Extend MVC view engine

In time of MVC1, I created a view engine to support customized view for shopping cart system. Recently I need this feature again in another project, that’s when I tried to make it a more generic way, and come across this post -

ASP.NET MVC 2 - Building extensible view engine.

It turn out to be working very well with my case with minor changes.

In my project, I want to be able to display a customized view for each carrier/program/language. The view engine will try to search the view in the following order:

// 0-View Name; 1-Controller; 2-Area, 3-Carrier; 4-Language

// 0-View Name; 1-Controller; 2-Area, 3-Carrier; 4-Language
        "~/Content/Views/{3}/{1}/{0}.{4}.aspx",
        "~/Content/Views/{3}/{1}/{0}.aspx",
        "~/Content/Views/{3}/Shared/{0}.{4}.aspx",
        "~/Content/Views/{3}/Shared/{0}.aspx",
        "~/Views/{1}/{0}.{4}.aspx",
        "~/Views/{1}/{0}.aspx",
        "~/Views/Shared/{0}.{4}.aspx",
        "~/Views/Shared/{0}.aspx",

Below is the extend view engine:

public class WebFormThemeViewEngine : WebFormExtensibleViewEngine
{

public WebFormThemeViewEngine()
{
    // initialize placeholders dictionary
    this.Config = new PlaceholdersDictionary();
    this.Config.Add(3, GetCarrierName );
    this.Config.Add(4, GetLanguageName);

    // calls ValidateAndPrepareConfig method of the base class
    ValidateAndPrepareConfig();

    // 0-View Name; 1-Controller; 2-Area, 3-Carrier; 4-Language
    // initialize *LocationFormats with appropriate values.
    this.ViewLocationFormats = new string[] {
        "~/Content/Views/{3}/{1}/{0}.{4}.aspx",
        "~/Content/Views/{3}/{1}/{0}.aspx",
        "~/Content/Views/{3}/Shared/{0}.{4}.aspx",
        "~/Content/Views/{3}/Shared/{0}.aspx",
        "~/Views/{1}/{0}.{4}.aspx",
        "~/Views/{1}/{0}.aspx",
        "~/Views/Shared/{0}.{4}.aspx",
        "~/Views/Shared/{0}.aspx",
    };
    this.AreaViewLocationFormats = new string[] {
        "~/Content/Areas/{2}/Views/{3}/{1}/{0}.{4}.aspx",
        "~/Content/Areas/{2}/Views/{3}/{1}/{0}.aspx",
        "~/Content/Areas/{2}/Views/{3}/Shared/{0}.{4}.aspx",
        "~/Content/Areas/{2}/Views/{3}/Shared/{0}.aspx",
        "~/Areas/{2}/Views/{1}/{0}.{4}.aspx",
        "~/Areas/{2}/Views/{1}/{0}.aspx",
        "~/Areas/{2}/Views/Shared/{0}.{4}.aspx",
        "~/Areas/{2}/Views/Shared/{0}.aspx",
    };
    this.MasterLocationFormats = new string[] {
        "~/Content/Views/{3}/Shared/{0}.{4}.master",
        "~/Content/Views/{3}/Shared/{0}.master",
        "~/Views/Shared/{0}.{4}.master",
        "~/Views/Shared/{0}.master",
    };
    this.AreaMasterLocationFormats = new string[] {
        "~/Content/Areas/{2}/Views/{3}/Shared/{0}.{4}.master",
        "~/Content/Areas/{2}/Views/{3}/Shared/{0}.master",
        "~/Areas/{2}/Views/Shared/{0}.{4}.master",
        "~/Areas/{2}/Views/Shared/{0}.master",
    };
    this.PartialViewLocationFormats = new string[] {
        "~/Content/Views/{3}/{1}/{0}.{4}.ascx",
        "~/Content/Views/{3}/{1}/{0}.ascx",
        "~/Content/Views/{3}/Shared/{0}.{4}.ascx",
        "~/Content/Views/{3}/Shared/{0}.ascx",
        "~/Views/{1}/{0}.{4}.ascx",
        "~/Views/{1}/{0}.ascx",
        "~/Views/Shared/{0}.{4}.ascx",
        "~/Views/Shared/{0}.ascx",
    };
    this.AreaPartialViewLocationFormats = new string[] {
        "~/Content/Areas/{2}/Views/{3}/{1}/{0}.{4}.ascx",
        "~/Content/Areas/{2}/Views/{3}/{1}/{0}.ascx",
        "~/Content/Areas/{2}/Views/{3}/Shared/{0}.{4}.ascx",
        "~/Content/Areas/{2}/Views/{3}/Shared/{0}.ascx",
        "~/Areas/{2}/Views/{1}/{0}.{4}.ascx",
        "~/Areas/{2}/Views/{1}/{0}.ascx",
        "~/Areas/{2}/Views/Shared/{0}.{4}.ascx",
        "~/Areas/{2}/Views/Shared/{0}.ascx",
    };


}


protected virtual object GetCarrierName(ControllerContext controllerContext, string locationFormat, ref bool skipLocation)
{
    string carrierName = controllerContext.RouteData.Values["Carrier"] as string;

    return carrierName;
}

protected virtual object GetLanguageName(ControllerContext controllerContext, string locationFormat, ref bool skipLocation)
{
    string language = System.Threading.Thread.CurrentThread.CurrentUICulture.TwoLetterISOLanguageName;
    return language;
}

}

Also include the WebFormExtensibleViewEngine class from Hennadiy Kurabko.

/// 

/// a PlaceholderValueFunc delegate that represents a signature of method used to calculate value of the placeholder.
///
/// 
/// a controller context
/// location format string
/// used to specify if we must skip processed location and does not perform any searching in it.
/// an object and sends a boolean skipLocation argument by reference. 
public delegate object PlaceholderValueFunc(ControllerContext controllerContext, string locationFormat, ref bool skipLocation);

/// 

/// link a placeholder number with calculation.
/// Integer key represents a placeholder number,
/// PlaceholderValueFunc delegate represents calculation logic.
/// 
public class PlaceholdersDictionary : Dictionary
{
}

/// 

///
/// 
public class WebFormExtensibleViewEngine : WebFormViewEngine
{

public WebFormExtensibleViewEngine()
    : this(new PlaceholdersDictionary())
{
}

/// 

/// Constructor takes an instance of the PlaceholdersDictionary class as an argument
/// 
/// 
public WebFormExtensibleViewEngine(PlaceholdersDictionary config)
    : base()
{
    Config = config;

    // calls to ValidateAndPrepareConfig method.
    // It checks if {0}, {1}, {2} placeholders are used in dictionary, that is denied.
    // And adds this three placeholders with anonymous delegates that simply return "{0}" for 0 placeholder, {1} for 1 placeholder
    ValidateAndPrepareConfig();
}

public new string[] AreaMasterLocationFormats { get; set; }
public new string[] AreaPartialViewLocationFormats { get; set; }
public new string[] AreaViewLocationFormats { get; set; }
public new string[] ViewLocationFormats { get; set; }
public new string[] MasterLocationFormats { get; set; }
public new string[] PartialViewLocationFormats { get; set; }

protected PlaceholdersDictionary Config { get; set; }

public override ViewEngineResult FindPartialView(ControllerContext controllerContext, string partialViewName, bool useCache)
{
    base.AreaPartialViewLocationFormats = PrepareLocationFormats(controllerContext, this.AreaPartialViewLocationFormats);
    base.PartialViewLocationFormats = PrepareLocationFormats(controllerContext, this.PartialViewLocationFormats);

    return base.FindPartialView(controllerContext, partialViewName, useCache);
}

public override ViewEngineResult FindView(ControllerContext controllerContext, string viewName, string masterName, bool useCache)
{
    base.AreaViewLocationFormats = PrepareLocationFormats(controllerContext, this.AreaViewLocationFormats);
    base.AreaMasterLocationFormats = PrepareLocationFormats(controllerContext, this.AreaMasterLocationFormats);
    base.ViewLocationFormats = PrepareLocationFormats(controllerContext, this.ViewLocationFormats);
    base.MasterLocationFormats = PrepareLocationFormats(controllerContext, this.MasterLocationFormats);

    //if (string.IsNullOrEmpty(masterName))
    //    masterName = "Site";

    return base.FindView(controllerContext, viewName, masterName, useCache);
}

/// 

/// checks if {0}, {1}, {2} placeholders are used in dictionary, that is denied.
/// 
protected virtual void ValidateAndPrepareConfig()
{
    // Validate
    if (Config.ContainsKey(0) || Config.ContainsKey(1) || Config.ContainsKey(2))
        throw new InvalidOperationException("Placeholder index must be greater than 2. Because {0} - view name, {1} - controller name, {2} - area name.");

    // Prepare
    Config[0] = (ControllerContext controllerContext, string location, ref bool skipLocation) => "{0}";
    Config[1] = (ControllerContext controllerContext, string location, ref bool skipLocation) => "{1}";
    Config[2] = (ControllerContext controllerContext, string location, ref bool skipLocation) => "{2}";
}

protected virtual string[] PrepareLocationFormats(ControllerContext controllerContext, string[] locationFormats)
{
    // First it checks if locationFormats array is null or contains no items
    // simply returns this array as result if so.
    if (locationFormats == null || locationFormats.Length == 0)
        return locationFormats;

    // it initializes locationFormatsPrepared local variable -
    // a list that will be used to store locations that was prepared
    // ready to be used by standard MVC mechanism.
    List locationFormatsPrepared = new List();

    // for every location format it creates an array of values that will be used to replace placeholders.
    // Each value calculated by invoking appropriate delegate. If delegate sets skipLocation flag to true,
    // processing of location will be stopped and such location will be skipped.
    foreach (string locationFormat in locationFormats)
    {
          object[] formatValues = new object[Config.Count];

          bool skipLocation = false;
          for (int i = 0; i < Config.Count; i++)
          {
               object formatValue = Config[i](controllerContext, locationFormat, ref skipLocation);

               if (skipLocation) break;

               formatValues[i] = formatValue;
          }
          if (skipLocation) continue;

          locationFormatsPrepared.Add(string.Format(locationFormat, formatValues));
      }

      return locationFormatsPrepared.ToArray();
   }
}