Add extensibility point for TH attribute code gen.

- Followed the "decorate" pattern that we use throughout Razor.
- Enabled the CSharpCodeBuilder to decorate the CSharpCodeVisitor which exposes a CSharpTagHelperCodeRenderer which is indirectly used to render attribute values.
- Fixed up some existing classes that just new'd up the CSharpCodeVisitor class to instead take in the decorated version.

#119

src/Microsoft.AspNet.Razor/Generator/Compiler/CodeBuilder/CSharp/CSharpCodeBuilder.cs

@@ -39,9 +39,13 @@ public override CodeBuilderResult Build()
                         writer.WriteLine("private static object @__o;");
                     }
 
+                    var csharpCodeVisitor = DecorateCSharpCodeVisitor(writer,
+                                                                      Context,
+                                                                      new CSharpCodeVisitor(writer, Context));
+
                     new CSharpTagHelperFieldDeclarationVisitor(writer, Context).Accept(Tree.Chunks);
-                    new CSharpHelperVisitor(writer, Context).Accept(Tree.Chunks);
-                    new CSharpTypeMemberVisitor(writer, Context).Accept(Tree.Chunks);
+                    new CSharpHelperVisitor(csharpCodeVisitor, writer, Context).Accept(Tree.Chunks);
+                    new CSharpTypeMemberVisitor(csharpCodeVisitor, writer, Context).Accept(Tree.Chunks);
                     new CSharpDesignTimeHelpersVisitor(writer, Context).AcceptTree(Tree);
 
                     BuildConstructor(writer);
@@ -53,7 +57,7 @@ public override CodeBuilderResult Build()
                     {
                         using (writer.BuildMethodDeclaration("public override async", "Task", Host.GeneratedClassContext.ExecuteMethodName))
                         {
-                            new CSharpCodeVisitor(writer, Context).Accept(Tree.Chunks);
+                            csharpCodeVisitor.Accept(Tree.Chunks);
                         }
                     }
                 }
@@ -62,6 +66,13 @@ public override CodeBuilderResult Build()
             return new CodeBuilderResult(writer.GenerateCode(), writer.LineMappingManager.Mappings);
         }
 
+        protected virtual CSharpCodeVisitor DecorateCSharpCodeVisitor([NotNull] CSharpCodeWriter writer,
+                                                                      [NotNull] CodeBuilderContext context,
+                                                                      [NotNull] CSharpCodeVisitor incomingVisitor)
+        {
+            return incomingVisitor;
+        }
+
         protected virtual CSharpCodeWritingScope BuildClassDeclaration(CSharpCodeWriter writer)
         {
             var baseTypeVisitor = new CSharpBaseTypeVisitor(writer, Context);
@@ -102,7 +113,7 @@ private void AddImports(CodeTree codeTree, CSharpCodeWriter writer, IEnumerable<
             string taskNamespace = typeof(Task).Namespace;
 
             // We need to add the task namespace but ONLY if it hasn't been added by the default imports or using imports yet.
-            if(!defaultImports.Contains(taskNamespace) && !usingVisitor.ImportedUsings.Contains(taskNamespace))
+            if (!defaultImports.Contains(taskNamespace) && !usingVisitor.ImportedUsings.Contains(taskNamespace))
             {
                 writer.WriteUsing(taskNamespace);
             }

src/Microsoft.AspNet.Razor/Generator/Compiler/CodeBuilder/CSharp/CSharpTagHelperCodeRenderer.cs

@@ -24,9 +24,7 @@ public class CSharpTagHelperCodeRenderer
         private static readonly TagHelperAttributeDescriptorComparer AttributeDescriptorComparer =
             new TagHelperAttributeDescriptorComparer();
 
-        // TODO: The work to properly implement this will be done in: https://github.com/aspnet/Razor/issues/74
-        private readonly TagHelperAttributeValueCodeRenderer _attributeValueCodeRenderer =
-            new TagHelperAttributeValueCodeRenderer();
+        private readonly TagHelperAttributeValueCodeRenderer _attributeValueCodeRenderer;
         private readonly CSharpCodeWriter _writer;
         private readonly CodeBuilderContext _context;
         private readonly IChunkVisitor _bodyVisitor;
@@ -40,12 +38,14 @@ public class CSharpTagHelperCodeRenderer
         /// <param name="context">A <see cref="CodeBuilderContext"/> instance that contains information about 
         /// the current code generation process.</param>
         public CSharpTagHelperCodeRenderer([NotNull] IChunkVisitor bodyVisitor,
+                                           [NotNull] TagHelperAttributeValueCodeRenderer attributeValueCodeRenderer,
                                            [NotNull] CSharpCodeWriter writer,
                                            [NotNull] CodeBuilderContext context)
         {
+            _bodyVisitor = bodyVisitor;
+            _attributeValueCodeRenderer = attributeValueCodeRenderer;
             _writer = writer;
             _context = context;
-            _bodyVisitor = bodyVisitor;
             _tagHelperContext = context.Host.GeneratedClassContext.GeneratedTagHelperContext;
         }
 

src/Microsoft.AspNet.Razor/Generator/Compiler/CodeBuilder/CSharp/Visitors/CSharpCodeVisitor.cs

@@ -17,18 +17,25 @@ public class CSharpCodeVisitor : CodeVisitor<CSharpCodeWriter>
         private const string TemplateWriterName = "__razor_template_writer";
 
         private CSharpPaddingBuilder _paddingBuilder;
-        private CSharpTagHelperCodeRenderer _tagHelperCodeRenderer;
 
         public CSharpCodeVisitor(CSharpCodeWriter writer, CodeBuilderContext context)
             : base(writer, context)
         {
             _paddingBuilder = new CSharpPaddingBuilder(context.Host);
-            _tagHelperCodeRenderer = new CSharpTagHelperCodeRenderer(this, writer, context);
+            TagHelperRenderer = new CSharpTagHelperCodeRenderer(this, 
+                                                                new TagHelperAttributeValueCodeRenderer(), 
+                                                                writer, 
+                                                                context);
         }
 
+        public CSharpTagHelperCodeRenderer TagHelperRenderer { get; set; }
+
         protected override void Visit(TagHelperChunk chunk)
         {
-            _tagHelperCodeRenderer.RenderTagHelper(chunk);
+            if (TagHelperRenderer != null)
+            {
+                TagHelperRenderer.RenderTagHelper(chunk);
+            }
         }
 
         protected override void Visit(ChunkBlock chunk)

src/Microsoft.AspNet.Razor/Generator/Compiler/CodeBuilder/CSharp/Visitors/CSharpHelperVisitor.cs

@@ -9,12 +9,14 @@ public class CSharpHelperVisitor : CodeVisitor<CSharpCodeWriter>
     {
         private const string HelperWriterName = "__razor_helper_writer";
 
-        private CSharpCodeVisitor _codeVisitor;
+        private CSharpCodeVisitor _csharpCodeVisitor;
 
-        public CSharpHelperVisitor(CSharpCodeWriter writer, CodeBuilderContext context)
+        public CSharpHelperVisitor([NotNull] CSharpCodeVisitor csharpCodeVisitor,
+                                   [NotNull] CSharpCodeWriter writer,
+                                   [NotNull] CodeBuilderContext context)
             : base(writer, context)
         {
-            _codeVisitor = new CSharpCodeVisitor(writer, context);
+            _csharpCodeVisitor = csharpCodeVisitor;
         }
 
         protected override void Visit(HelperChunk chunk)
@@ -46,7 +48,7 @@ protected override void Visit(HelperChunk chunk)
             Context.TargetWriterName = HelperWriterName;
 
             // Generate children code
-            _codeVisitor.Accept(chunk.Children);
+            _csharpCodeVisitor.Accept(chunk.Children);
 
             Context.TargetWriterName = currentTargetWriterName;
 

src/Microsoft.AspNet.Razor/Generator/Compiler/CodeBuilder/CSharp/Visitors/CSharpTypeMemberVisitor.cs

@@ -9,10 +9,12 @@ public class CSharpTypeMemberVisitor : CodeVisitor<CSharpCodeWriter>
     {
         private CSharpCodeVisitor _csharpCodeVisitor;
 
-        public CSharpTypeMemberVisitor(CSharpCodeWriter writer, CodeBuilderContext context)
+        public CSharpTypeMemberVisitor([NotNull] CSharpCodeVisitor csharpCodeVisitor, 
+                                       [NotNull] CSharpCodeWriter writer, 
+                                       [NotNull] CodeBuilderContext context)
             : base(writer, context)
         {
-            _csharpCodeVisitor = new CSharpCodeVisitor(writer, context);
+            _csharpCodeVisitor = csharpCodeVisitor;
         }
 
         protected override void Visit(TypeMemberChunk chunk)

src/Microsoft.AspNet.Razor/TagHelpers/TagHelperAttributeValueCodeRenderer.cs

@@ -2,9 +2,6 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
-using System.Diagnostics;
-using System.Linq;
-using System.Reflection;
 using Microsoft.AspNet.Razor.Generator;
 using Microsoft.AspNet.Razor.Generator.Compiler.CSharp;
 
@@ -31,10 +28,10 @@ public class TagHelperAttributeValueCodeRenderer
         /// Writes the string: "new MyPropertyType(...)" to the output where the "..." is rendered by calling the
         /// <paramref name="renderAttributeValue"/> <see cref="Action"/>.
         /// </remarks>
-        public void RenderAttributeValue([NotNull] TagHelperAttributeDescriptor attributeDescriptor,
-                                         [NotNull] CSharpCodeWriter writer,
-                                         [NotNull] CodeBuilderContext context,
-                                         [NotNull] Action<CSharpCodeWriter> renderAttributeValue)
+        public virtual void RenderAttributeValue([NotNull] TagHelperAttributeDescriptor attributeDescriptor,
+                                                 [NotNull] CSharpCodeWriter writer,
+                                                 [NotNull] CodeBuilderContext context,
+                                                 [NotNull] Action<CSharpCodeWriter> renderAttributeValue)
         {
             renderAttributeValue(writer);
         }