Introduced new [TopicResponseCache] attribute

The `[TopicResponseCache]` attribute operates off of the `TopicController` (or derivative) and will identify any `CacheProfile` topics associated with the `CurrentTopic` (defaulting to the `Default` cache profile) and use its attributes to set the HTTP response headers for the current request. This is compatible with the ASP.NET Core Response Caching Middleware.

This satisfies the core of Issue #89 .

OnTopic.AspNetCore.Mvc/_filters/TopicResponseCacheAttribute.cs

@@ -0,0 +1,173 @@
+/*==============================================================================================================================
+| Author        Ignia, LLC
+| Client        Ignia, LLC
+| Project       Topics Library
+\=============================================================================================================================*/
+using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Mvc.Filters;
+using Microsoft.AspNetCore.ResponseCaching;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Net.Http.Headers;
+using OnTopic.AspNetCore.Mvc.Controllers;
+using OnTopic.Attributes;
+
+namespace OnTopic.AspNetCore.Mvc {
+
+  /*============================================================================================================================
+  | CLASS: RESPONSE CACHE FILTER
+  \---------------------------------------------------------------------------------------------------------------------------*/
+  /// <summary>
+  ///   When applied to a <see cref="TopicController"/>—or derived class—will apply any configured <c>CacheProfile</c> reference
+  ///   associated with the current topic.
+  /// </summary>
+  /// <remarks>
+  ///   <para>
+  ///     The <c>Page</c> content type has a topic reference to a <c>CacheProfile</c> content type, which contains settings for
+  ///     configuring HTTP response headers. The <see cref="TopicResponseCacheAttribute"/> evaluates the current <see cref="Topic"/> to
+  ///     determine which, if any, <c>CacheProfile</c> it is associated with, and applies the settings to the HTTP response
+  ///     headers. If a <c>CacheProfile</c> is not configured, it will default to the <c>CacheProfile</c> with the <see cref="
+  ///     Topic.Key"/> of <c>Default</c>.
+  ///   </para>
+  ///   <para>
+  ///     This filter is enabled automatically when <see cref="ServiceCollectionExtensions.AddTopicSupport(IMvcBuilder)"/> is
+  ///     configured. It is only applied to actions on controllers derived from <see cref="TopicController"/>, which is needed
+  ///     in order to ensure access to the current <see cref="Topic"/> and a configured <see cref="ITopicRepository"/>.
+  ///   </para>
+  ///   <para>
+  ///     If the ASP.NET Core Response Caching Middleware is configured via e.g. <see cref="ResponseCachingServicesExtensions
+  ///     .AddResponseCaching(IServiceCollection)"/>, then the page content may be eligible for output caching, depending on the
+  ///     configuration used. This allows the same processed page content to be used for multiple clients without the server
+  ///     needing to rerender them. This reduces response time and CPU usages at a cost of increased memory.
+  ///   </para>
+  /// </remarks>
+  [AttributeUsage(AttributeTargets.Class | AttributeTargets.Method)]
+  public sealed class TopicResponseCacheAttribute : ActionFilterAttribute {
+
+    /*==========================================================================================================================
+    | PRIVATE VARIABLES
+    \-------------------------------------------------------------------------------------------------------------------------*/
+    private static              Topic?                          _defaultCacheProfile;
+
+    /*==========================================================================================================================
+    | EVENT: ON ACTION EXECUTING
+    \-------------------------------------------------------------------------------------------------------------------------*/
+    /// <inheritdoc />
+    public override void OnActionExecuting(ActionExecutingContext context) {
+
+      /*------------------------------------------------------------------------------------------------------------------------
+      | Validate parameters
+      \-----------------------------------------------------------------------------------------------------------------------*/
+      Contract.Requires(context, nameof(context));
+
+      /*------------------------------------------------------------------------------------------------------------------------
+      | Validate controller
+      \-----------------------------------------------------------------------------------------------------------------------*/
+      var controller            = context.Controller as TopicController;
+
+      if (controller is null) {
+        return;
+      }
+
+      /*------------------------------------------------------------------------------------------------------------------------
+      | Ensure default cache profile
+      \-----------------------------------------------------------------------------------------------------------------------*/
+
+      // Lookup the default cache profile for reference
+      if (_defaultCacheProfile is null) {
+        _defaultCacheProfile    = controller.TopicRepository.Load("Configuration:CacheProfiles:Default");
+      }
+
+      // Ensure the above lookup is only performed once per application
+      if (_defaultCacheProfile is null) {
+        _defaultCacheProfile    = new Topic("ImplicitDefault", "CacheProfile");
+      }
+
+      /*------------------------------------------------------------------------------------------------------------------------
+      | Identify cache profile
+      \-----------------------------------------------------------------------------------------------------------------------*/
+      var cacheProfile          = controller.CurrentTopic?.References.GetValue("CacheProfile")?? _defaultCacheProfile;
+
+      // If the empty cache profile is returned
+      if (cacheProfile.Key is "ImplicitDefault") {
+        return;
+      }
+
+      /*------------------------------------------------------------------------------------------------------------------------
+      | Establish variables
+      \-----------------------------------------------------------------------------------------------------------------------*/
+      var headers               = context.HttpContext.Response.Headers;
+      var duration              = cacheProfile.Attributes.GetInteger("Duration");
+      var location              = Enum.Parse<ResponseCacheLocation>(cacheProfile.Attributes.GetValue("Location")?? "None");
+      var noStore               = cacheProfile.Attributes.GetBoolean("NoStore");
+      var varyByHeader          = cacheProfile.Attributes.GetValue("VaryByHeader");
+      var varyByQueryKeys       = cacheProfile.Attributes.GetValue("VaryByQueryKeys");
+
+      /*------------------------------------------------------------------------------------------------------------------------
+      | Exit if the cache profile is effectively empty
+      \-----------------------------------------------------------------------------------------------------------------------*/
+      if (duration is 0 && location is 0 && !noStore && String.IsNullOrEmpty(varyByHeader + varyByQueryKeys)) {
+        return;
+      }
+
+      /*------------------------------------------------------------------------------------------------------------------------
+      | Validate metadata
+      \-----------------------------------------------------------------------------------------------------------------------*/
+      if (!noStore && duration is 0) {
+        throw new InvalidOperationException(
+          $"The {nameof(duration)} attribute must be set to a positive value if the {nameof(noStore)} attribute is not enabled."
+        );
+      }
+
+      /*------------------------------------------------------------------------------------------------------------------------
+      | Clear existing headers
+      \-----------------------------------------------------------------------------------------------------------------------*/
+      headers.Remove(HeaderNames.Vary);
+      headers.Remove(HeaderNames.CacheControl);
+      headers.Remove(HeaderNames.Pragma);
+
+      /*------------------------------------------------------------------------------------------------------------------------
+      | Vary by keys, if appropriate
+      \-----------------------------------------------------------------------------------------------------------------------*/
+      if (varyByQueryKeys is not null) {
+        var responseCachingFeature = context.HttpContext.Features.Get<IResponseCachingFeature>();
+        if (responseCachingFeature == null) {
+          throw new InvalidOperationException(
+            "VaryByQueryKeys depends on the ASP.NET Response Caching Middleware, which is not currently configured."
+          );
+        }
+        responseCachingFeature.VaryByQueryKeys = varyByQueryKeys.Split(',', StringSplitOptions.RemoveEmptyEntries);
+      }
+
+      /*------------------------------------------------------------------------------------------------------------------------
+      | Set standard HTTP headers
+      \-----------------------------------------------------------------------------------------------------------------------*/
+      if (!string.IsNullOrEmpty(varyByHeader)) {
+        headers[HeaderNames.Vary]                               = varyByHeader;
+      }
+
+      if (noStore) {
+        headers[HeaderNames.CacheControl]                       = "no-store";
+        if (location is ResponseCacheLocation.None) {
+          headers.AppendCommaSeparatedValues(HeaderNames.CacheControl, "no-cache");
+          headers[HeaderNames.Pragma]                           = "no-cache";
+        }
+        return;
+      }
+
+      if (location is ResponseCacheLocation.None) {
+        headers[HeaderNames.Pragma]                             = "no-cache";
+      }
+
+      string? cacheControl                                      = location switch {
+        ResponseCacheLocation.Any                               => "public",
+        ResponseCacheLocation.Client                            => "private",
+        ResponseCacheLocation.None                              => "no-cache",
+        _                                                       => null
+      };
+
+      headers[HeaderNames.CacheControl]                         = $"{cacheControl},max-age={duration}";
+
+    }
+
+  } //Class
+} //Namespace