WebFormViewEngine及用户控件寻址bug

浏览数：225 / 时间：2015年06月09日

在做我的网站的时候遇到了主题切换的问题，特总结与大家共享。

熟悉asp.net mvc的朋友都知道，mvc中，默认情况下视图都在views文件夹下放着。要想改变文件必须重写WebFormViewEngine，也就是从WebFormViewEngine继承。对于razor模板是从RazorViewEngine继承。

首选我们来回顾一下，传统的做法。假若一个网站有前台、个人中心、企业中心、后台组成（对于类似博客系统不同用户不同主题的现象问题一样），那么，每个功能都需要定义一个视图引擎。个人中心代码如下：

 1 public class UserViewEngine : WebFormViewEngine
 2 
 3      {
 4          public UserViewEngine()
 5          {
 6              MasterLocationFormats = new[]{
 7                  "~/Views/Users/{1}/{0}.master",
 8                  "~/Views/Users/shared/{0}.master"
 9              };
10  
11              ViewLocationFormats = new[]{
12                  "~/Views/Users/{1}/{0}.aspx",
13                  "~/Views/Users/{1}/{0}.ascx",
14                  "~/Views/Users/shared/{0}.aspx",
15                  "~/Views/Users/shared/{0}.ascx"
16              };
17  
18              PartialViewLocationFormats = new[]{
19                  "~/Views/Users/{1}/{0}.ascx",
20                  "~/Views/Users/shared/{0}.ascx"
21              };
22          }
23  
24          public override ViewEngineResult FindView(ControllerContext controllerContext, string viewName, string masterName, bool useCache)
25          {
26              return base.FindView(controllerContext, viewName, masterName, useCache);
27          }
28  
29          public override ViewEngineResult FindPartialView(ControllerContext controllerContext, string partialViewName, bool useCache)
30          {
31              return base.FindPartialView(controllerContext, partialViewName, useCache);
32          }
33      }

View Code

企业中心代码如下：

 1 public class MemberViewEngine : WebFormViewEngine
 2 
 3      {
 4          public MemberViewEngine()
 5          {
 6              MasterLocationFormats = new[]{
 7                  "~/Views/Member/{1}/{0}.master",
 8                  "~/Views/Member/shared/{0}.master"
 9              };
10  
11              ViewLocationFormats = new[]{
12                  "~/Views/Member/{1}/{0}.aspx",
13                  "~/Views/Member/shared/{0}.aspx"
14              };
15  
16              PartialViewLocationFormats = new[]{
17                  "~/Views/Member/{1}/{0}.ascx",
18                  "~/Views/Member/shared/{0}.ascx",
19                  "~/Views/shared/{0}.ascx"
20              };
21          }
22  
23          public override ViewEngineResult FindView(ControllerContext controllerContext, string viewName, string masterName, bool useCache)
24          {
25              return base.FindView(controllerContext, viewName, masterName, useCache);
26          }
27  
28          public override ViewEngineResult FindPartialView(ControllerContext controllerContext, string partialViewName, bool useCache)
29          {
30              return base.FindPartialView(controllerContext, partialViewName, useCache);
31          }
32      }

View Code

对于前台和后台的模板引擎，可以自己通过简单的修改去实现。

视图引擎能被调用的地方有三处，第一处是在globals中，也就是在application_start中注册视图引擎；第二个地方时在父类的构造函数中初始化；第三个是在view函数被调用前，也就是利用过滤器。

application_start中代码如下：

 protected void Application_Start()

         {
             ViewEngines.Engines.Clear();
             ViewEngines.Engines.Add(new UserViewEngine());
             AreaRegistration.RegisterAllAreas();
             RegisterRoutes(RouteTable.Routes);
             
         }

View Code

构造函数中如下：

 public ctor()

         {
             ViewEngines.Engines.Clear();
             ViewEngines.Engines.Add(new UserViewEngine());
             
         }

View Code

过滤器中如下:

1  public override void OnActionExecuting(ActionExecutingContext filterContext)
2 
3      {
4          //其他操作
5          ViewEngines.Engines.Clear();
6          ViewEngines.Engines.Add(new UserViewEngine());
7      }

View Code

对于第一种调用方法，显然满足不了我们的需求。这样只有求助于第二种和第三种调用方法。也就是在view函数被调用之前进行对当前视图引擎的替换。view函数一般如下：

1  public ActionResult About()
2 
3          {
4              return View();
5          }

View Code

这样便实现了我们的需求。

但是，目前实现的只能在单线程下使用，或者在同时都访问前台或者都放我用户中心的情况下是对的。假若同时出现并发现象。也就是一个用户访问企业中心，一个用户访问用户中心。为了说明问题（简单代码实现），贴代码如下，用户中心类似代码：

1   public ActionResult List()
2 
3          {
4          ViewEngines.Engines.Clear();
5          ViewEngines.Engines.Add(new UserViewEngine());
6  
7          Thread.Sleep(500000)
8              return View();
9          }

View Code

企业中心类似代码

1  public ActionResult List()
2 
3          {
4          ViewEngineCollection.Clear();
5          ViewEngineCollection.Add(new MemberViewEngine());
6  
7          Thread.Sleep(10000)
8              return View();
9          }

View Code

为了说明问题，我故意在上面加了线程睡眠时间。

现在，个人中心先被访问，然后企业中心再被访问。这样运行之后是会报错的，说找不到视图。

究其原因，是由于个人中心运行慢，企业中心运行快。那么用户中心的视图引擎就变成了企业中心的视图引擎。但我们明明都在函数里加了如下代码，为啥还报错呢？

1  ViewEngines.Engines.Clear();
2 
3          ViewEngines.Engines.Add(new UserViewEngine());

View Code

1  ViewEngines.Clear();
2 
3          ViewEngines.Add(new MemberViewEngine());

View Code

我抱着究根问底的态度去研究程序的源码，发觉微软的实现如下：

 1  // Copyright (c) Microsoft Open Technologies, Inc. All rights reserved. See License.txt in the project root for license information.
 2 
 3  
 4  namespace System.Web.Mvc
 5  {
 6      public static class ViewEngines
 7      {
 8          private static readonly ViewEngineCollection _engines = new ViewEngineCollection
 9          {
10              new WebFormViewEngine(),
11              new RazorViewEngine(),
12          };
13  
14          public static ViewEngineCollection Engines
15          {
16              get { return _engines; }
17          }
18      }
19  }

View Code

看到上面的代码，大家应该都明白了问题出现的根源，是静态类造成的。

那么，我们有没解决问题的办法呢？幸运的是，微软给出了另外一个调用办法，这个方法是在controller中有一个非静态变量，定义如下：

1  private ViewEngineCollection _viewEngineCollection;
2 
3  [SuppressMessage("Microsoft.Usage", "CA2227:CollectionPropertiesShouldBeReadOnly", Justification = "This entire type is meant to be mutable.")]
4          public ViewEngineCollection ViewEngineCollection
5          {
6              get { return _viewEngineCollection ?? ViewEngines.Engines; }
7              set { _viewEngineCollection = value; }
8          }

View Code

看到这里，大家是不是感觉到微软考虑问题是很周全了呢？

别急，我们看看 get { return _viewEngineCollection ?? ViewEngines.Engines; }

也就是说 _viewEngineCollection 为空的时候取的是ViewEngines.Engines。那么，我们在每个view函数被调用前都把ViewEngineCollection赋值不就行了吗？类似于下面的代码：

1  ViewEngineCollection.Clear();
2 
3          ViewEngineCollection.Add(new MemberViewEngine());

View Code

有的朋友可能发觉，这样实现是不对的，他们会提出下面的实现办法：

1 ViewEngineCollection = new ViewEngineCollection { new MemberViewEngine() };

View Code

这样的代码一运行，确实正确，我们ok！

但这是在我们没用到用户控件的情况下，假若一个母版页中加载了用户控件，类似下面的代码

1 <%Html.RenderPartial("Menu");%>

View Code

上面的代码在运行中还是会报错的。

这个问题困扰了我一段时间，我想微软不会错吧。是不是我的程序出错了，郁闷了好久。

然后，我就想到了看看RenderPartial 是怎么实现的。一查源码，发觉代码如下：

 1  // Copyright (c) Microsoft Open Technologies, Inc. All rights reserved. See License.txt in the project root for license information.
 2 
 3  
 4  namespace System.Web.Mvc.Html
 5  {
 6      public static class RenderPartialExtensions
 7      {
 8          // Renders the partial view with the parent‘s view data and model
 9          public static void RenderPartial(this HtmlHelper htmlHelper, string partialViewName)
10          {
11              htmlHelper.RenderPartialInternal(partialViewName, htmlHelper.ViewData, null /* model */, htmlHelper.ViewContext.Writer, ViewEngines.Engines);
12          }
13  
14          // Renders the partial view with the given view data and, implicitly, the given view data‘s model
15          public static void RenderPartial(this HtmlHelper htmlHelper, string partialViewName, ViewDataDictionary viewData)
16          {
17              htmlHelper.RenderPartialInternal(partialViewName, viewData, null /* model */, htmlHelper.ViewContext.Writer, ViewEngines.Engines);
18          }
19  
20          // Renders the partial view with an empty view data and the given model
21          public static void RenderPartial(this HtmlHelper htmlHelper, string partialViewName, object model)
22          {
23              htmlHelper.RenderPartialInternal(partialViewName, htmlHelper.ViewData, model, htmlHelper.ViewContext.Writer, ViewEngines.Engines);
24          }
25  
26          // Renders the partial view with a copy of the given view data plus the given model
27          public static void RenderPartial(this HtmlHelper htmlHelper, string partialViewName, object model, ViewDataDictionary viewData)
28          {
29              htmlHelper.RenderPartialInternal(partialViewName, viewData, model, htmlHelper.ViewContext.Writer, ViewEngines.Engines);
30          }
31      }
32  }

View Code

以上是微软asp.net mvc 4中实现的源码，我们看到了，微软用的还是ViewEngines.Engines，也就是说用的还是静态变量。这样，我们便找到了问题的根源。

找到问题的根源后，我就在想，能不能把该函数重写了呢？我就跟踪到源码中去，结果发觉它的实现如下：

 1  internal virtual void RenderPartialInternal(string partialViewName, ViewDataDictionary viewData, object model, TextWriter writer, ViewEngineCollection viewEngineCollection)
 2 
 3          {
 4              if (String.IsNullOrEmpty(partialViewName))
 5              {
 6                  throw new ArgumentException(MvcResources.Common_NullOrEmpty, "partialViewName");
 7              }
 8  
 9              ViewDataDictionary newViewData = null;
10  
11              if (model == null)
12              {
13                  if (viewData == null)
14                  {
15                      newViewData = new ViewDataDictionary(ViewData);
16                  }
17                  else
18                  {
19                      newViewData = new ViewDataDictionary(viewData);
20                  }
21              }
22              else
23              {
24                  if (viewData == null)
25                  {
26                      newViewData = new ViewDataDictionary(model);
27                  }
28                  else
29                  {
30                      newViewData = new ViewDataDictionary(viewData) { Model = model };
31                  }
32              }
33  
34              ViewContext newViewContext = new ViewContext(ViewContext, ViewContext.View, newViewData, ViewContext.TempData, writer);
35              IView view = FindPartialView(newViewContext, partialViewName, viewEngineCollection);
36              view.Render(newViewContext, writer);
37          }

View Code

也即使说RenderPartialInternal 只能在程序集内部被调用，外部没法调用。这样的情况让我郁闷了好久。

然后我就把 RenderPartialInternal及其相关的源码复制了出来，组成了一个自己的类，幸运的是，复制出来的代码在略微修改后可以运行，实现如下：

 1 public static class HtmlHelperExtension
 2      {
 3          public static void RenderPartial2(this HtmlHelper htmlHelper, string partialViewName)
 4          {
 5              RenderPartialInternal(htmlHelper.ViewContext,partialViewName, htmlHelper.ViewData, null /* model */, htmlHelper.ViewContext.Writer, ((Controller)htmlHelper.ViewContext.Controller).ViewEngineCollection);
 6          }
 7  
 8          internal static IView FindPartialView(ViewContext viewContext, string partialViewName, ViewEngineCollection viewEngineCollection)
 9          {
10              ViewEngineResult result = viewEngineCollection.FindPartialView(viewContext, partialViewName);
11              if (result.View != null)
12              {
13                  return result.View;
14              }
15  
16              StringBuilder locationsText = new StringBuilder();
17              foreach (string location in result.SearchedLocations)
18              {
19                  locationsText.AppendLine();
20                  locationsText.Append(location);
21              }
22  
23              throw new InvalidOperationException(partialViewName+locationsText);
24          }
25  
26          internal static void RenderPartialInternal(ViewContext ViewContext, string partialViewName, ViewDataDictionary viewData, object model, TextWriter writer, ViewEngineCollection viewEngineCollection)
27          {
28              if (String.IsNullOrEmpty(partialViewName))
29              {
30                  throw new ArgumentException("partialViewName");
31              }
32  
33              ViewDataDictionary newViewData = null;
34  
35              if (model == null)
36              {
37                  newViewData = new ViewDataDictionary(viewData);
38              }
39              else
40              {
41                  if (viewData == null)
42                  {
43                      newViewData = new ViewDataDictionary(model);
44                  }
45                  else
46                  {
47                      newViewData = new ViewDataDictionary(viewData) { Model = model };
48                  }
49              }
50  
51              ViewContext newViewContext = new ViewContext(ViewContext, ViewContext.View, newViewData, ViewContext.TempData, writer);
52              IView view = FindPartialView(newViewContext, partialViewName, viewEngineCollection);
53              view.Render(newViewContext, writer);
54          }
55      }