Update `HelperResult` to take in an async func.

- The corresponding Razor change results in `HelperResult`s being rendered with async lambdas.
- This change enables `TagHelper`s and other async code to exist inside of `HelperResult` blocks.
- Added test to validate Templates (they generate `HelperResult`s) can utilize `TagHelper`s correctly.
- Rename `RazorPage`s `RenderBodyDelegate` to `RenderBodyDelegateAsync`.

aspnet/Razor#494

src/Microsoft.AspNet.Mvc.Razor/HelperResult.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.IO;
+using System.Threading.Tasks;
 using Microsoft.AspNet.Html.Abstractions;
 using Microsoft.Framework.Internal;
 using Microsoft.Framework.WebEncoders;
@@ -14,24 +15,26 @@ namespace Microsoft.AspNet.Mvc.Razor
     /// </summary>
     public class HelperResult : IHtmlContent
     {
-        private readonly Action<TextWriter> _action;
+        private readonly Func<TextWriter, Task> _asyncAction;
 
         /// <summary>
         /// Creates a new instance of <see cref="HelperResult"/>.
         /// </summary>
-        /// <param name="action">The delegate to invoke when
+        /// <param name="asyncAction">The asynchronous delegate to invoke when
         /// <see cref="WriteTo(TextWriter, IHtmlEncoder)"/> is called.</param>
-        public HelperResult([NotNull] Action<TextWriter> action)
+        /// <remarks>Calls to <see cref="WriteTo(TextWriter, IHtmlEncoder)"/> result in a blocking invocation of
+        /// <paramref name="asyncAction"/>.</remarks>
+        public HelperResult([NotNull] Func<TextWriter, Task> asyncAction)
         {
-            _action = action;
+            _asyncAction = asyncAction;
         }
 
         /// <summary>
-        /// Gets the delegate to invoke when <see cref="WriteTo(TextWriter, IHtmlEncoder)"/> is called.
+        /// Gets the asynchronous delegate to invoke when <see cref="WriteTo(TextWriter, IHtmlEncoder)"/> is called.
         /// </summary>
-        public Action<TextWriter> WriteAction
+        public Func<TextWriter, Task> WriteAction
         {
-            get { return _action; }
+            get { return _asyncAction; }
         }
 
         /// <summary>
@@ -41,7 +44,7 @@ namespace Microsoft.AspNet.Mvc.Razor
         /// <param name="encoder">The <see cref="IHtmlEncoder"/> to encode the content.</param>
         public virtual void WriteTo([NotNull] TextWriter writer, [NotNull] IHtmlEncoder encoder)
         {
-            _action(writer);
+            _asyncAction(writer).GetAwaiter().GetResult();
         }
     }
 }

src/Microsoft.AspNet.Mvc.Razor/IRazorPage.cs

@@ -22,7 +22,7 @@ namespace Microsoft.AspNet.Mvc.Razor
         /// <summary>
         /// Gets or sets the action invoked to render the body.
         /// </summary>
-        Action<TextWriter> RenderBodyDelegate { get; set; }
+        Func<TextWriter, Task> RenderBodyDelegateAsync { get; set; }
 
         /// <summary>
         /// Gets or sets a flag that determines if the layout of this page is being rendered.

src/Microsoft.AspNet.Mvc.Razor/RazorPage.cs

@@ -126,7 +126,7 @@ namespace Microsoft.AspNet.Mvc.Razor
         }
 
         /// <inheritdoc />
-        public Action<TextWriter> RenderBodyDelegate { get; set; }
+        public Func<TextWriter, Task> RenderBodyDelegateAsync { get; set; }
 
         /// <inheritdoc />
         public bool IsLayoutBeingRendered { get; set; }
@@ -685,14 +685,14 @@ namespace Microsoft.AspNet.Mvc.Razor
 
         protected virtual HelperResult RenderBody()
         {
-            if (RenderBodyDelegate == null)
+            if (RenderBodyDelegateAsync == null)
             {
                 var message = Resources.FormatRazorPage_MethodCannotBeCalled(nameof(RenderBody), Path);
                 throw new InvalidOperationException(message);
             }
 
             _renderedBody = true;
-            return new HelperResult(RenderBodyDelegate);
+            return new HelperResult(RenderBodyDelegateAsync);
         }
 
         /// <summary>
@@ -860,7 +860,7 @@ namespace Microsoft.AspNet.Mvc.Razor
                     throw new InvalidOperationException(Resources.FormatSectionsNotRendered(Path, sectionNames));
                 }
             }
-            else if (RenderBodyDelegate != null && !_renderedBody)
+            else if (RenderBodyDelegateAsync != null && !_renderedBody)
             {
                 // There are no sections defined, but RenderBody was NOT called.
                 // If a body was defined, then RenderBody should have been called.

src/Microsoft.AspNet.Mvc.Razor/RazorView.cs

@@ -189,7 +189,7 @@ namespace Microsoft.AspNet.Mvc.Razor
                 // in the layout.
                 previousPage.IsLayoutBeingRendered = true;
                 layoutPage.PreviousSectionWriters = previousPage.SectionWriters;
-                layoutPage.RenderBodyDelegate = bodyWriter.CopyTo;
+                layoutPage.RenderBodyDelegateAsync = bodyWriter.CopyToAsync;
                 bodyWriter = await RenderPageAsync(layoutPage, context, executeViewStart: false);
 
                 renderedLayouts.Add(layoutPage);

test/Microsoft.AspNet.Mvc.FunctionalTests/compiler/resources/TagHelpersWebSite.Home.Index.html

@@ -27,6 +27,8 @@
 
 
 
+
+
 <div>
     <p>This website has <em><strong style="font-size: 1.25em;text-decoration: underline;">not</strong></em> been approved yet. Visit <strong><a target="_blank" href="http://www.contoso.com">www.contoso.com</a></strong> for <strong>more</strong> information.</p>
 </div>
@@ -36,6 +38,8 @@
     <div>["Lorem","ipsum","dolor","sit","amet","consectetur","adipisicing","elit","sed","do","eiusmod","tempor","incididunt","ut","labore","et","dolore","magna","aliquaUt","enim"]</div>
     <h3 style="font-family: cursive;">Current Tag Cloud from ViewComponentHelper:</h3>
     <section><b>["Lorem","ipsum","dolor","sit","amet","consectetur","adipisicing","elit","sed","do","eiusmod","tempor","incididunt","ut","labore"]</b></section>
+<br /><p><em>Rendering Template:</em></p>
+<div><h3 style="font-family: cursive;">Tag Cloud from Template: </h3>["Lorem","ipsum","dolor","sit","amet","consectetur","adipisicing","elit","sed","do","eiusmod","tempor","incididunt","ut","labore","et","dolore","magna","aliquaUt","enim"]</div>
 </div>
 
 

test/Microsoft.AspNet.Mvc.Razor.Test/RazorPageTest.cs

@@ -301,7 +301,7 @@ namespace Microsoft.AspNet.Mvc.Razor
             {
                 { "baz", _nullRenderAsyncDelegate }
             };
-            page.RenderBodyDelegate = CreateBodyAction("body-content");
+            page.RenderBodyDelegateAsync = CreateBodyAction("body-content");
 
             // Act
             await page.ExecuteAsync();
@@ -325,7 +325,7 @@ namespace Microsoft.AspNet.Mvc.Razor
             {
                 { "baz", _nullRenderAsyncDelegate }
             };
-            page.RenderBodyDelegate = CreateBodyAction("body-content");
+            page.RenderBodyDelegateAsync = CreateBodyAction("body-content");
 
             // Act
             await page.ExecuteAsync();
@@ -338,7 +338,7 @@ namespace Microsoft.AspNet.Mvc.Razor
         public async Task RenderSection_ThrowsIfSectionIsRenderedMoreThanOnce()
         {
             // Arrange
-            var expected = new HelperResult(action: null);
+            var expected = new HelperResult(asyncAction: null);
             var page = CreatePage(v =>
             {
                 v.Path = "/Views/TestPath/Test.cshtml";
@@ -362,7 +362,7 @@ namespace Microsoft.AspNet.Mvc.Razor
         public async Task RenderSectionAsync_ThrowsIfSectionIsRenderedMoreThanOnce()
         {
             // Arrange
-            var expected = new HelperResult(action: null);
+            var expected = new HelperResult(asyncAction: null);
             var page = CreatePage(async v =>
             {
                 v.Path = "/Views/TestPath/Test.cshtml";
@@ -386,7 +386,7 @@ namespace Microsoft.AspNet.Mvc.Razor
         public async Task RenderSectionAsync_ThrowsIfSectionIsRenderedMoreThanOnce_WithSyncMethod()
         {
             // Arrange
-            var expected = new HelperResult(action: null);
+            var expected = new HelperResult(asyncAction: null);
             var page = CreatePage(async v =>
             {
                 v.Path = "/Views/TestPath/Test.cshtml";
@@ -410,7 +410,7 @@ namespace Microsoft.AspNet.Mvc.Razor
         public async Task RenderSectionAsync_ThrowsIfNotInvokedFromLayoutPage()
         {
             // Arrange
-            var expected = new HelperResult(action: null);
+            var expected = new HelperResult(asyncAction: null);
             var page = CreatePage(async v =>
             {
                 v.Path = "/Views/TestPath/Test.cshtml";
@@ -434,7 +434,7 @@ namespace Microsoft.AspNet.Mvc.Razor
             {
             });
             page.Path = path;
-            page.RenderBodyDelegate = CreateBodyAction("some content");
+            page.RenderBodyDelegateAsync = CreateBodyAction("some content");
 
             // Act
             await page.ExecuteAsync();
@@ -454,7 +454,7 @@ namespace Microsoft.AspNet.Mvc.Razor
             {
             });
             page.Path = path;
-            page.RenderBodyDelegate = CreateBodyAction("some content");
+            page.RenderBodyDelegateAsync = CreateBodyAction("some content");
             page.PreviousSectionWriters = new Dictionary<string, RenderAsyncDelegate>
             {
                 { sectionName, _nullRenderAsyncDelegate }
@@ -480,7 +480,7 @@ namespace Microsoft.AspNet.Mvc.Razor
                 v.RenderSection(sectionA);
                 v.RenderSection(sectionB);
             });
-            page.RenderBodyDelegate = CreateBodyAction("some content");
+            page.RenderBodyDelegateAsync = CreateBodyAction("some content");
             page.PreviousSectionWriters = new Dictionary<string, RenderAsyncDelegate>
             {
                 { sectionA, _nullRenderAsyncDelegate },
@@ -514,7 +514,7 @@ namespace Microsoft.AspNet.Mvc.Razor
                 v.Write(v.RenderSection("footer"));
                 v.WriteLiteral("Layout end");
             });
-            page.RenderBodyDelegate = CreateBodyAction("body content" + Environment.NewLine);
+            page.RenderBodyDelegateAsync = CreateBodyAction("body content" + Environment.NewLine);
             page.PreviousSectionWriters = new Dictionary<string, RenderAsyncDelegate>
             {
                 {
@@ -1834,9 +1834,9 @@ namespace Microsoft.AspNet.Mvc.Razor
                 new HtmlHelperOptions());
         }
 
-        private static Action<TextWriter> CreateBodyAction(string value)
+        private static Func<TextWriter, Task> CreateBodyAction(string value)
         {
-            return (writer) => writer.Write(value);
+            return async (writer) => await writer.WriteAsync(value);
         }
 
         public abstract class TestableRazorPage : RazorPage

test/WebSites/TagHelpersWebSite/Views/Home/Index.cshtml

@@ -1,4 +1,6 @@
 @using TagHelpersWebSite.Models
+@using Microsoft.AspNet.Mvc.Razor
+
 @model WebsiteContext
 
 @{
@@ -16,6 +18,15 @@
     </style>
 }
 
+@functions {
+    public void RenderTemplate(string title, Func<string, HelperResult> template)
+    {
+        Output.WriteLine("<br /><p><em>Rendering Template:</em></p>");
+        var helperResult = template(title);
+        helperResult.WriteTo(Output, HtmlEncoder);
+    }
+}
+
 <div condition="!Model.Approved">
     <p>This website has <strong surround="em">not</strong> been approved yet. Visit www.contoso.com for <strong make-pretty="false">more</strong> information.</p>
 </div>
@@ -25,6 +36,11 @@
     <tag-cloud count="Model.TagsToShow" surround="div" />
     <h3>Current Tag Cloud from ViewComponentHelper:</h3>
     <section bold>@await Component.InvokeAsync("Tags", 15)</section>
+    @{
+        RenderTemplate(
+            "Tag Cloud from Template: ",
+            @<div condition="true"><h3>@item</h3><tag-cloud count="Model.TagsToShow"></tag-cloud></div>);
+    }
 </div>
 
 @section footerContent {