Add order property to control TagHelper execution order.

- Added order property to the ITagHelper class. Lower values indicates higher execution priority.

#94

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

@@ -10,6 +10,12 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
     /// </summary>
     public interface ITagHelper
     {
+        /// <summary>
+        /// Gets the execution order of this <see cref= "ITagHelper" /> relative to others targeting the same element.
+        /// <see cref="ITagHelper"/> instances with lower values are executed first.
+        /// </summary>
+        int Order { get; }
+
         /// <summary>
         /// Asynchronously executes the <see cref="ITagHelper"/> with the given <paramref name="context"/> and
         /// <paramref name="output"/>.

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

@@ -10,6 +10,10 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
     /// </summary>
     public abstract class TagHelper : ITagHelper
     {
+        /// <inheritdoc />
+        /// <remarks>Default order is <c>0</c>.</remarks>
+        public virtual int Order { get; } = 0;
+
         /// <summary>
         /// Synchronously executes the <see cref="TagHelper"/> with the given <paramref name="context"/> and
         /// <paramref name="output"/>.

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

@@ -1,6 +1,7 @@
 // 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.Linq;
 using System.Threading.Tasks;
 
 namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
@@ -24,8 +25,9 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
                 executionContext.UniqueId,
                 executionContext.GetChildContentAsync);
             var tagHelperOutput = new TagHelperOutput(executionContext.TagName, executionContext.HTMLAttributes);
+            var orderedTagHelpers = executionContext.TagHelpers.OrderBy(tagHelper => tagHelper.Order);
 
-            foreach (var tagHelper in executionContext.TagHelpers)
+            foreach (var tagHelper in orderedTagHelpers)
             {
                 await tagHelper.ProcessAsync(tagHelperContext, tagHelperOutput);
             }