Part II The Core of ASP.NET MVC in Visual Basic .NET

Printing QR in Visual Basic .NET Part II The Core of ASP.NET MVC

The VirtualPathProviderViewEngine class doesn t personally take care of the creation of the IView object; instead, it delegates that task to derived classes. The method CreateView used in the following listing is, in fact, marked as abstract on the VirtualPathProviderViewEngine class:

public virtual ViewEngineResult FindView( ControllerContext context, string viewName, string masterName, bool useCache) { string[] searchedViewLocations; string[] searchedMasterLocations; ... // Get the controller name for the current action String requiredString = context.RouteData.GetRequiredString("controller"); // Get the physical path for the ASPX template to use for the specified view String viewTemplatePath = this.GetPath(context, this.ViewLocationFormats, "ViewLocationFormats", viewName, requiredString, "View", useCache, out searchedViewLocations); // Get the physical path for the MASTER template to use for the specified master String masterTemplatePath = this.GetPath(context, this.MasterLocationFormats, "MasterLocationFormats", masterName, requiredString, "Master", useCache, out searchedMasterLocations); // Check physical paths if (!String.IsNullOrEmpty(viewTemplatePath) && (!String.IsNullOrEmpty(masterTemplatePath) || String.IsNullOrEmpty(masterName))) { // Create the view object, and pack it into a ViewEngineResult container return new ViewEngineResult( this.CreateView(context, viewTemplatePath, masterTemplatePath), this); } // If here, then view or master couldn't be resolved. The ViewEngineResult // being returned then contains only the list of locations unsuccessfully // searched. This information is used to arrange the exception message. return new ViewEngineResult( searchedViewLocations.Union<String>(searchedMasterLocations)); }

GetPath is a private member of the VirtualPathProviderViewEngine class, which contains the logic for resolving names to files. Ultimately, the method loops through a predefined list of location names and attempts to see whether a match can be found according to naming

convention rules currently set. For example, this method is responsible for implementing the rule that says the view Bar invoked by controller XXX must be a bar.aspx file located under the Views\XXX folder. The list of locations to search for views, partial views, and master views is stored in ad hoc public properties on the VirtualPathProviderViewEngine class. These properties named ViewLocationFormats, PartialViewLocationFormats, and MasterLocationFormats, respectively are string arrays left unassigned in the VirtualPathProviderViewEngine class. They are set, instead, by the actual view engine class doing the real job of creating IView objects. The GetPath method also uses a cache to speed up the search. Any view name that is successfully resolved is stored in a location view cache. The cache is then checked first on any subsequent access. The view cache is abstracted by the IViewLocationCache interface and is exposed as a public read/write property named ViewLocationCache. The class that provides view location cache services by default is DefaultViewLocationCache. It stores any resolved view names in the ASP.NET Cache object. Note Just like any other class in ASP.NET MVC, the DefaultViewLocationCache class doesn t

use any ASP.NET intrinsic objects directly. Instead, it uses the HttpContextBase class as an intermediary, which gains you isolation from the ASP.NET runtime during testing.

It s interesting to notice that the view engine doesn t throw any exception when the view name can t be resolved. As you might have noticed, the GetPath method provides an output argument at the bottom of the signature. This argument is expected to be an array of strings containing the searched locations.

private string GetPath( ControllerContext controllerContext, string[] locations, string locationsPropertyName, string name, string controllerName, string cacheKeyPrefix, bool useCache, string[] searchedLocations)

The argument is filled with any locations on the Web server where the GetPath method attempts to find a match between existing files and any provided view name. If the view engine detects that it doesn t hold enough information to create the view, it performs a set union operation between the paths that were searched for view and master view and packs that information into the returned ViewEngineResult container. That information is then displayed in the subsequent exception, as illustrated in Figure 5-2.