Add AnchorTagHelper.

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

#1247

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

@@ -0,0 +1,162 @@
+// 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.Linq;
+using Microsoft.AspNet.Mvc.Rendering;
+using Microsoft.AspNet.Mvc.TagHelpers.Internal;
+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-";
+
+        [Activate]
+        private ViewContext ViewContext { get; set; }
+
+        [Activate]
+        private IHtmlGenerator Generator { get; set; }
+
+        /// <summary>
+        /// The name of the action method.
+        /// </summary>
+        /// <remarks>Cannot be provided if <see cref="Route"/> is specified.</remarks>
+        public string Action { get; set; }
+
+        /// <summary>
+        /// The name of the controller.
+        /// </summary>
+        /// <remarks>Cannot be provided if <see cref="Route"/> is specified.</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 for the URL.
+        /// </summary>
+        public string Host { get; set; }
+
+        /// <summary>
+        /// The URL fragment name (the anchor name).
+        /// </summary>
+        public string Fragment { get; set; }
+
+        /// <summary>
+        /// Name of the route.
+        /// </summary>
+        /// <remarks>Cannot be provided if <see cref="Action"/> or <see cref="Controller"/> is specified.</remarks>
+        public string Route { get; set; }
+
+        /// <inheritdoc />
+        /// <remarks>Does nothing if user provides a "href" attribute. Cannot specify a "href" attribute AND 
+        /// <see cref="Action"/>, <see cref="Controller"/> or <see cref="Route"/>.</remarks>
+        public override void Process(TagHelperContext context, TagHelperOutput output)
+        {
+            // 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)
+                {
+                    // User specified an href AND a Action, Controller or Route; can't determine the href attribute.
+                    throw new InvalidOperationException(
+                        Resources.FormatAnchorTagHelper_CannotDetermineHrefOneSpecified(
+                            nameof(AnchorTagHelper),
+                            nameof(Action),
+                            nameof(Controller),
+                            nameof(Route)));
+                }
+
+                // User is using the AnchorTagHelper as normal anchor tag, restore any provided attributes.
+                RestoreBoundHtmlAttributes(context, output);
+            }
+            else
+            {
+                TagBuilder tagBuilder;
+
+                var prefixedValues = TagHelperOutputHelper.PullPrefixedAttributes(RouteAttributePrefix, output);
+
+                // Generator.GenerateActionLink || GenerateRouteLink does not accept a Dictionary<string, string> for
+                // route values.
+                var routeValues = prefixedValues.ToDictionary(
+                    attribute => attribute.Key.Substring(RouteAttributePrefix.Length),
+                    attribute => (object)attribute.Value);
+
+                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(
+                            nameof(AnchorTagHelper),
+                            nameof(Route),
+                            nameof(Action),
+                            nameof(Controller)));
+                }
+                else
+                {
+                    tagBuilder = Generator.GenerateRouteLink(linkText: string.Empty,
+                                                             routeName: Route,
+                                                             protocol: Protocol,
+                                                             hostName: Host,
+                                                             fragment: Fragment,
+                                                             routeValues: routeValues,
+                                                             htmlAttributes: null);
+                }
+
+                TagHelperOutputHelper.MergeAttributes(tagBuilder, output);
+            }
+        }
+
+        private void RestoreBoundHtmlAttributes(TagHelperContext context, TagHelperOutput output)
+        {
+            if (Action != null)
+            {
+                TagHelperOutputHelper.RestoreBoundHtmlAttribute(nameof(Action), context, output);
+            }
+
+            if (Controller != null)
+            {
+                TagHelperOutputHelper.RestoreBoundHtmlAttribute(nameof(Controller), context, output);
+            }
+
+            if (Protocol != null)
+            {
+                TagHelperOutputHelper.RestoreBoundHtmlAttribute(nameof(Protocol), context, output);
+            }
+
+            if (Host != null)
+            {
+                TagHelperOutputHelper.RestoreBoundHtmlAttribute(nameof(Host), context, output);
+            }
+
+            if (Fragment != null)
+            {
+                TagHelperOutputHelper.RestoreBoundHtmlAttribute(nameof(Fragment), context, output);
+            }
+
+            if (Route != null)
+            {
+                TagHelperOutputHelper.RestoreBoundHtmlAttribute(nameof(Route), context, output);
+            }
+        }
+    }
+}

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_CannotDetermineHrefOneSpecified" xml:space="preserve">
+    <value>Cannot determine an href for {0}. {0}s with a specified href must not have a {1}, {2} or {3} attribute.</value>
+  </data>
+  <data name="AnchorTagHelper_CannotDetermineHrefRouteActionOrControllerSpecified" xml:space="preserve">
+    <value>Cannot determine an href for {0}. {0}s with a specified {1} must not have a {2} or {3} attribute.</value>
+  </data>
   <data name="FormTagHelper_CannotDetermineAction" xml:space="preserve">
     <value>Cannot determine an {1} for {0}. {0}s with a url based {1} must not have a {2} attribute.</value>
   </data>