Add extensions for TagHelperOutput to copy and merge from TagBuilder.

- Added tests to validate functionality of all TagHelperOutputExtension methods.

#1319

Author: N. Taylor Mullen

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

@@ -0,0 +1,118 @@
+// 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>
+    /// Utility related extensions for <see cref="TagHelperOutput"/>.
+    /// </summary>
+    public static class TagHelperOutputExtensions
+    {
+        /// <summary>
+        /// Copies a user-provided attribute from <paramref name="context"/>'s 
+        /// <see cref="TagHelperContext.AllAttributes"/> to <paramref name="tagHelperOutput"/>'s
+        /// <see cref="TagHelperOutput.Attributes"/>.
+        /// </summary>
+        /// <param name="tagHelperOutput">The <see cref="TagHelperOutput"/> this method extends.</param>
+        /// <param name="attributeName">The name of the bound attribute.</param>
+        /// <param name="context">The <see cref="TagHelperContext"/>.</param>
+        /// <remarks>Only copies the attribute if <paramref name="tagHelperOutput"/>'s 
+        /// <see cref="TagHelperOutput.Attributes"/> does not contain an attribute with the given 
+        /// <paramref name="attributeName"/></remarks>
+        public static void CopyHtmlAttribute(this TagHelperOutput tagHelperOutput,
+                                             string attributeName,
+                                             TagHelperContext context)
+        {
+            // We look for the original attribute so we can restore the exact attribute name the user typed.
+            var entry = context.AllAttributes.First(attribute =>
+                attribute.Key.Equals(attributeName, StringComparison.OrdinalIgnoreCase));
+
+            if (!tagHelperOutput.Attributes.ContainsKey(entry.Key))
+            {
+                tagHelperOutput.Attributes.Add(entry.Key, entry.Value.ToString());
+            }
+        }
+
+        /// <summary>
+        /// Returns all attributes from <paramref name="tagHelperOutput"/>'s 
+        /// <see cref="TagHelperOutput.Attributes"/> that have the given <paramref name="prefix"/>.
+        /// </summary>
+        /// <param name="tagHelperOutput">The <see cref="TagHelperOutput"/> this method extends.</param>
+        /// <param name="prefix">A prefix to look for.</param>
+        /// <returns><see cref="KeyValuePair{string, string}"/>s with <see cref="KeyValuePair{string, string}.Key"/>
+        /// starting with the given <paramref name="prefix"/>.</returns>
+        public static IEnumerable<KeyValuePair<string, string>> FindPrefixedAttributes(
+            this TagHelperOutput tagHelperOutput, string prefix)
+        {
+            // TODO: We will not need this method once https://github.com/aspnet/Razor/issues/89 is completed.
+
+            // We're only interested in HTML attributes that have the desired prefix.
+            var prefixedAttributes = tagHelperOutput.Attributes
+                .Where(attribute => attribute.Key.StartsWith(prefix, StringComparison.OrdinalIgnoreCase))
+                .ToArray();
+
+            return prefixedAttributes;
+        }
+
+        /// <summary>
+        /// Merges the given <paramref name="tagBuilder"/> into the <paramref name="tagHelperOutput"/>.
+        /// </summary>
+        /// <param name="tagHelperOutput">The <see cref="TagHelperOutput"/> this method extends.</param>
+        /// <param name="tagBuilder">The <see cref="TagBuilder"/> to merge.</param>
+        /// <remarks><paramref name="tagHelperOutput"/>'s <see cref="TagHelperOutput.Content"/> has the given
+        /// <paramref name="tagBuilder"/>s <see cref="TagBuilder.InnerHtml"/> appended to it. This is to ensure
+        /// multiple <see cref="ITagHelper"/>s running on the same HTML tag don't overwrite each other; therefore,
+        /// this method may not be appropriate for all <see cref="ITagHelper"/> scenarios.</remarks>
+        public static void Merge(this TagHelperOutput tagHelperOutput, TagBuilder tagBuilder)
+        {
+            tagHelperOutput.TagName = tagBuilder.TagName;
+            tagHelperOutput.Content += tagBuilder.InnerHtml;
+
+            MergeAttributes(tagHelperOutput, tagBuilder);
+        }
+
+        /// <summary>
+        /// Merges the given <see cref="tagBuilder"/>'s <see cref="TagBuilder.Attributes"/> into the 
+        /// <paramref name="tagHelperOutput"/>.
+        /// </summary>
+        /// <param name="tagHelperOutput">The <see cref="TagHelperOutput"/> this method extends.</param>
+        /// <param name="tagBuilder">The <see cref="TagBuilder"/> to merge attributes from.</param>
+        /// <remarks>Existing <see cref="TagHelperOutput.Attributes"/> on the given <paramref name="tagHelperOutput"/>
+        /// are not overriden; "class" attributes are merged with spaces.</remarks>
+        public static void MergeAttributes(this TagHelperOutput tagHelperOutput, TagBuilder tagBuilder)
+        {
+            foreach (var attribute in tagBuilder.Attributes)
+            {
+                if (!tagHelperOutput.Attributes.ContainsKey(attribute.Key))
+                {
+                    tagHelperOutput.Attributes.Add(attribute.Key, attribute.Value);
+                }
+                else if (attribute.Key.Equals("class", StringComparison.Ordinal))
+                {
+                    tagHelperOutput.Attributes["class"] += " " + attribute.Value;
+                }
+            }
+        }
+
+        /// <summary>
+        /// Removes the given <paramref name="attributes"/> from <paramref name="tagHelperOutput"/>'s
+        /// <see cref="TagHelperOutput.Attributes"/>.
+        /// </summary>
+        /// <param name="tagHelperOutput">The <see cref="TagHelperOutput"/> this method extends.</param>
+        /// <param name="attributes">Attributes to remove.</param>
+        public static void RemoveRange(
+            this TagHelperOutput tagHelperOutput, IEnumerable<KeyValuePair<string, string>> attributes)
+        {
+            foreach (var attribute in attributes)
+            {
+                tagHelperOutput.Attributes.Remove(attribute);
+            }
+        }
+    }
+}