Add AnchorTagHelper.

- Added a TagHelper that targets the <a> tag and allows users to do the equivalent of Html.ActionLink or Html.RouteLink.
- Added tests to validate AnchorTagHelper functionality.

#1247

src/Microsoft.AspNet.Mvc.TagHelpers/AnchorTagHelper.cs

@@ -0,0 +1,156 @@
+// 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;
+using System.Linq;
+using Microsoft.AspNet.Mvc.Rendering;
+using Microsoft.AspNet.Razor.Runtime.TagHelpers;
+
+namespace Microsoft.AspNet.Mvc.TagHelpers
+{
+    /// <summary>
+    /// <see cref="ITagHelper"/> implementation targeting &lt;a&gt; elements.
+    /// </summary>
+    [TagName("a")]
+    public class AnchorTagHelper : TagHelper
+    {
+        private const string RouteAttributePrefix = "route-";
+        private const string Href = "href";
+
+        [Activate]
+        private IHtmlGenerator Generator { get; set; }
+
+        /// <summary>
+        /// The name of the action method.
+        /// </summary>
+        /// <remarks>Must be <c>null</c> if <see cref="Route"/> is non-<c>null</c>.</remarks>
+        public string Action { get; set; }
+
+        /// <summary>
+        /// The name of the controller.
+        /// </summary>
+        /// <remarks>Must be <c>null</c> if <see cref="Route"/> is non-<c>null</c>.</remarks>
+        public string Controller { get; set; }
+
+        /// <summary>
+        /// The protocol for the URL, such as &quot;http&quot; or &quot;https&quot;.
+        /// </summary>
+        public string Protocol { get; set; }
+
+        /// <summary>
+        /// The host name.
+        /// </summary>
+        public string Host { get; set; }
+
+        /// <summary>
+        /// The URL fragment name.
+        /// </summary>
+        public string Fragment { get; set; }
+
+        /// <summary>
+        /// Name of the route.
+        /// </summary>
+        /// <remarks>
+        /// Must be <c>null</c> if <see cref="Action"/> or <see cref="Controller"/> is non-<c>null</c>.
+        /// </remarks>
+        public string Route { get; set; }
+
+        /// <inheritdoc />
+        /// <remarks>Does nothing if user provides an "href" attribute. Throws an 
+        /// <see cref="InvalidOperationException"/> if "href" attribute is provided and <see cref="Action"/>, 
+        /// <see cref="Controller"/>, or <see cref="Route"/> are non-<c>null</c>.</remarks>
+        public override void Process(TagHelperContext context, TagHelperOutput output)
+        {
+            var routePrefixedAttributes = output.FindPrefixedAttributes(RouteAttributePrefix);
+
+            // If there's an "href" on the tag it means it's being used as a normal anchor.
+            if (output.Attributes.ContainsKey(Href))
+            {
+                if (Action != null || 
+                    Controller != null || 
+                    Route != null || 
+                    Protocol != null || 
+                    Host != null || 
+                    Fragment != null || 
+                    routePrefixedAttributes.Any())
+                {
+                    // User specified an href and one of the bound attributes; can't determine the href attribute.
+                    throw new InvalidOperationException(
+                        Resources.FormatAnchorTagHelper_CannotOverrideSpecifiedHref(
+                            "<a>",
+                            nameof(Action).ToLowerInvariant(),
+                            nameof(Controller).ToLowerInvariant(),
+                            nameof(Route).ToLowerInvariant(),
+                            nameof(Protocol).ToLowerInvariant(),
+                            nameof(Host).ToLowerInvariant(),
+                            nameof(Fragment).ToLowerInvariant(),
+                            RouteAttributePrefix,
+                            Href));
+                }
+            }
+            else
+            {
+                TagBuilder tagBuilder;
+                var routeValues = GetRouteValues(output, routePrefixedAttributes);
+
+                if (Route == null)
+                {
+                    tagBuilder = Generator.GenerateActionLink(linkText: string.Empty,
+                                                              actionName: Action,
+                                                              controllerName: Controller,
+                                                              protocol: Protocol,
+                                                              hostname: Host,
+                                                              fragment: Fragment,
+                                                              routeValues: routeValues,
+                                                              htmlAttributes: null);
+                }
+                else if (Action != null || Controller != null)
+                {
+                    // Route and Action or Controller were specified. Can't determine the href attribute.
+                    throw new InvalidOperationException(
+                        Resources.FormatAnchorTagHelper_CannotDetermineHrefRouteActionOrControllerSpecified(
+                            "<a>",
+                            nameof(Route).ToLowerInvariant(),
+                            nameof(Action).ToLowerInvariant(),
+                            nameof(Controller).ToLowerInvariant(),
+                            Href));
+                }
+                else
+                {
+                    tagBuilder = Generator.GenerateRouteLink(linkText: string.Empty,
+                                                             routeName: Route,
+                                                             protocol: Protocol,
+                                                             hostName: Host,
+                                                             fragment: Fragment,
+                                                             routeValues: routeValues,
+                                                             htmlAttributes: null);
+                }
+
+                if (tagBuilder != null)
+                {
+                    output.MergeAttributes(tagBuilder);
+                }
+            }
+        }
+
+        // TODO: We will not need this method once https://github.com/aspnet/Razor/issues/89 is completed.
+        private static Dictionary<string, object> GetRouteValues(
+            TagHelperOutput output, IEnumerable<KeyValuePair<string, string>> routePrefixedAttributes)
+        {
+            Dictionary<string, object> routeValues = null;
+            if (routePrefixedAttributes.Any())
+            {
+                // Prefixed values should be treated as bound attributes, remove them from the output.
+                output.RemoveRange(routePrefixedAttributes);
+
+                // Generator.GenerateForm does not accept a Dictionary<string, string> for route values.
+                routeValues = routePrefixedAttributes.ToDictionary(
+                    attribute => attribute.Key.Substring(RouteAttributePrefix.Length),
+                    attribute => (object)attribute.Value);
+            }
+
+            return routeValues;
+        }
+    }
+}

src/Microsoft.AspNet.Mvc.TagHelpers/Resources.resx

@@ -117,6 +117,12 @@
   <resheader name="writer">
     <value>System.Resources.ResXResourceWriter, System.Windows.Forms, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</value>
   </resheader>
+  <data name="AnchorTagHelper_CannotDetermineHrefRouteActionOrControllerSpecified" xml:space="preserve">
+    <value>Cannot determine an {4} for {0}. An {0} with a specified {1} must not have an {2} or {3} attribute.</value>
+  </data>
+  <data name="AnchorTagHelper_CannotOverrideSpecifiedHref" xml:space="preserve">
+    <value>Cannot determine an {8} for {0}. An {0} with a specified {8} must not have attributes starting with {7} or an {1}, {2}, {3}, {4}, {5} or {6} attribute.</value>
+  </data>
   <data name="FormTagHelper_CannotDetermineAction" xml:space="preserve">
     <value>Cannot determine an {1} for {0}. A {0} with a URL-based {1} must not have attributes starting with {3} or a {2} attribute.</value>
   </data>