React to aspnet/Razor#221: Modify TagHelpers to utilize new ContentMode design.

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

@@ -153,6 +153,18 @@ private ITagHelperActivator TagHelperActivator
         /// 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)
@@ -162,7 +174,7 @@ public void StartWritingScope()
 
             // 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.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 @@ public override void Process(TagHelperContext context, TagHelperOutput output)
                 if (tagBuilder != null)
                 {
                     output.MergeAttributes(tagBuilder);
-                    output.Content += tagBuilder.InnerHtml;
+                    output.PostContent += tagBuilder.InnerHtml;
                     output.SelfClosing = false;
                 }
             }
@@ -116,7 +114,7 @@ public override void Process(TagHelperContext context, TagHelperOutput output)
                 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

@@ -1,6 +1,7 @@
 // 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;
@@ -10,7 +11,6 @@ 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 +31,7 @@ public class LabelTagHelper : TagHelper
 
         /// <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)
             {
@@ -45,10 +45,12 @@ public override void Process(TagHelperContext context, TagHelperOutput output)
                 {
                     output.MergeAttributes(tagBuilder);
 
+                    var childContent = await context.GetChildContentAsync();
+
                     // We check for whitespace to detect scenarios such as:
                     // <label for="Name">
                     // </label>
-                    if (string.IsNullOrWhiteSpace(output.Content))
+                    if (string.IsNullOrWhiteSpace(childContent) && string.IsNullOrEmpty(output.Content))
                     {
                         output.Content = tagBuilder.InnerHtml;
                     }

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;
@@ -17,7 +18,6 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
     /// <see cref="ContentBehavior.Modify"/> in order to read element's content but does not modify that content. The
     /// only modification it makes is to add a <c>selected</c> attribute 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 @@ public class OptionTagHelper : TagHelper
         /// <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 @@ public override void Process(TagHelperContext context, TagHelperOutput output)
                     }
 
                     // 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 @@ public override void Process(TagHelperContext context, TagHelperOutput output)
                 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)]
     public class TextAreaTagHelper : TagHelper
     {
         private const string ForAttributeName = "asp-for";

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

@@ -1,6 +1,7 @@
 // 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;
@@ -12,7 +13,6 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
     /// attribute.
     /// </summary>
     [TagName("span")]
-    [ContentBehavior(ContentBehavior.Modify)]
     public class ValidationMessageTagHelper : TagHelper
     {
         private const string ValidationForAttributeName = "asp-validation-for";
@@ -33,7 +33,7 @@ public class ValidationMessageTagHelper : TagHelper
 
         /// <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)
             {
@@ -47,10 +47,12 @@ public override void Process(TagHelperContext context, TagHelperOutput output)
                 {
                     output.MergeAttributes(tagBuilder);
 
+                    var childContent = await context.GetChildContentAsync();
+
                     // We check for whitespace to detect scenarios such as:
                     // <span validation-for="Name">
                     // </span>
-                    if (string.IsNullOrWhiteSpace(output.Content))
+                    if (string.IsNullOrWhiteSpace(childContent) && string.IsNullOrEmpty(output.Content))
                     {
                         output.Content = tagBuilder.InnerHtml;
                     }

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

@@ -13,7 +13,6 @@ namespace Microsoft.AspNet.Mvc.TagHelpers
     /// attribute.
     /// </summary>
     [TagName("div")]
-    [ContentBehavior(ContentBehavior.Append)]
     public class ValidationSummaryTagHelper : TagHelper
     {
         private const string ValidationSummaryAttributeName = "asp-validation-summary";
@@ -71,7 +70,7 @@ public override void Process(TagHelperContext context, TagHelperOutput output)
                 if (tagBuilder != null)
                 {
                     output.MergeAttributes(tagBuilder);
-                    output.Content += tagBuilder.InnerHtml;
+                    output.PostContent += tagBuilder.InnerHtml;
                 }
             }
         }

test/Microsoft.AspNet.Mvc.Razor.Host.Test/TestFiles/Output/Runtime/ModelExpressionTagHelper.cs

@@ -43,13 +43,29 @@ public override async Task ExecuteAsync()
             BeginContext(120, 2, true);
             WriteLiteral("\r\n");
             EndContext();
-            __tagHelperExecutionContext = __tagHelperScopeManager.Begin("inputTest", "test");
+            __tagHelperExecutionContext = __tagHelperScopeManager.Begin("inputTest", "test", async() => {
+            }
+            , StartWritingScope, EndWritingScope);
             __Microsoft_AspNet_Mvc_Razor_InputTestTagHelper = CreateTagHelper<Microsoft.AspNet.Mvc.Razor.InputTestTagHelper>();
             __tagHelperExecutionContext.Add(__Microsoft_AspNet_Mvc_Razor_InputTestTagHelper);
             __Microsoft_AspNet_Mvc_Razor_InputTestTagHelper.For = CreateModelExpression(__model => __model.Now);
             __tagHelperExecutionContext.AddTagHelperAttribute("For", __Microsoft_AspNet_Mvc_Razor_InputTestTagHelper.For);
             __tagHelperExecutionContext.Output = __tagHelperRunner.RunAsync(__tagHelperExecutionContext).Result;
             WriteLiteral(__tagHelperExecutionContext.Output.GenerateStartTag());
+            WriteLiteral(__tagHelperExecutionContext.Output.GeneratePreContent());
+            if (__tagHelperExecutionContext.Output.ContentSet)
+            {
+                WriteLiteral(__tagHelperExecutionContext.Output.GenerateContent());
+            }
+            else if (__tagHelperExecutionContext.ChildContentRetrieved)
+            {
+                WriteLiteral(__tagHelperExecutionContext.GetChildContentAsync().Result);
+            }
+            else
+            {
+                __tagHelperExecutionContext.ExecuteChildContentAsync().Wait();
+            }
+            WriteLiteral(__tagHelperExecutionContext.Output.GeneratePostContent());
             WriteLiteral(__tagHelperExecutionContext.Output.GenerateEndTag());
             __tagHelperExecutionContext = __tagHelperScopeManager.End();
         }

test/Microsoft.AspNet.Mvc.TagHelpers.Test/AnchorTagHelperTest.cs

@@ -32,15 +32,18 @@ public async Task ProcessAsync_GeneratesExpectedOutput()
                     { "asp-host", "contoso.com" },
                     { "asp-protocol", "http" }
                 },
-                uniqueId: "test");
+                uniqueId: "test",
+                getChildContentAsync: () => Task.FromResult("Something Else"));
             var output = new TagHelperOutput(
                 expectedTagName,
                 attributes: new Dictionary<string, string>
                 {
                     { "id", "myanchor" },
                     { "asp-route-foo", "bar" },
-                },
-                content: "Something");
+                })
+            {
+                Content = "Something"
+            };
 
             var urlHelper = new Mock<IUrlHelper>();
             urlHelper
@@ -85,11 +88,15 @@ public async Task ProcessAsync_CallsIntoRouteLinkWithExpectedParameters()
         {
             // Arrange
             var context = new TagHelperContext(
-                allAttributes: new Dictionary<string, object>(), uniqueId: "test");
+                allAttributes: new Dictionary<string, object>(),
+                uniqueId: "test",
+                getChildContentAsync: () => Task.FromResult("Something"));
             var output = new TagHelperOutput(
                 "a",
-                attributes: new Dictionary<string, string>(),
-                content: string.Empty);
+                attributes: new Dictionary<string, string>())
+            {
+                Content = string.Empty
+            };
 
             var generator = new Mock<IHtmlGenerator>(MockBehavior.Strict);
             generator
@@ -119,11 +126,15 @@ public async Task ProcessAsync_CallsIntoActionLinkWithExpectedParameters()
         {
             // Arrange
             var context = new TagHelperContext(
-                allAttributes: new Dictionary<string, object>(), uniqueId: "test");
+                allAttributes: new Dictionary<string, object>(),
+                uniqueId: "test",
+                getChildContentAsync: () => Task.FromResult("Something"));
             var output = new TagHelperOutput(
                 "a",
-                attributes: new Dictionary<string, string>(),
-                content: string.Empty);
+                attributes: new Dictionary<string, string>())
+                {
+                    Content = string.Empty
+                };
 
             var generator = new Mock<IHtmlGenerator>();
             generator
@@ -166,8 +177,7 @@ public async Task ProcessAsync_ThrowsIfHrefConflictsWithBoundAttributes(string p
                 attributes: new Dictionary<string, string>()
                 {
                     { "href", "http://www.contoso.com" }
-                },
-                content: string.Empty);
+                });
             if (propertyName == "asp-route-")
             {
                 output.Attributes.Add("asp-route-foo", "bar");
@@ -202,8 +212,7 @@ public async Task ProcessAsync_ThrowsIfRouteAndActionOrControllerProvided(string
             typeof(AnchorTagHelper).GetProperty(propertyName).SetValue(anchorTagHelper, "Home");
             var output = new TagHelperOutput(
                 "a",
-                attributes: new Dictionary<string, string>(),
-                content: string.Empty);
+                attributes: new Dictionary<string, string>());
             var expectedErrorMessage = "Cannot determine an 'href' attribute for <a>. An <a> with a specified " +
                 "'asp-route' must not have an 'asp-action' or 'asp-controller' attribute.";