Add ValidationSummaryTagHelper.

#1251

Author: NTaylorMullen

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

@@ -0,0 +1,72 @@
+// 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 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;div&gt; elements with <c>validation-summary</c> or
+    /// <c>validation-model-errors-only</c> attributes.
+    /// </summary>S
+    [TagName("div")]
+    [ContentBehavior(ContentBehavior.Append)]
+    public class ValidationSummaryTagHelper : TagHelper
+    {
+        private static readonly bool DefaultModelErrorsOnlyBehavior = false;
+
+        [Activate]
+        private ViewContext ViewContext { get; set; }
+
+        [Activate]
+        private IHtmlGenerator Generator { get; set; }
+
+        /// <summary>
+        /// If <c>true</c> appends a validation summary.
+        /// </summary>
+        /// <remarks>If <see cref="ValidationModelErrorsOnly"/> is provided this is assumed <c>true</c> unless 
+        /// specified <c>false</c>.</remarks>
+        [HtmlAttributeName("validation-summary")]
+        public bool? ValidationSummary { get; set; }
+
+        /// <summary>
+        /// If <c>true</c>, display model-level errors only; otherwise display all errors. Defaults behavior is 
+        /// <c>false</c>.
+        /// </summary>
+        /// <remarks>
+        /// If provided <see cref="ValidationSummary"/> is assumed <c>true</c> unless specified <c>false</c>.
+        /// </remarks>
+        [HtmlAttributeName("validation-model-errors-only")]
+        public bool? ValidationModelErrorsOnly { get; set; }
+
+        /// <inheritdoc />
+        public override void Process(TagHelperContext context, TagHelperOutput output)
+        {
+            // If the user specified validation-summary, use the provided value. If validation-summary was not 
+            // specified and validation-model-errors-only was, assume they want a validation summary.
+            var generateValidationSummary = (ValidationSummary.HasValue && ValidationSummary.Value) ||
+                                            (!ValidationSummary.HasValue && ValidationModelErrorsOnly.HasValue);
+
+            if (generateValidationSummary)
+            {
+                // If the user specified their validation-model-errors-only behavior, use it; otherwise revert to
+                // the default behavior (false).
+                var validationModelErrorsOnly = ValidationModelErrorsOnly.HasValue ?
+                                                ValidationModelErrorsOnly.Value :
+                                                DefaultModelErrorsOnlyBehavior;
+
+                var tagBuilder = Generator.GenerateValidationSummary(
+                    ViewContext,
+                    validationModelErrorsOnly,
+                    // We don't have access to the users message, it's implied by the content of the <div> tag.
+                    message: string.Empty,
+                    headerTag: output.TagName,
+                    htmlAttributes: null);
+
+                output.Merge(tagBuilder);
+            }
+        }
+    }
+}