Made INavigationTopicViewModelCore generic

This allows it to have a `Children` element that is a collection of `INavigationTopicViewModelCore` implementations, instead of limiting children exclusively to the `INavigationTopicViewModelCore` interface. This is useful in case implementers need to extend the data model.

Author: JeremyCaney

Ignia.Topics.ViewModels/NavigationTopicViewModel.cs

@@ -15,13 +15,22 @@ namespace Ignia.Topics.ViewModels {
   ///   Provides a strongly-typed data transfer object for feeding views with information about the navigation.
   /// </summary>
   /// <remarks>
-  ///   No topics are expected to have a <c>Navigation</c> content type. Instead, this view model is expected to be manually
-  ///   constructed by e.g. a <c>LayoutController</c>.
+  ///   <para>
+  ///     No topics are expected to have a <c>Navigation</c> content type. Instead, this view model is expected to be manually
+  ///     constructed by e.g. a <c>LayoutController</c>.
+  ///   </para>
+  ///   <para>
+  ///     Since C# doesn't support return-type covariance, this class can't be derived in a meaningful way (i.e., if it were to
+  ///     be, the <see cref="NavigationTopicViewModel.Children"/> property would still return a <see cref="Collection{T}"/> of
+  ///     <see cref="NavigationTopicViewModel"/> instances). Instead, the preferred way to extend the functionality is to create
+  ///     a new implementation of <see cref="INavigationTopicViewModelCore{T}"/>. To help communicate this, the <see
+  ///     cref="NavigationTopicViewModel"/> class is marked as <c>sealed</c>.
+  ///   </para>
   /// </remarks>
-  public class NavigationTopicViewModel : PageTopicViewModel, INavigationTopicViewModelCore {
+  public sealed class NavigationTopicViewModel : PageTopicViewModel, INavigationTopicViewModelCore<NavigationTopicViewModel> {
 
-    public Collection<INavigationTopicViewModelCore> Children { get; set; }
-    public bool IsSelected(string uniqueKey) => uniqueKey?.StartsWith(this.UniqueKey) ?? false;
+    public Collection<NavigationTopicViewModel> Children { get; set; }
+    public bool IsSelected(string uniqueKey) => uniqueKey?.StartsWith(UniqueKey) ?? false;
 
   } // Class
 

Ignia.Topics.Web.Mvc/Controllers/LayoutController.cs

@@ -21,14 +21,17 @@ namespace Ignia.Topics.Web.Mvc.Controllers {
   ///   Provides access to views for populating specific layout dependencies, such as the <see cref="Menu"/>.
   /// </summary>
   /// <remarks>
-  ///   As a best practice, global data required by the layout view are requested independently of the current page. This allows
-  ///   each layout element to be provided with its own layout data, in the form of <see cref="NavigationViewModel{T}"/>s,
-  ///   instead of needing to add this data to every view model returned by <see cref="TopicController"/>. The <see
-  ///   cref="LayoutController{T}"/> facilitates this by not only providing a default implementation for <see cref="Menu"/>, but
-  ///   additionally providing protected helper methods that aid in locating and assembling <see cref="Topic"/> and <see
-  ///   cref="INavigationTopicViewModelCore"/> references that are relevant to specific layout elements.
+  ///   <para>
+  ///     As a best practice, global data required by the layout view are requested independently of the current page. This
+  ///     allows each layout element to be provided with its own layout data, in the form of <see
+  ///     cref="NavigationViewModel{T}"/>s, instead of needing to add this data to every view model returned by <see
+  ///     cref="TopicController"/>. The <see cref="LayoutController{T}"/> facilitates this by not only providing a default
+  ///     implementation for <see cref="Menu"/>, but additionally providing protected helper methods that aid in locating and
+  ///     assembling <see cref="Topic"/> and <see cref="INavigationTopicViewModelCore"/> references that are relevant to
+  ///     specific layout elements.
+  ///   </para>
   /// </remarks>
-  public class LayoutController<T> : Controller where T : class, INavigationTopicViewModelCore, new() {
+  public class LayoutController<T> : Controller where T : class, INavigationTopicViewModelCore<T>, new() {
 
     /*==========================================================================================================================
     | PRIVATE VARIABLES

Ignia.Topics/INavigationTopicViewModelCore.cs

@@ -18,15 +18,15 @@ namespace Ignia.Topics {
   ///   No topics are expected to have a <c>Navigation</c> content type. Instead, implementers of this view model are expected
   ///   to manually construct instances.
   /// </remarks>
-  public interface INavigationTopicViewModelCore : IPageTopicViewModelCore {
+  public interface INavigationTopicViewModelCore<T> : IPageTopicViewModelCore where T: INavigationTopicViewModelCore<T> {
 
     /*==========================================================================================================================
     | PROPERTY: CHILDREN
     \-------------------------------------------------------------------------------------------------------------------------*/
     /// <summary>
-    ///   Represents a collection of child <see cref="INavigationTopicViewModelCore"/> instances.
+    ///   Represents a collection of child <see cref="INavigationTopicViewModelCore{T}"/> instances.
     /// </summary>
-    Collection<INavigationTopicViewModelCore> Children { get; set; }
+    Collection<T> Children { get; set; }
 
     /*==========================================================================================================================
     | METHOD: ISSELECTED
@@ -35,7 +35,7 @@ public interface INavigationTopicViewModelCore : IPageTopicViewModelCore {
     ///   Determines if the current item is selected based on the provided <paramref name="uniqueKey"/>.
     /// </summary>
     /// <param name="uniqueKey">
-    ///   The unique key to compare against the current <see cref="INavigationTopicViewModelCore"/>
+    ///   The unique key to compare against the current <see cref="INavigationTopicViewModelCore{T}"/>
     /// </param>
     bool IsSelected(string uniqueKey);