Modify TagHelpers to use new content mode design.

- React to aspnet/Razor#221
- Modified existing TagHelpers to no longer rely on ContentBehavior and to instead utilize GetChildContentAsync, PreContent, Content and PostContent.

samples/TagHelperSample.Web/Views/Home/Create.cshtml

@@ -13,7 +13,7 @@
 <form asp-anti-forgery="false" asp-action="Create">
     <div class="form-horizontal">
         @* validation summary tag helper will target just <div/> elements and append the list of errors *@
-        @* - i.e. this helper, like <select/> helper, has ContentBehavior.Append *@
+        @* - i.e. this helper, like <select/> helper appends content. *@
         @* helper does nothing if model is valid and (client-side validation is disabled or
            asp-validation-summary="ValidationSummary.ModelOnly") *@
         @* don't need a bound attribute to match Html.ValidationSummary()'s headerTag parameter; users wrap message as they wish *@
@@ -48,7 +48,7 @@
         <div class="form-group">
             <label asp-for="YearsEmployeed" class="control-label col-md-2" />
             <div class="col-md-10">
-                @* <select/> tag helper has ContentBehavior.Append -- items render after static options *@
+                @* <select/> items render after static options *@
                 @{ var @object = "multiple"; }
                 <select asp-for="@(Model.YearsEmployeed)" asp-items="@((IEnumerable<SelectListItem>)ViewBag.Items)" size="2" class="form-control" multiple="@(@object)">
                     @* Static use of "selected" attribute may cause HTML errors if in a single-selection <select/> *@

src/Microsoft.AspNet.Mvc.Razor/RazorPage.cs

@@ -155,6 +155,18 @@ namespace Microsoft.AspNet.Mvc.Razor
         /// be buffered until <see cref="EndWritingScope"/> is called.
         /// </remarks>
         public void StartWritingScope()
+        {
+            StartWritingScope(new StringWriter());
+        }
+
+        /// <summary>
+        /// Starts a new writing scope with the given <paramref name="writer"/>.
+        /// </summary>
+        /// <remarks>
+        /// All writes to the <see cref="Output"/> or <see cref="ViewContext.Writer"/> after calling this method will
+        /// be buffered until <see cref="EndWritingScope"/> is called.
+        /// </remarks>
+        public void StartWritingScope(TextWriter writer)
         {
             // If there isn't a base writer take the ViewContext.Writer
             if (_originalWriter == null)
@@ -164,7 +176,7 @@ namespace Microsoft.AspNet.Mvc.Razor
 
             // We need to replace the ViewContext's Writer to ensure that all content (including content written
             // from HTML helpers) is redirected.
-            ViewContext.Writer = new StringWriter();
+            ViewContext.Writer = writer;
 
             _writerScopes.Push(ViewContext.Writer);
         }

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

@@ -6,14 +6,12 @@ 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
     {
         private const string ActionAttributeName = "asp-action";
@@ -106,7 +104,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                 if (tagBuilder != null)
                 {
                     output.MergeAttributes(tagBuilder);
-                    output.Content += tagBuilder.InnerHtml;
+                    output.PostContent += tagBuilder.InnerHtml;
                     output.SelfClosing = false;
                 }
             }
@@ -116,7 +114,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                 var antiForgeryTagBuilder = Generator.GenerateAntiForgery(ViewContext);
                 if (antiForgeryTagBuilder != null)
                 {
-                    output.Content += antiForgeryTagBuilder.ToString(TagRenderMode.SelfClosing);
+                    output.PostContent += antiForgeryTagBuilder.ToString(TagRenderMode.SelfClosing);
                 }
             }
         }

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

@@ -15,7 +15,6 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
     /// <summary>
     /// <see cref="ITagHelper"/> implementation targeting &lt;input&gt; elements with an <c>asp-for</c> attribute.
     /// </summary>
-    [ContentBehavior(ContentBehavior.Replace)]
     public class InputTagHelper : TagHelper
     {
         private const string ForAttributeName = "asp-for";

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

@@ -1,16 +1,15 @@
 // 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.Threading.Tasks;
 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;label&gt; elements with an <c>asp-for</c> attribute.
     /// </summary>
-    [ContentBehavior(ContentBehavior.Modify)]
     public class LabelTagHelper : TagHelper
     {
         private const string ForAttributeName = "asp-for";
@@ -31,7 +30,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
 
         /// <inheritdoc />
         /// <remarks>Does nothing if <see cref="For"/> is <c>null</c>.</remarks>
-        public override void Process(TagHelperContext context, TagHelperOutput output)
+        public override async Task ProcessAsync(TagHelperContext context, TagHelperOutput output)
         {
             if (For != null)
             {
@@ -48,9 +47,19 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                     // We check for whitespace to detect scenarios such as:
                     // <label for="Name">
                     // </label>
-                    if (string.IsNullOrWhiteSpace(output.Content))
+                    if (!output.ContentSet)
                     {
-                        output.Content = tagBuilder.InnerHtml;
+                        var childContent = await context.GetChildContentAsync();
+
+                        if (string.IsNullOrWhiteSpace(childContent))
+                        {
+                            // Provide default label text since there was nothing useful in the Razor source.
+                            output.Content = tagBuilder.InnerHtml;
+                        }
+                        else
+                        {
+                            output.Content = childContent;
+                        }
                     }
                 }
             }

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

@@ -3,6 +3,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Threading.Tasks;
 using Microsoft.AspNet.Mvc.Rendering;
 using Microsoft.AspNet.Razor.Runtime.TagHelpers;
 using Microsoft.AspNet.Razor.TagHelpers;
@@ -13,11 +14,10 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
     /// <see cref="ITagHelper"/> implementation targeting &lt;option&gt; elements.
     /// </summary>
     /// <remarks>
-    /// This <see cref="ITagHelper"/> works in conjunction with <see cref="SelectTagHelper"/>. It has
+    /// This <see cref="ITagHelper"/> works in conjunction with <see cref="SelectTagHelper"/>. It reads elements 
-    /// <see cref="ContentBehavior.Modify"/> in order to read element's content but does not modify that content. The
+    /// content but does not modify that content. The only modification it makes is to add a <c>selected</c> attribute 
-    /// only modification it makes is to add a <c>selected</c> attribute in some cases.
+    /// in some cases.
     /// </remarks>
-    [ContentBehavior(ContentBehavior.Modify)]
     public class OptionTagHelper : TagHelper
     {
         // Protected to ensure subclasses are correctly activated. Internal for ease of use when testing.
@@ -51,7 +51,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
         /// <see cref="ICollection{string}"/> instance. Also does nothing if the associated &lt;option&gt; is already
         /// selected.
         /// </remarks>
-        public override void Process(TagHelperContext context, TagHelperOutput output)
+        public override async Task ProcessAsync(TagHelperContext context, TagHelperOutput output)
         {
             // Pass through attributes that are also well-known HTML attributes.
             if (Value != null)
@@ -84,9 +84,9 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                     }
 
                     // Select this <option/> element if value attribute or content matches a selected value. Callers
-                    // encode values as-needed before setting TagHelperOutput.Content. But TagHelperOutput itself
+                    // encode values as-needed while executing child content. But TagHelperOutput itself
                     // encodes attribute values later, when GenerateStartTag() is called.
-                    var text = output.Content;
+                    var text = await context.GetChildContentAsync();
                     var selected = (Value != null) ? selectedValues.Contains(Value) : encodedValues.Contains(text);
                     if (selected)
                     {

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

@@ -15,7 +15,6 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
     /// <summary>
     /// <see cref="ITagHelper"/> implementation targeting &lt;select&gt; elements with an <c>asp-for</c> attribute.
     /// </summary>
-    [ContentBehavior(ContentBehavior.Append)]
     public class SelectTagHelper : TagHelper
     {
         private const string ForAttributeName = "asp-for";
@@ -146,7 +145,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                 if (tagBuilder != null)
                 {
                     output.MergeAttributes(tagBuilder);
-                    output.Content += tagBuilder.InnerHtml;
+                    output.PostContent += tagBuilder.InnerHtml;
                     output.SelfClosing = false;
                 }
 

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

@@ -10,7 +10,6 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
     /// <summary>
     /// <see cref="ITagHelper"/> implementation targeting &lt;textarea&gt; elements with an <c>asp-for</c> attribute.
     /// </summary>
-    [ContentBehavior(ContentBehavior.Replace)]
     [HtmlElementName("textarea")]
     public class TextAreaTagHelper : TagHelper
     {

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

@@ -1,9 +1,9 @@
 // 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.Threading.Tasks;
 using Microsoft.AspNet.Mvc.Rendering;
 using Microsoft.AspNet.Razor.Runtime.TagHelpers;
-using Microsoft.AspNet.Razor.TagHelpers;
 
 namespace Microsoft.AspNet.Mvc.TagHelpers
 {
@@ -12,7 +12,6 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
     /// attribute.
     /// </summary>
     [HtmlElementName("span")]
-    [ContentBehavior(ContentBehavior.Modify)]
     public class ValidationMessageTagHelper : TagHelper
     {
         private const string ValidationForAttributeName = "asp-validation-for";
@@ -33,7 +32,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
 
         /// <inheritdoc />
         /// <remarks>Does nothing if <see cref="For"/> is <c>null</c>.</remarks>
-        public override void Process(TagHelperContext context, TagHelperOutput output)
+        public override async Task ProcessAsync(TagHelperContext context, TagHelperOutput output)
         {
             if (For != null)
             {
@@ -50,9 +49,19 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
                     // We check for whitespace to detect scenarios such as:
                     // <span validation-for="Name">
                     // </span>
-                    if (string.IsNullOrWhiteSpace(output.Content))
+                    if (!output.ContentSet)
                     {
-                        output.Content = tagBuilder.InnerHtml;
+                        var childContent = await context.GetChildContentAsync();
+
+                        if (string.IsNullOrWhiteSpace(childContent))
+                        {
+                            // Provide default label text since there was nothing useful in the Razor source.
+                            output.Content = tagBuilder.InnerHtml;
+                        }
+                        else
+                        {
+                            output.Content = childContent;
+                        }
                     }
                 }
             }

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

@@ -13,7 +13,6 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
     /// attribute.
     /// </summary>
     [HtmlElementName("div")]
-    [ContentBehavior(ContentBehavior.Append)]
     public class ValidationSummaryTagHelper : TagHelper
     {
         private const string ValidationSummaryAttributeName = "asp-validation-summary";
@@ -81,7 +80,7 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
             if (tagBuilder != null)
             {
                 output.MergeAttributes(tagBuilder);
-                output.Content += tagBuilder.InnerHtml;
+                output.PostContent += tagBuilder.InnerHtml;
             }
         }
     }

test/WebSites/ActivatorWebSite/TagHelpers/HiddenTagHelper.cs

@@ -4,12 +4,11 @@
 using Microsoft.AspNet.Mvc;
 using Microsoft.AspNet.Mvc.Rendering;
 using Microsoft.AspNet.Razor.Runtime.TagHelpers;
-using Microsoft.AspNet.Razor.TagHelpers;
+using System.Threading.Tasks;
 
 namespace ActivatorWebSite.TagHelpers
 {
     [HtmlElementName("span")]
-    [ContentBehavior(ContentBehavior.Modify)]
     public class HiddenTagHelper : TagHelper
     {
         public string Name { get; set; }
@@ -17,9 +16,11 @@ namespace ActivatorWebSite.TagHelpers
         [Activate]
         public IHtmlHelper HtmlHelper { get; set; }
 
-        public override void Process(TagHelperContext context, TagHelperOutput output)
+        public override async Task ProcessAsync(TagHelperContext context, TagHelperOutput output)
         {
-            output.Content = HtmlHelper.Hidden(Name, output.Content).ToString();
+            var content = await context.GetChildContentAsync();
+
+            output.Content = HtmlHelper.Hidden(Name, content).ToString();
         }
     }
 }

test/WebSites/ActivatorWebSite/TagHelpers/RepeatContentTagHelper.cs

@@ -4,12 +4,11 @@
 using Microsoft.AspNet.Mvc;
 using Microsoft.AspNet.Mvc.Rendering;
 using Microsoft.AspNet.Razor.Runtime.TagHelpers;
-using Microsoft.AspNet.Razor.TagHelpers;
+using System.Threading.Tasks;
 
 namespace ActivatorWebSite.TagHelpers
 {
     [HtmlElementName("div")]
-    [ContentBehavior(ContentBehavior.Modify)]
     public class RepeatContentTagHelper : TagHelper
     {
         public int RepeatContent { get; set; }
@@ -19,14 +18,14 @@ namespace ActivatorWebSite.TagHelpers
         [Activate]
         public IHtmlHelper HtmlHelper { get; set; }
 
-        public override void Process(TagHelperContext context, TagHelperOutput output)
+        public override async Task ProcessAsync(TagHelperContext context, TagHelperOutput output)
         {
+            var content = await context.GetChildContentAsync();
             var repeatContent = HtmlHelper.Encode(Expression.Metadata.Model.ToString());
 
             if (string.IsNullOrEmpty(repeatContent))
             {
-                repeatContent = output.Content;
+                repeatContent = content;
-                output.Content = string.Empty;
             }
 
             for (int i = 0; i < RepeatContent; i++)

test/WebSites/ActivatorWebSite/TagHelpers/TitleTagHelper.cs

@@ -9,7 +9,6 @@ using Microsoft.AspNet.Razor.TagHelpers;
 namespace ActivatorWebSite.TagHelpers
 {
     [HtmlElementName("body")]
-    [ContentBehavior(ContentBehavior.Prepend)]
     public class TitleTagHelper : TagHelper
     {
         [Activate]
@@ -23,7 +22,7 @@ namespace ActivatorWebSite.TagHelpers
             var builder = new TagBuilder("h2");
             var title = ViewContext.ViewBag.Title;
             builder.InnerHtml = HtmlHelper.Encode(title);
-            output.Content = builder.ToString();
+            output.PreContent = builder.ToString();
         }
     }
 }