Umbraco-CMS/src/Umbraco.Web.Common/Extensions/ControllerActionEndpointConventionBuilderExtensions.cs

using System;
using System.Linq;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Mvc.Controllers;
using Microsoft.AspNetCore.Mvc.Filters;
using Microsoft.AspNetCore.Routing;
using Umbraco.Cms.Core.Models.PublishedContent;
using Umbraco.Cms.Web.Common.Filters;
using Umbraco.Cms.Web.Common.Routing;

namespace Umbraco.Extensions
{
    public static class ControllerActionEndpointConventionBuilderExtensions
    {
        /// <summary>
        /// Allows for defining a callback to set the returned <see cref="IPublishedContent"/> for the current request for this route
        /// </summary>
        public static void ForUmbracoPage(
            this ControllerActionEndpointConventionBuilder builder,
            Func<ActionExecutingContext, IPublishedContent> findContent)
            => builder.Add(convention =>
            {
                // filter out matched endpoints that are suppressed
                if (convention.Metadata.OfType<ISuppressMatchingMetadata>().FirstOrDefault()?.SuppressMatching != true)
                {
                    // Get the controller action descriptor
                    ControllerActionDescriptor actionDescriptor = convention.Metadata.OfType<ControllerActionDescriptor>().FirstOrDefault();
                    if (actionDescriptor != null)
                    {
                        // This is more or less like the IApplicationModelProvider, it allows us to add filters, etc... to the ControllerActionDescriptor
                        // dynamically. Here we will add our custom virtual page filter along with a callback in the endpoint's metadata
                        // to execute in order to find the IPublishedContent for the request.

                        var filter = new UmbracoVirtualPageFilterAttribute();

                        // Check if this already contains this filter since we don't want it applied twice.
                        // This could occur if the controller being routed is IVirtualPageController AND
                        // is being routed with ForUmbracoPage. In that case, ForUmbracoPage wins
                        // because the UmbracoVirtualPageFilterAttribute will check for the metadata first since
                        // that is more explicit and flexible in case the same controller is routed multiple times.
                        if (!actionDescriptor.FilterDescriptors.Any(x => x.Filter is UmbracoVirtualPageFilterAttribute))
                        {
                            actionDescriptor.FilterDescriptors.Add(new FilterDescriptor(filter, 0));
                            convention.Metadata.Add(filter);
                        }

                        convention.Metadata.Add(new CustomRouteContentFinderDelegate(findContent));
                    }
                }
            });
    }
}