Add FormTagHelper.

- Added the FormTagHelper.
- Utilized the IHtmlGenerator to share base functionality with the HTMLHelper counterparts.

#1246

Author: N. Taylor Mullen

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

@@ -0,0 +1,125 @@
+// 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;
+using Microsoft.AspNet.Razor.TagHelpers;
+
+namespace Microsoft.AspNet.Mvc.TagHelpers
+{
+    /// <summary>
+    /// <see cref="ITagHelper"/> implementation targeting &lt;form&gt; elements.
+    /// </summary>
+    [ContentBehavior(ContentBehavior.Append)]
+    public class FormTagHelper : TagHelper
+    {
+        [Activate]
+        private ViewContext ViewContext { get; set; }
+
+        [Activate]
+        private AntiForgery AntiForgery { get; set; }
+
+        [Activate]
+        private IHtmlGenerator Generator { get; set; }
+
+        /// <summary>
+        /// The name of the action method.
+        /// </summary>
+        /// <remarks>
+        /// If value contains a '/' this <see cref="ITagHelper"/> will do nothing.
+        /// </remarks>
+        public string Action { get; set; }
+
+        /// <summary>
+        /// The name of the controller.
+        /// </summary>
+        public string Controller { get; set; }
+
+        /// <summary>
+        /// The HTTP method for processing the form, either GET or POST.
+        /// </summary>
+        public string Method { get; set; }
+
+        /// <inheritdoc />
+        /// <remarks>Does nothing if <see cref="Action"/> contains a '/'.</remarks>
+        public override void Process(TagHelperContext context, TagHelperOutput output)
+        {
+            // If Action contains a '/' it means the user is attempting to use the FormTagHelper as a normal form.
+            if (Action != null && Action.Contains('/'))
+            {
+                RestoreBoundHtmlAttributes(context, output);
+            }
+            else
+            {
+                // TODO: Make this behavior optional once https://github.com/aspnet/Razor/issues/121 is completed.
+                output.Content = AntiForgery.GetHtml(ViewContext.HttpContext).ToString();
+
+                var routeValues = PullRouteValues(output.Attributes);
+                var tagBuilder = Generator.GenerateForm(ViewContext,
+                                                        Action,
+                                                        Controller,
+                                                        routeValues,
+                                                        Method,
+                                                        htmlAttributes: new Dictionary<string, object>());
+
+                TagHelperOutputHelper.MergeAttributes(output, tagBuilder);
+            }
+        }
+
+        // TODO: We will not need this method once https://github.com/aspnet/Razor/issues/89 is completed.
+        private static Dictionary<string, object> PullRouteValues(IDictionary<string, string> htmlAttributes)
+        {
+            var routeAttributePrefix = "route-";
+
+            // We're only interested in HTML attributes that have the desired routeAttributePrefix.
+            var routeAttributes = htmlAttributes.Where(attribute =>
+                attribute.Key.StartsWith(routeAttributePrefix, StringComparison.OrdinalIgnoreCase));
+
+            // We need to remove any route based HTML attributes from the HTML attributes dictionary because
+            // they shouldn't be treated as HTML attributes, they're route values.
+            foreach (var attribute in routeAttributes)
+            {
+                htmlAttributes.Remove(attribute.Key);
+            }
+
+            // We build a Dictionary<string, object> because Generator.GenerateForm does not accept a 
+            // Dictionary <string, string>.
+            return routeAttributes.ToDictionary(attribute => attribute.Key.Substring(routeAttributePrefix.Length),
+                                                attribute => (object)attribute.Value);
+        }
+
+        private void RestoreBoundHtmlAttributes(TagHelperContext context, TagHelperOutput output)
+        {
+            var attributesToRestore = new List<string>();
+
+            if (Action != null)
+            {
+                attributesToRestore.Add(nameof(Action));
+            }
+
+            if (Controller != null)
+            {
+                attributesToRestore.Add(nameof(Controller));
+            }
+
+            if (Method != null)
+            {
+                attributesToRestore.Add(nameof(Method));
+            }
+
+            foreach (var attributeName in attributesToRestore)
+            {
+                // We need to look for the KeyValuePair<string, object> attribute so we can ensure that the attribute
+                // that we re-add to the output object has the same attribute name as the one the user typed.
+                var entry = context.AllAttributes.Single(attribute =>
+                    attribute.Key.Equals(attributeName, StringComparison.OrdinalIgnoreCase));
+                var originalAttribute = new KeyValuePair<string, string>(entry.Key, entry.Value.ToString());
+
+                output.Attributes.Add(originalAttribute);
+            }
+        }
+    }
+}