为什么ASP.NET View Engine会检查.Mobile.chstml视图路径？

Question

对于我的ASP.NET MVC 4项目,我正在尝试实现一个自定义视图引擎来查找文件夹中存在的"Index.cshtml"视图文件.另外,我为所有未找到的视图路径抛出了404.

当视图文件不存在时,404工作.当视图文件存在时,视图引擎将尝试使用FileExists()函数查找.Mobile.cshtml文件.没有.mobile.cshtml文件,因此它会抛出异常.为什么视图引擎在找到非移动文件后仍然会查找.mobile.cshtml文件？

例如,当视图引擎能够在"〜/ Views/About/History/Index.cshtml"中找到视图路径时,它将尝试查找文件"〜/ Views/About/History/Index.Mobile.cshtml ".下面是我的自定义视图引擎的完整代码.

namespace System.Web.Mvc
{
  // Extend where RazorViewEngine looks for view files.
  // This looks for path/index.ext file if no path.ext file is found
  // Ex:  looks for "about/history/index.chstml" if "about/history.cshtml" is not found.
  public class CustomViewEngine : RazorViewEngine
  {
    public BeckmanViewEngine()
    {
      AreaViewLocationFormats = new[] 
      {
        "~/Areas/{2}/Views/{1}/{0}/Index.cshtml",
      };

      ViewLocationFormats = new[] 
      {
        "~/Views/{1}/{0}/Index.cshtml",
      };
    }

    // Return 404 Exception if viewpath file in existing path is not found
    protected override bool FileExists(ControllerContext context, string path)
    {
      if (!base.FileExists(context, path))
      {
        throw new HttpException(404, "HTTP/1.1 404 Not Found");
      }

      return true;
    }
  }
}

Answer 1

在MVC 4源代码中挖掘了一下之后,我找到了答案.

该RazorViewEngine派生自BuildManagerViewEngine,而这一次轮流从派生VirtualPathProviderViewEngine.它是VirtualPathProviderViewEngine实现该方法的那个FindView:

    public virtual ViewEngineResult FindView(ControllerContext controllerContext, string viewName, string masterName, bool useCache)
    {
        if (controllerContext == null)
        {
            throw new ArgumentNullException("controllerContext");
        }
        if (String.IsNullOrEmpty(viewName))
        {
            throw new ArgumentException(MvcResources.Common_NullOrEmpty, "viewName");
        }

        string[] viewLocationsSearched;
        string[] masterLocationsSearched;

        string controllerName = controllerContext.RouteData.GetRequiredString("controller");
        string viewPath = GetPath(controllerContext, ViewLocationFormats, AreaViewLocationFormats, "ViewLocationFormats", viewName, controllerName, CacheKeyPrefixView, useCache, out viewLocationsSearched);
        string masterPath = GetPath(controllerContext, MasterLocationFormats, AreaMasterLocationFormats, "MasterLocationFormats", masterName, controllerName, CacheKeyPrefixMaster, useCache, out masterLocationsSearched);

        if (String.IsNullOrEmpty(viewPath) || (String.IsNullOrEmpty(masterPath) && !String.IsNullOrEmpty(masterName)))
        {
            return new ViewEngineResult(viewLocationsSearched.Union(masterLocationsSearched));
        }

        return new ViewEngineResult(CreateView(controllerContext, viewPath, masterPath), this);
    }

这GetPath当视图路径尚未缓存使用会有做这样的事情还没有方法:

return nameRepresentsPath
           ? GetPathFromSpecificName(controllerContext, name, cacheKey, ref searchedLocations)
           : GetPathFromGeneralName(controllerContext, viewLocations, name, controllerName, areaName, cacheKey, ref searchedLocations);

到达那里!有趣的方法是GetPathFromGeneralName,尝试构建视图的整个路径并检查该路径是否存在.该方法循环遍历在View Engine中注册的每个视图位置,使用对当前HttpContext有效的显示模式更新视图路径,然后检查已解析的路径是否存在.如果是,则找到视图,分配给结果,缓存并返回结果路径.

private string GetPathFromGeneralName(ControllerContext controllerContext, List<ViewLocation> locations, string name, string controllerName, string areaName, string cacheKey, ref string[] searchedLocations)
{
    string result = String.Empty;
    searchedLocations = new string[locations.Count];

    for (int i = 0; i < locations.Count; i++)
    {
        ViewLocation location = locations[i];
        string virtualPath = location.Format(name, controllerName, areaName);
        DisplayInfo virtualPathDisplayInfo = DisplayModeProvider.GetDisplayInfoForVirtualPath(virtualPath, controllerContext.HttpContext, path => FileExists(controllerContext, path), controllerContext.DisplayMode);

        if (virtualPathDisplayInfo != null)
        {
            string resolvedVirtualPath = virtualPathDisplayInfo.FilePath;

            searchedLocations = _emptyLocations;
            result = resolvedVirtualPath;
            ViewLocationCache.InsertViewLocation(controllerContext.HttpContext, AppendDisplayModeToCacheKey(cacheKey, virtualPathDisplayInfo.DisplayMode.DisplayModeId), result);

            if (controllerContext.DisplayMode == null)
            {
                    controllerContext.DisplayMode = virtualPathDisplayInfo.DisplayMode;
            }

            // Populate the cache for all other display modes. We want to cache both file system hits and misses so that we can distinguish
            // in future requests whether a file's status was evicted from the cache (null value) or if the file doesn't exist (empty string).
            IEnumerable<IDisplayMode> allDisplayModes = DisplayModeProvider.Modes;
            foreach (IDisplayMode displayMode in allDisplayModes)
            {
                if (displayMode.DisplayModeId != virtualPathDisplayInfo.DisplayMode.DisplayModeId)
                {
                    DisplayInfo displayInfoToCache = displayMode.GetDisplayInfo(controllerContext.HttpContext, virtualPath, virtualPathExists: path => FileExists(controllerContext, path));

                    string cacheValue = String.Empty;
                    if (displayInfoToCache != null && displayInfoToCache.FilePath != null)
                    {
                        cacheValue = displayInfoToCache.FilePath;
                    }
                    ViewLocationCache.InsertViewLocation(controllerContext.HttpContext, AppendDisplayModeToCacheKey(cacheKey, displayMode.DisplayModeId), cacheValue);
                }
            }
            break;
        }

        searchedLocations[i] = virtualPath;
    }

    return result;
}

你可能已经注意到我还没有谈到一段带有以下注释的代码(为了清楚起见重新格式化):

// Populate the cache for all other display modes. 
// We want to cache both file system hits and misses so that we can distinguish
// in future requests whether a file's status was evicted from the cache 
// (null value) or if the file doesn't exist (empty string).

那个(和注释:)下面的代码片段)意味着一旦MVC 4找到了View Engine中注册的View Locations的第一个有效路径,它还会检查视图文件是否适用于所有其他显示模式未经测试存在,因此信息可以包含在缓存中(尽管只是针对该视图位置而不是视图引擎中可用的所有位置).另请注意,如何将lambda传递给每个测试的显示模式,以检查该模式的文件是否存在:

DisplayInfo displayInfoToCache = displayMode.GetDisplayInfo(
                      controllerContext.HttpContext, 
                      virtualPath, 
                      virtualPathExists: path => FileExists(controllerContext, path));

这就解释了为什么当你覆盖FileExists它时,也会为移动视图调用它,即使它已经找到了非移动视图.

在任何情况下,可以按照添加方式删除显示模式:通过在应用程序启动时更新DisplayModes集合.例如,删除移动显示模式并仅保留默认和非特定模式(您无法清除集合或将找不到视图):

...
using System.Web.WebPages;
...

protected void Application_Start()
{
    DisplayModeProvider.Instance.Modes.Remove(
              DisplayModeProvider.Instance.Modes
                                 .Single(m => m.DisplayModeId == "Mobile"));

相当长的答案,但希望它是有道理的!