ViewLocators experiments (Umbraco 9) - NOTE: THIS DOES NOT FULLY WORK.

ThemeLayoutExpander.cs

/*
 LayoutExpander - adds extra paths to where Asp.net Core looks for .cshtml files.
*/

using Microsoft.AspNetCore.Mvc.Razor;
using Microsoft.Extensions.Options;

using System;
using System.Collections.Generic;
using System.Linq;

namespace Aubergine.Core.Themes
{
    public class AubergineThemesRazorViewEngineOptionsSetup : IConfigureOptions<RazorViewEngineOptions>
    {
        public void Configure(RazorViewEngineOptions options)
        {
            if (options == null)
            {
                throw new ArgumentNullException(nameof(options));
            }

            options.ViewLocationExpanders.Add(new ThemeLayoutExpander());
        }
    }

    public class ThemeLayoutExpander : IViewLocationExpander
    {
        private const string THEME_KEY = "theme";

        public IEnumerable<string> ExpandViewLocations(ViewLocationExpanderContext context, IEnumerable<string> viewLocations)
        {
            if (context.Values.TryGetValue(THEME_KEY, out string theme))
            {

                string[] themeLocations = new[]  {
                        $"/Themes/{theme}/{{0}}.cshtml",
                        $"/Views/Themes/{theme}/{{0}}.cshtml"
                    };

                viewLocations = themeLocations.Concat(viewLocations);
            }
            return viewLocations;
        }

        public void PopulateValues(ViewLocationExpanderContext context)
        {
            // something clever (configy), to workout the theme for the site.
            context.Values[THEME_KEY] = "Test";
        }
    }
}

ThemesBuilderExtension.cs

/*
 IUmbracoBuilder Extension - so we can add themes to a site. 

 this only works if it the AddAubergineThemes is called before Umbraco's AddWebsite() call
 suspect something inside that call is stopping the adding of new options to the razor views
 
*/
using Microsoft.Extensions.DependencyInjection;

using Umbraco.Cms.Core.DependencyInjection;

namespace Aubergine.Core.Themes
{
    public static class ThemesBuilderExtension
    {
        public static IUmbracoBuilder AddAubergineThemes(this IUmbracoBuilder builder)
        {
            builder.Services.ConfigureOptions<AubergineThemesRazorViewEngineOptionsSetup>();
            return builder;
        }
    }
}