Add Templated Razor delegates

Author: ltrzesniewski

src/RazorBlade.Analyzers/RazorBladeSourceGenerator.cs

@@ -148,6 +148,8 @@ private static RazorCSharpDocument GenerateRazorCode(SourceText sourceText, Inpu
                 });
 
                 cfg.Features.Add(new ErrorOnTagHelperSyntaxTreePass());
+
+                cfg.AddTargetExtension(new TemplateTargetExtension { TemplateTypeName = "HelperResult" });
             }
         );
 

src/RazorBlade.IntegrationTest/TestTemplate.cshtml

@@ -1,4 +1,4 @@
-@using RazorBlade.IntegrationTest
+@using System
 @inherits RazorBlade.HtmlTemplate
 
 <b>Hello, @Name!</b>
@@ -12,6 +12,13 @@
 @br
 @Html.Raw("<br>")
 
+@{
+    Func<string, IEncodedContent> bold = @<b>@item</b>;
+}
+
+@bold("Bold text")
+@bold("Other bold text")
+
 @(new Footer())
 
 @functions {

src/RazorBlade.Library.Tests/RazorTemplateTests.cs

@@ -233,6 +233,30 @@ public async Task should_buffer_output_until_flushed()
         output.ToString().ShouldEqual("foo bar baz");
     }
 
+    [Test]
+    public void should_execute_templated_delegate()
+    {
+        var template = new Template(t =>
+        {
+            Func<object, object> bold = item => new RazorTemplate.HelperResult(writer =>
+                {
+                    t.PushWriter(writer);
+                    t.WriteLiteral("<b>");
+                    t.Write(item);
+                    t.WriteLiteral("</b>");
+                    t.PopWriter();
+                    return Task.CompletedTask;
+                }
+            );
+
+            t.Write(bold("Bold text"));
+            t.WriteLiteral(" - ");
+            t.Write(bold("Other bold text"));
+        });
+
+        template.Render().ShouldEqual("<b>Bold text</b> - <b>Other bold text</b>");
+    }
+
     private class Template(Func<Template, Task> executeAction) : RazorTemplate
     {
         public Template(Action<Template> executeAction)
@@ -252,6 +276,10 @@ protected internal override async Task ExecuteAsync()
 
         protected internal override void Write(object? value)
         {
+            if (value is IEncodedContent encodedContent)
+                encodedContent.WriteTo(Output);
+            else
+                WriteLiteral(value?.ToString());
         }
 
         protected internal override void BeginWriteAttribute(string name, string prefix, int prefixOffset, string suffix, int suffixOffset, int attributeValuesCount)

src/RazorBlade.Library/RazorTemplate.cs

@@ -521,4 +521,24 @@ public void WriteTo(TextWriter textWriter)
         public override string ToString()
             => Output.ToString();
     }
+
+    /// <summary>
+    /// Represents a deferred write operation.
+    /// </summary>
+    [PublicAPI]
+    protected internal class HelperResult : IEncodedContent
+    {
+        private readonly Func<TextWriter, Task> _action;
+
+        /// <summary>
+        /// Creates a deferred operation.
+        /// </summary>
+        /// <param name="action">The action to execute.</param>
+        public HelperResult(Func<TextWriter, Task> action)
+            => _action = action;
+
+        /// <inheritdoc />
+        public void WriteTo(TextWriter textWriter)
+            => _action.Invoke(textWriter).GetAwaiter().GetResult();
+    }
 }