Reimplementing EnsurePartialViewMacroViewContextFilterAttribute and other required classes

src/Umbraco.Web.Common/Constants/Constants.cs

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+namespace Umbraco.Web.Common.Constants
+{
+    /// <summary>
+    /// constants
+    /// </summary>
+    internal static class Constants
+    {
+        internal const string ViewLocation = "~/Views";
+
+        internal const string DataTokenCurrentViewContext = "umbraco-current-view-context";
+    }
+}

src/Umbraco.Web.Common/Controllers/RenderController.cs

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+using Microsoft.AspNetCore.Mvc;
+
+namespace Umbraco.Web.Common.Controllers
+{
+    public abstract class RenderController : Controller
+    {
+
+    }
+}

src/Umbraco.Web.Common/Filters/EnsurePartialViewMacroViewContextFilterAttribute.cs

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+using System.IO;
+using System.Threading.Tasks;
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.AspNetCore.Mvc.Filters;
+using Microsoft.AspNetCore.Mvc.Rendering;
+using Microsoft.AspNetCore.Mvc.ViewEngines;
+using Microsoft.AspNetCore.Mvc.ViewFeatures;
+using Umbraco.Web.Common.Controllers;
+
+namespace Umbraco.Web.Common.Filters
+{
+    /// <summary>
+    /// This is a special filter which is required for the RTE to be able to render Partial View Macros that
+    /// contain forms when the RTE value is resolved outside of an MVC view being rendered
+    /// </summary>
+    /// <remarks>
+    /// The entire way that we support partial view macros that contain forms isn't really great, these forms
+    /// need to be executed as ChildActions so that the ModelState,ViewData,TempData get merged into that action
+    /// so the form can show errors, viewdata, etc...
+    /// Under normal circumstances, macros will be rendered after a ViewContext is created but in some cases
+    /// developers will resolve the RTE value in the controller, in this case the Form won't be rendered correctly
+    /// with merged ModelState from the controller because the special DataToken hasn't been set yet (which is
+    /// normally done in the UmbracoViewPageOfModel when a real ViewContext is available.
+    /// So we need to detect if the currently rendering controller is IRenderController and if so we'll ensure that
+    /// this DataToken exists before the action executes in case the developer resolves an RTE value that contains
+    /// a partial view macro form.
+    /// </remarks>
+    internal class EnsurePartialViewMacroViewContextFilterAttribute : ActionFilterAttribute
+    {
+        
+        /// <summary>
+        /// Ensures the custom ViewContext datatoken is set before the RenderController action is invoked,
+        /// this ensures that any calls to GetPropertyValue with regards to RTE or Grid editors can still
+        /// render any PartialViewMacro with a form and maintain ModelState
+        /// </summary>
+        /// <param name="context"></param>
+        public override void OnActionExecuting(ActionExecutingContext context)
+        {
+            if (!(context.Controller is Controller controller)) return;
+
+            //ignore anything that is not IRenderController
+            if (!(controller is RenderController)) return;
+
+            SetViewContext(context, controller);
+        }
+
+        /// <summary>
+        /// Ensures that the custom ViewContext datatoken is set after the RenderController action is invoked,
+        /// this ensures that any custom ModelState that may have been added in the RenderController itself is
+        /// passed onwards in case it is required when rendering a PartialViewMacro with a form
+        /// </summary>
+        /// <param name="context">The filter context.</param>
+        public override void OnResultExecuting(ResultExecutingContext context)
+        {
+            if (!(context.Controller is Controller controller)) return;
+
+            //ignore anything that is not IRenderController
+            if (!(controller is RenderController)) return;
+
+            SetViewContext(context, controller);
+        }
+
+        private void SetViewContext(ActionContext context, Controller controller)
+        {
+            var viewCtx = new ViewContext(
+                context,
+                new DummyView(),
+                controller.ViewData,
+                controller.TempData,
+                new StringWriter(),
+                new HtmlHelperOptions());
+
+            //set the special data token
+            context.RouteData.DataTokens[Constants.Constants.DataTokenCurrentViewContext] = viewCtx;
+        }
+
+        private class DummyView : IView
+        {
+            public Task RenderAsync(ViewContext context)
+            {
+                return Task.CompletedTask;
+            }
+
+            public string Path { get; }
+        }
+    }
+}