Add a <partial /> tag helper

Fixes #5916

src/Microsoft.AspNetCore.Mvc.TagHelpers/PartialTagHelper.cs

@@ -0,0 +1,113 @@
+// Copyright (c) .NET Foundation. 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.IO;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using Microsoft.AspNetCore.Mvc.Rendering;
+using Microsoft.AspNetCore.Mvc.ViewEngines;
+using Microsoft.AspNetCore.Mvc.ViewFeatures;
+using Microsoft.AspNetCore.Mvc.ViewFeatures.Internal;
+using Microsoft.AspNetCore.Razor.TagHelpers;
+
+namespace Microsoft.AspNetCore.Mvc.TagHelpers
+{
+    /// <summary>
+    /// Renders a partial view.
+    /// </summary>
+    [HtmlTargetElement("partial", Attributes = "name", TagStructure = TagStructure.WithoutEndTag)]
+    public class PartialTagHelper : TagHelper
+    {
+        private readonly ICompositeViewEngine _viewEngine;
+        private readonly IViewBufferScope _viewBufferScope;
+
+        public PartialTagHelper(
+            ICompositeViewEngine viewEngine,
+            IViewBufferScope viewBufferScope)
+        {
+            _viewEngine = viewEngine ?? throw new ArgumentNullException(nameof(viewEngine));
+            _viewBufferScope = viewBufferScope ?? throw new ArgumentNullException(nameof(viewBufferScope));
+        }
+
+        /// <summary>
+        /// The name of the partial view used to create the HTML markup.
+        /// </summary>
+        public string Name { get; set; }
+
+        /// <summary>
+        /// A model to pass into the partial view.
+        /// </summary>
+        public object Model { get; set; }
+
+        /// <summary>
+        /// A <see cref="ViewDataDictionary"/> to pass into the partial view.
+        /// </summary>
+        public ViewDataDictionary ViewData { get; set; }
+
+        [HtmlAttributeNotBound]
+        [ViewContext]
+        public ViewContext ViewContext { get; set; }
+
+        /// <inheritdoc />
+        public override async Task ProcessAsync(TagHelperContext context, TagHelperOutput output)
+        {
+            if (context == null)
+            {
+                throw new ArgumentNullException(nameof(context));
+            }
+
+            if (output == null)
+            {
+                throw new ArgumentNullException(nameof(context));
+            }
+
+            var viewBuffer = new ViewBuffer(_viewBufferScope, Name, ViewBuffer.PartialViewPageSize);
+            using (var writer = new ViewBufferTextWriter(viewBuffer, Encoding.UTF8))
+            {
+                await RenderPartialViewAsync(writer);
+
+                // Reset the TagName. We don't want `partial` to render.
+                output.TagName = null;
+                output.Content.SetHtmlContent(viewBuffer);
+            }
+        }
+
+        private async Task RenderPartialViewAsync(TextWriter writer)
+        {
+            var viewEngineResult = _viewEngine.GetView(ViewContext.ExecutingFilePath, Name, isMainPage: false);
+            var getViewLocations = viewEngineResult.SearchedLocations;
+            if (!viewEngineResult.Success)
+            {
+                viewEngineResult = _viewEngine.FindView(ViewContext, Name, isMainPage: false);
+            }
+
+            if (!viewEngineResult.Success)
+            {
+                var searchedLocations = Enumerable.Concat(getViewLocations, viewEngineResult.SearchedLocations);
+                var locations = string.Empty;
+                if (searchedLocations.Any())
+                {
+                    locations += Environment.NewLine + string.Join(Environment.NewLine, searchedLocations);
+                }
+
+                throw new InvalidOperationException(
+                    Resources.FormatViewEngine_PartialViewNotFound(Name, locations));
+            }
+
+            var view = viewEngineResult.View;
+            using (view as IDisposable)
+            {
+                // Determine which ViewData we should use to construct a new ViewData
+                var baseViewData = ViewData ?? ViewContext.ViewData;
+                var model = Model ?? ViewContext.ViewData.Model;
+
+                var newViewData = new ViewDataDictionary<object>(baseViewData, model);
+                var partialViewContext = new ViewContext(ViewContext, view, newViewData, writer);
+
+                await view.RenderAsync(partialViewContext);
+            }
+        }
+    }
+}

src/Microsoft.AspNetCore.Mvc.TagHelpers/Resources.resx

@@ -153,4 +153,7 @@
   <data name="ArgumentCannotContainHtmlSpace" xml:space="preserve">
     <value>Value cannot contain HTML space characters.</value>
   </data>
+  <data name="ViewEngine_PartialViewNotFound" xml:space="preserve">
+    <value>The partial view '{0}' was not found. The following locations were searched:{1}</value>
+  </data>
 </root>

test/Microsoft.AspNetCore.Mvc.FunctionalTests/compiler/resources/HtmlGenerationWebSite.HtmlGeneration_Home.ProductListUsingTagHelpers.html

@@ -0,0 +1,64 @@
+<html>
+<head>
+    <meta charset="utf-8" />
+</head>
+<body>
+    <form action="/HtmlGeneration_Product" method="post">
+            <div>
+                <label class="product" for="z0__product_HomePage">HomePage</label>
+                <input type="url" size="50" disabled="disabled" readonly="readonly" id="z0__product_HomePage" name="[0].product.HomePage" value="http://www.contoso.com/" />
+            </div>
+
+<div>
+    <label class="product" for="z0__Number">Number</label>
+    <input type="number" data-val="true" data-val-required="The Number field is required." id="z0__Number" name="[0].Number" value="0" />
+</div>
+<div>
+    <label class="product" for="z0__ProductName">ProductName</label>
+    <input type="text" data-val="true" data-val-required="The ProductName field is required." id="z0__ProductName" name="[0].ProductName" value="Product_0" />
+</div>
+<div>
+    <label class="product" for="z0__Description">Description</label>
+    <textarea rows="4" cols="50" class="product" id="z0__Description" name="[0].Description">
+</textarea>
+</div>
+            <div>
+                <label class="product" for="z1__product_HomePage">HomePage</label>
+                <input type="url" size="50" disabled="disabled" readonly="readonly" id="z1__product_HomePage" name="[1].product.HomePage" value="" />
+            </div>
+
+<div>
+    <label class="product" for="z1__Number">Number</label>
+    <input type="number" data-val="true" data-val-required="The Number field is required." id="z1__Number" name="[1].Number" value="1" />
+</div>
+<div>
+    <label class="product" for="z1__ProductName">ProductName</label>
+    <input type="text" data-val="true" data-val-required="The ProductName field is required." id="z1__ProductName" name="[1].ProductName" value="Product_1" />
+</div>
+<div>
+    <label class="product" for="z1__Description">Description</label>
+    <textarea rows="4" cols="50" class="product" id="z1__Description" name="[1].Description">
+</textarea>
+</div>
+            <div>
+                <label class="product" for="z2__product_HomePage">HomePage</label>
+                <input type="url" size="50" disabled="disabled" readonly="readonly" id="z2__product_HomePage" name="[2].product.HomePage" value="" />
+            </div>
+
+<div>
+    <label class="product" for="z2__Number">Number</label>
+    <input type="number" data-val="true" data-val-required="The Number field is required." id="z2__Number" name="[2].Number" value="2" />
+</div>
+<div>
+    <label class="product" for="z2__ProductName">ProductName</label>
+    <input type="text" data-val="true" data-val-required="The ProductName field is required." id="z2__ProductName" name="[2].ProductName" value="Product_2" />
+</div>
+<div>
+    <label class="product" for="z2__Description">Description</label>
+    <textarea rows="4" cols="50" class="product" id="z2__Description" name="[2].Description">
+Product_2 description</textarea>
+</div>
+        <input type="submit" />
+    <input name="__RequestVerificationToken" type="hidden" value="{0}" /></form>
+</body>
+</html>