumerable")  
26![InBlock.gif][]EndIf  
27![InBlock.gif][]  
28![InBlock.gif][]Dim EnumItems As IEnumerable =CType(items, IEnumerable)  
29![InBlock.gif][]Dim builder AsNew StringBuilder()  
30![InBlock.gif][]  
31![InBlock.gif][]ForEach item AsObjectIn EnumItems  
32![InBlock.gif][] builder.AppendFormat("<li>\{0\}</li>", HttpUtility.HtmlEncode(item.ToString()))  
33![InBlock.gif][]Next  
34![InBlock.gif][]  
35![InBlock.gif][]Return builder.ToString()  
36![ExpandedSubBlockEnd.gif][]End Function  
37![ExpandedSubBlockEnd.gif][]End Module  
38![ExpandedBlockEnd.gif][]End Namespace

清单1 - ListExtensions.cs (C\#)

![None.gif][] using  System;  
![None.gif][] using  System.Collections;  
![None.gif][] using  System.Text;  
![None.gif][] using  System.Web;  
![None.gif][] using  System.Web.Mvc;  
![None.gif][]  
![None.gif][] namespace  BulletedListHelper.HtmlHelpers  
![ExpandedBlockStart.gif][]![ContractedBlock.gif][] ![dot.gif][] \{  
![InBlock.gif][]publicstaticclass ListExtensions  
![ExpandedSubBlockStart.gif][]![ContractedSubBlock.gif][]![dot.gif][]\{  
![InBlock.gif][]publicstaticstring OrderedList(this HtmlHelper helper, Object items)  
![ExpandedSubBlockStart.gif][]![ContractedSubBlock.gif][]![dot.gif][]\{  
![InBlock.gif][]return"<ol>"\+ ListExtensions.GetListItems(items) \+"</ol>";  
![ExpandedSubBlockEnd.gif][] \}  
![InBlock.gif][]  
![InBlock.gif][]publicstaticstring UnorderedList(this HtmlHelper helper, Object items)  
![ExpandedSubBlockStart.gif][]![ContractedSubBlock.gif][]![dot.gif][]\{  
![InBlock.gif][]return"<ul>"\+ ListExtensions.GetListItems(items) \+"</ul>";  
![ExpandedSubBlockEnd.gif][] \}  
![InBlock.gif][]  
![InBlock.gif][]privatestaticstring GetListItems(Object items)  
![ExpandedSubBlockStart.gif][]![ContractedSubBlock.gif][]![dot.gif][]\{  
![InBlock.gif][]if (items ==null)  
![InBlock.gif][]thrownew ArgumentNullException("items");  
![InBlock.gif][]if (items is IEnumerable ==false)  
![InBlock.gif][]thrownew InvalidCastException("items must be IEnumerable");  
![InBlock.gif][]  
![InBlock.gif][] var enumItems = (IEnumerable)items;  
![InBlock.gif][] var builder =new StringBuilder();  
![InBlock.gif][]foreach (Object item in enumItems)  
![InBlock.gif][] builder.AppendFormat("<li>\{0\}</li>", HttpUtility.HtmlEncode(item.ToString()));  
![InBlock.gif][]return builder.ToString();  
![ExpandedSubBlockEnd.gif][] \}  
![InBlock.gif][]  
![ExpandedSubBlockEnd.gif][] \}  
![ExpandedBlockEnd.gif][]\}

ListExtensions类有两个公共方法——OrderedList()和UnorderedList()。将集合分别传入两个方法，可以分别显示有序和无序列表。注意这些方法返回的是字符串。的确，HTML Helper方法的作用就是向浏览器呈现一个格式化的字符串。  
  
创建完扩展方法后，你可以在视图中像下面这样使用它：  
  
清单2 - Index.aspx

![ExpandedBlockStart.gif][] ![ContractedBlock.gif][] <% ![dot.gif][] @ Page Language="VB" MasterPageFile="~/Views/Shared/Site.Master" AutoEventWireup="false" CodeBehind="Index.aspx.vb" Inherits="BulletedListHelper.Index" %>   
![ExpandedBlockStart.gif][]![ContractedBlock.gif][] <% ![dot.gif][] @ Import Namespace="BulletedListHelper.HtmlHelpers" %>   
![None.gif][]  
![None.gif][] < asp:Content  ID ="indexContent"  ContentPlaceHolderID ="MainContent"  runat ="server" >   
![None.gif][] < h1 > Movies (Ordered) </ h1 >   
![None.gif][] <% =  Html.OrderedList(ViewData.Model)  %>   
![None.gif][] < h1 > Movies (Unordered) </ h1 >   
![None.gif][] <% =  Html.UnorderedList(ViewData.Model)  %>   
![None.gif][] </ asp:Content >

注意在文件的顶部引入了BulletedList.HtmlHelpers命名空间。Html.OrderedList()用于呈现有序列表，而Html.UnorderedList()用于呈现无序列表。注意在View的Html属性所暴露的HtmlHelper对象上调用了这些方法，和其他扩展方法一样。当你在浏览器中打开该视图时，可以得到如图1所示的页面：  
  
图1 - 使用自定义HTML Helper呈现的Index.aspx  
![clip_image002_thumb.jpg][]  
  
最后清单3给出了HomeController所暴露的Index()方法，该方法展示了如何将一个电影记录集合传递给Index.aspx视图。电影记录是利用Linq to SQL查询得到的。  
  
清单3 - HomeController.vb (VB.NET)

![ExpandedBlockStart.gif][] ![ContractedBlock.gif][] Public  Class HomeController Class HomeController  
![InBlock.gif][]Inherits System.Web.Mvc.Controller  
![InBlock.gif][]  
![InBlock.gif][]Private db AsNew MoviesDataContext()  
![InBlock.gif][]  
![ExpandedSubBlockStart.gif][]![ContractedSubBlock.gif][]Function Index()Function Index()  
![InBlock.gif][]Dim movies = From m In db.Movies Select m.Title  
![InBlock.gif][]Return View(movies)  
![ExpandedSubBlockEnd.gif][]End Function  
![ExpandedBlockEnd.gif][]End Class

清单3 - HomeController.cs (C\#)

![None.gif][] using  System;  
![None.gif][] using  System.Collections.Generic;  
![None.gif][] using  System.Linq;  
![None.gif][] using  System.Web;  
![None.gif][] using  System.Web.Mvc;  
![None.gif][] using  BulletedListHelper.Models;  
![None.gif][]  
![None.gif][] namespace  BulletedListHelper.Controllers  
![ExpandedBlockStart.gif][]![ContractedBlock.gif][] ![dot.gif][] \{  
![InBlock.gif][]publicclass HomeController : Controller  
![ExpandedSubBlockStart.gif][]![ContractedSubBlock.gif][]![dot.gif][]\{  
![InBlock.gif][]private MoviesDataContext db =new MoviesDataContext();  
![InBlock.gif][]  
![InBlock.gif][]public ActionResult Index()  
![ExpandedSubBlockStart.gif][]![ContractedSubBlock.gif][]![dot.gif][]\{  
![InBlock.gif][] var movies = from m in db.Movies select m.Title;  
![InBlock.gif][]return View(movies);  
![ExpandedSubBlockEnd.gif][] \}  
![ExpandedSubBlockEnd.gif][] \}  
![ExpandedBlockEnd.gif][]\}

你可以使用这种方式向ASP.NET MVC视图中呈现几乎任何东西。例如，你可以使用类似的方法创建TreeView、Menu、TabStrip等等。  
  
此处下载源代码：[http://weblogs.asp.net/blogs/stephenwalther/Downloads/Tip1/ListHtmlHelpers.zip][http_weblogs.asp.net_blogs_stephenwalther_Downloads_Tip1_ListHtmlHelpers.zip]  
  
\----  
  
广告：欢迎光临\[.NET正则表达式库\] [http://regex-lib.net/][http_regex-lib.net]。

转载于:https://www.cnblogs.com/AndersLiu/archive/2008/07/03/asp-net-mvc-tip-1-creating-new-html-helpers-with-extension-methods.html

[http_weblogs.asp.net_stephenwalther_archive_2008_06_13_asp-net-mvc-tip-1-creating-new-html-helpers-with-extension-methods.aspx]: http://weblogs.asp.net/stephenwalther/archive/2008/06/13/asp-net-mvc-tip-1-creating-new-html-helpers-with-extension-methods.aspx
[None.gif]: /images/20211207/170fd0e38ab04f5aab0fcfd2cc5b3847.png
[ExpandedBlockStart.gif]: https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif
[ContractedBlock.gif]: /images/20211207/94165db13c264cfa81ad9cd408521c55.png
[ExpandedSubBlockStart.gif]: https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif
[ContractedSubBlock.gif]: /images/20211207/ff00e2ae9b15401a9b21874c3cf3ce9c.png
[InBlock.gif]: /images/20211207/f3e9c02bd7924f69a8e8ad2728b4f409.png
[ExpandedSubBlockEnd.gif]: /images/20211207/513e01531c1f4ef8a8241a46a7f10b83.png
[ExpandedBlockEnd.gif]: /images/20211207/74fd989f6d394c9db870eb87a412cff0.png
[dot.gif]: /images/20211207/4689e6456fbd4fcebdf5f40f928e62e1.png
[clip_image002_thumb.jpg]: /images/20211207/3f9a51351ed44f198bf18af5a559474c.png
[http_weblogs.asp.net_blogs_stephenwalther_Downloads_Tip1_ListHtmlHelpers.zip]: http://weblogs.asp.net/blogs/stephenwalther/Downloads/Tip1/ListHtmlHelpers.zip
[http_regex-lib.net]: http://regex-lib.net/