Add TagHelperScopeManager for runtime.

- Also added an interface ITagHelperScopeManager so we can activate the type via DI.

#155

src/Microsoft.AspNet.Razor.Runtime/Properties/Resources.Designer.cs

@@ -62,6 +62,22 @@ namespace Microsoft.AspNet.Razor.Runtime
             return GetString("TagHelperTypeResolver_TagHelperAssemblyNameCannotBeEmptyOrNull");
         }
 
+        /// <summary>
+        /// Must call '{2}.{1}' before calling '{2}.{0}'.
+        /// </summary>
+        internal static string ScopeManager_EndCannotBeCalledWithoutACallToBegin
+        {
+            get { return GetString("ScopeManager_EndCannotBeCalledWithoutACallToBegin"); }
+        }
+
+        /// <summary>
+        /// Must call '{2}.{1}' before calling '{2}.{0}'.
+        /// </summary>
+        internal static string FormatScopeManager_EndCannotBeCalledWithoutACallToBegin(object p0, object p1, object p2)
+        {
+            return string.Format(CultureInfo.CurrentCulture, GetString("ScopeManager_EndCannotBeCalledWithoutACallToBegin"), p0, p1, p2);
+        }
+
         private static string GetString(string name, params string[] formatterNames)
         {
             var value = _resourceManager.GetString(name);

src/Microsoft.AspNet.Razor.Runtime/Resources.resx

@@ -128,4 +128,7 @@
   <data name="TagHelperTypeResolver_TagHelperAssemblyNameCannotBeEmptyOrNull" xml:space="preserve">
     <value>Tag helper directive assembly name cannot be null or empty.</value>
   </data>
+  <data name="ScopeManager_EndCannotBeCalledWithoutACallToBegin" xml:space="preserve">
+    <value>Must call '{2}.{1}' before calling '{2}.{0}'.</value>
+  </data>
 </root>

src/Microsoft.AspNet.Razor.Runtime/TagHelpers/TagHelperExecutionContext.cs

@@ -9,15 +9,15 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
     /// <summary>
     /// Class used to store information about a <see cref="ITagHelper"/>'s execution lifetime.
     /// </summary>
-    public class TagHelpersExecutionContext
+    public class TagHelperExecutionContext
     {
         private readonly List<ITagHelper> _tagHelpers;
 
         /// <summary>
-        /// Instantiates a new <see cref="TagHelpersExecutionContext"/>.
+        /// Instantiates a new <see cref="TagHelperExecutionContext"/>.
         /// </summary>
         /// <param name="tagName">The HTML tag name in the Razor source.</param>
-        public TagHelpersExecutionContext([NotNull] string tagName)
+        public TagHelperExecutionContext([NotNull] string tagName)
         {
             AllAttributes = new Dictionary<string, object>(StringComparer.Ordinal);
             HTMLAttributes = new Dictionary<string, string>(StringComparer.Ordinal);

src/Microsoft.AspNet.Razor.Runtime/TagHelpers/TagHelperRunner.cs

@@ -17,7 +17,7 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
         /// <param name="context">Contains information associated with running <see cref="ITagHelper"/>s.</param>
         /// <returns>Resulting <see cref="TagHelperOutput"/> from processing all of the 
         /// <paramref name="context"/>'s <see cref="ITagHelper"/>s.</returns>
-        public async Task<TagHelperOutput> RunAsync([NotNull] TagHelpersExecutionContext context)
+        public async Task<TagHelperOutput> RunAsync([NotNull] TagHelperExecutionContext context)
         {
             return await RunAsyncCore(context, string.Empty);
         }
@@ -29,13 +29,13 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
         /// <param name="bufferedBody">Contains the buffered content of the current HTML tag.</param>
         /// <returns>Resulting <see cref="TagHelperOutput"/> from processing all of the 
         /// <paramref name="context"/>'s <see cref="ITagHelper"/>s.</returns>
-        public async Task<TagHelperOutput> RunAsync([NotNull] TagHelpersExecutionContext context,
+        public async Task<TagHelperOutput> RunAsync([NotNull] TagHelperExecutionContext context,
                                                     [NotNull] TextWriter bufferedBody)
         {
             return await RunAsyncCore(context, bufferedBody.ToString());
         }
 
-        private async Task<TagHelperOutput> RunAsyncCore(TagHelpersExecutionContext executionContext, string outputContent)
+        private async Task<TagHelperOutput> RunAsyncCore(TagHelperExecutionContext executionContext, string outputContent)
         {
             var tagHelperContext = new TagHelperContext(executionContext.AllAttributes);
             var tagHelperOutput = new TagHelperOutput(executionContext.TagName,

src/Microsoft.AspNet.Razor.Runtime/TagHelpers/TagHelperScopeManager.cs

@@ -0,0 +1,64 @@
+// Copyright (c) Microsoft Open Technologies, Inc. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using System.Collections.Generic;
+
+namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
+{
+    /// <summary>
+    /// Class that manages <see cref="TagHelperExecutionContext"/> scopes.
+    /// </summary>
+    public class TagHelperScopeManager
+    {
+        private readonly Stack<TagHelperExecutionContext> _executionScopes;
+
+        /// <summary>
+        /// Instantiates a new <see cref="TagHelperScopeManager"/>.
+        /// </summary>
+        public TagHelperScopeManager()
+        {
+            _executionScopes = new Stack<TagHelperExecutionContext>();
+        }
+
+        /// <summary>
+        /// Starts a <see cref="TagHelperExecutionContext"/> scope.
+        /// </summary>
+        /// <param name="tagName">The HTML tag name that the scope is associated with.</param>
+        /// <returns>A <see cref="TagHelperExecutionContext"/> to use.</returns>
+        public TagHelperExecutionContext Begin(string tagName)
+        {
+            var executionContext = new TagHelperExecutionContext(tagName);
+
+            _executionScopes.Push(executionContext);
+
+            return executionContext;
+        }
+
+        /// <summary>
+        /// Ends a <see cref="TagHelperExecutionContext"/> scope.
+        /// </summary>
+        /// <returns>If the current scope is nested, the parent <see cref="TagHelperExecutionContext"/>.
+        /// <c>null</c> otherwise.</returns>
+        public TagHelperExecutionContext End()
+        {
+            if (_executionScopes.Count == 0)
+            {
+                throw new InvalidOperationException(
+                    Resources.FormatScopeManager_EndCannotBeCalledWithoutACallToBegin(
+                        nameof(End),
+                        nameof(Begin),
+                        nameof(TagHelperScopeManager)));
+            }
+
+            _executionScopes.Pop();
+
+            if (_executionScopes.Count != 0)
+            {
+                return _executionScopes.Peek();
+            }
+
+            return null;
+        }
+    }
+}