Add PartialAsync and RenderPartialAsync.

This enables Html.PartialAsync and Html.RenderPartialAsync.  It also includes a sample to showcase the features.  This is in reference to WEBFX-95.

samples/MvcSample.Web/Views/Shared/HelloWorldPartial.cshtml

@@ -0,0 +1,4 @@
+@using MvcSample.Web.Models
+@model User
+
+<strong>Hello @Model.Name from Partial</strong>

samples/MvcSample.Web/Views/Shared/MyView.cshtml

@@ -11,6 +11,12 @@
     {
         return "Hello World";
     }
+    
+    public Task RenderHelloWorldPartial(User model)
+    {
+        // Imagine this method was a super complex method that was used as a helper method.
+        return Html.RenderPartialAsync("HelloWorldPartial", model);
+    }
 }
 
 <div class="jumbotron">
@@ -21,6 +27,12 @@
 <div class="row">
     <h3 title="@Model.Name" class="@nullValue">Hello @Model.Name! Happy @Model.Age birthday.</h3>
     <h3>This value was retrieved asynchronously: @(await AsyncValueRetrieval())</h3>
+    <h3>Partial Async: @await Html.PartialAsync("HelloWorldPartial", Model)</h3>
+    <h3>Render Partial Async (Custom model): @{ await RenderHelloWorldPartial(new User()
+                                                {
+                                                    Name="Bob"
+                                                }); 
+                                              }</h3>
 
     <div class="col-md-4">
         <h2>Getting started</h2>

src/Microsoft.AspNet.Mvc.Rendering/Html/HtmlHelper.cs

@@ -1,9 +1,11 @@
 using System;
 using System.Collections.Generic;
 using System.Diagnostics.CodeAnalysis;
+using System.Globalization;
+using System.IO;
 using System.Net;
 using System.Reflection;
-using System.Text;
+using System.Threading.Tasks;
 using Microsoft.AspNet.Abstractions;
 
 namespace Microsoft.AspNet.Mvc.Rendering
@@ -21,12 +23,15 @@ namespace Microsoft.AspNet.Mvc.Rendering
         public static readonly string ValidationSummaryValidCssClassName = "validation-summary-valid";
 
         private ViewContext _viewContext;
+        private IViewEngine _viewEngine;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="HtmlHelper"/> class.
         /// </summary>
-        public HtmlHelper()
+        public HtmlHelper(IViewEngine viewEngine)
         {
+            _viewEngine = viewEngine;
+
             // Underscores are fine characters in id's.
             IdAttributeDotReplacement = "_";
         }
@@ -125,6 +130,42 @@ namespace Microsoft.AspNet.Mvc.Rendering
             return TagBuilder.CreateSanitizedId(name, IdAttributeDotReplacement);
         }
 
+        public async Task<HtmlString> PartialAsync([NotNull] string partialViewName, object model, ViewDataDictionary viewData)
+        {
+            using (var writer = new StringWriter(CultureInfo.CurrentCulture))
+            {
+                await RenderPartialCoreAsync(partialViewName, model, viewData, writer);
+
+                return new HtmlString(writer.ToString());
+            }
+        }
+
+        public Task RenderPartialAsync([NotNull] string partialViewName, object model, ViewDataDictionary viewData)
+        {
+            return RenderPartialCoreAsync(partialViewName, model, viewData, ViewContext.Writer);
+        }
+
+        protected virtual async Task RenderPartialCoreAsync([NotNull] string partialViewName, 
+                                                               object model, 
+                                                               ViewDataDictionary viewData, 
+                                                               TextWriter writer)
+        {
+            // Determine which ViewData we should use to construct a new ViewData
+            var baseViewData = viewData ?? ViewData;
+
+            var newViewData = new ViewDataDictionary(baseViewData, model);
+
+            var newViewContext = new ViewContext(ViewContext)
+            {
+                ViewData = newViewData,
+                Writer = writer
+            };
+
+            var viewEngineResult = await _viewEngine.FindPartialView(newViewContext.ViewEngineContext, partialViewName);
+
+            await viewEngineResult.View.RenderAsync(newViewContext);
+        }
+
         /// <summary>
         /// Returns the HTTP method that handles form input (GET or POST) as a string.
         /// </summary>

src/Microsoft.AspNet.Mvc.Rendering/Html/HtmlHelperOfT.cs

@@ -8,8 +8,8 @@ namespace Microsoft.AspNet.Mvc.Rendering
         /// <summary>
         /// Initializes a new instance of the <see cref="HtmlHelper{TModel}"/> class.
         /// </summary>
-        public HtmlHelper()
-            : base()
+        public HtmlHelper(IViewEngine viewEngine)
+            : base(viewEngine)
         {
         }
 

src/Microsoft.AspNet.Mvc.Rendering/Html/Partials/PartialAsyncExtensions.cs

@@ -0,0 +1,45 @@
+using System.Threading.Tasks;
+
+namespace Microsoft.AspNet.Mvc.Rendering
+{
+    public static class PartialAsyncExtensions
+    {
+        /// <summary>
+        /// Renders the partial view with the parent's view data and model to a string.
+        /// </summary>
+        /// <typeparam name="T">The <see cref="Type"/> of the model.</typeparam>
+        /// <param name="htmlHelper">The <see cref="HtmlHelper"/> instance that this method extends.</param>
+        /// <param name="partialViewName">The name of the partial view to render.</param>
+        /// <returns>A <see cref="Task{T}"/> that represents when rendering to the <see cref="HtmlString"/> has completed.</returns>
+        public static Task<HtmlString> PartialAsync<T>(this IHtmlHelper<T> htmlHelper, [NotNull] string partialViewName)
+        {
+            return htmlHelper.PartialAsync(partialViewName, htmlHelper.ViewData.Model, viewData: null);
+        }
+
+        /// <summary>
+        /// Renders the partial view with the given view data and, implicitly, the given view data's model to a string.
+        /// </summary>
+        /// <typeparam name="T">The <see cref="Type"/> of the model.</typeparam>
+        /// <param name="htmlHelper">The <see cref="HtmlHelper"/> instance that this method extends.</param>
+        /// <param name="partialViewName">The name of the partial view to render.</param>
+        /// <param name="viewData">The <see cref="ViewDataDictionary"/> that is provided to the partial view that will be rendered.</param>
+        /// <returns>A <see cref="Task{T}"/> that represents when rendering to the <see cref="HtmlString"/> has completed.</returns>
+        public static Task<HtmlString> PartialAsync<T>(this IHtmlHelper<T> htmlHelper, [NotNull] string partialViewName, ViewDataDictionary viewData)
+        {
+            return htmlHelper.PartialAsync(partialViewName, htmlHelper.ViewData.Model, viewData: viewData);
+        }
+
+        /// <summary>
+        /// Renders the partial view with an empty view data and the given model to a string.
+        /// </summary>
+        /// <typeparam name="T">The <see cref="Type"/> of the model.</typeparam>
+        /// <param name="htmlHelper">The <see cref="HtmlHelper"/> instance that this method extends.</param>
+        /// <param name="partialViewName">The name of the partial view to render.</param>
+        /// <param name="model">The model to provide to the partial view that will be rendered.</param>
+        /// <returns>A <see cref="Task{T}"/> that represents when rendering to the <see cref="HtmlString"/> has completed.</returns>
+        public static Task<HtmlString> PartialAsync<T>(this IHtmlHelper<T> htmlHelper, [NotNull] string partialViewName, object model)
+        {
+            return htmlHelper.PartialAsync(partialViewName, model, viewData: null);
+        }
+    }
+}

src/Microsoft.AspNet.Mvc.Rendering/Html/Partials/RenderPartialAsyncExtensions.cs

@@ -0,0 +1,46 @@
+using System.Threading.Tasks;
+
+namespace Microsoft.AspNet.Mvc.Rendering
+{
+    public static class RenderPartialAsyncExtensions
+    {
+        
+        /// <summary>
+        /// Renders the partial view with the parent's view data and model.
+        /// </summary>
+        /// <typeparam name="T">The <see cref="Type"/> of the model.</typeparam>
+        /// <param name="htmlHelper">The <see cref="HtmlHelper"/> instance that this method extends.</param>
+        /// <param name="partialViewName">The name of the partial view to render.</param>
+        /// <returns>A <see cref="Task"/> that represents when rendering has completed.</returns>
+        public static Task RenderPartialAsync<T>(this IHtmlHelper<T> htmlHelper, [NotNull] string partialViewName)
+        {
+            return htmlHelper.RenderPartialAsync(partialViewName, htmlHelper.ViewData.Model, viewData: htmlHelper.ViewData);
+        }
+
+        /// <summary>
+        /// Renders the partial view with the given view data and, implicitly, the given view data's model.
+        /// </summary>
+        /// <typeparam name="T">The <see cref="Type"/> of the model.</typeparam>
+        /// <param name="htmlHelper">The <see cref="HtmlHelper"/> instance that this method extends.</param>
+        /// <param name="partialViewName">The name of the partial view to render.</param>
+        /// <param name="viewData">The <see cref="ViewDataDictionary"/> that is provided to the partial view that will be rendered.</param>
+        /// <returns>A <see cref="Task"/> that represents when rendering has completed.</returns>
+        public static Task RenderPartialAsync<T>(this IHtmlHelper<T> htmlHelper, [NotNull] string partialViewName, ViewDataDictionary viewData)
+        {
+            return htmlHelper.RenderPartialAsync(partialViewName, htmlHelper.ViewData.Model, viewData: viewData);
+        }
+
+        /// <summary>
+        /// Renders the partial view with an empty view data and the given model.
+        /// </summary>
+        /// <typeparam name="T">The <see cref="Type"/> of the model.</typeparam>
+        /// <param name="htmlHelper">The <see cref="HtmlHelper"/> instance that this method extends.</param>
+        /// <param name="partialViewName">The name of the partial view to render.</param>
+        /// <param name="model">The model to provide to the partial view that will be rendered.</param>
+        /// <returns>A <see cref="Task"/> that represents when rendering has completed.</returns>
+        public static Task RenderPartialAsync<T>(this IHtmlHelper<T> htmlHelper, [NotNull] string partialViewName, object model)
+        {
+            return htmlHelper.RenderPartialAsync(partialViewName, model, htmlHelper.ViewData);
+        }
+    }
+}

src/Microsoft.AspNet.Mvc.Rendering/IHtmlHelperOfT.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Threading.Tasks;
 
 namespace Microsoft.AspNet.Mvc.Rendering
 {
@@ -64,5 +65,24 @@ namespace Microsoft.AspNet.Mvc.Rendering
         /// <param name="value">object with string representation as HTML markup.</param>
         /// <returns>An <see cref="HtmlString"/> that represents HTML markup.</returns>
         HtmlString Raw(object value);
+
+        
+        /// <summary>
+        /// Returns a partial view in string format.
+        /// </summary>
+        /// <param name="partialViewName">The name of the partial view to render and return.</param>
+        /// <param name="model">A model to pass into the partial view.</param>
+        /// <param name="viewData">A <see cref="ViewDataDictionary"/> to pass into the partial view.</param>
+        /// <returns>A task that represents when rendering of the partial view into a string has completed.</returns>
+        Task<HtmlString> PartialAsync([NotNull] string partialViewName, object model, ViewDataDictionary viewData);
+
+        /// <summary>
+        /// Renders a partial view.
+        /// </summary>
+        /// <param name="partialViewName">The name of the partial view to render.</param>
+        /// <param name="model">A model to pass into the partial view.</param>
+        /// <param name="viewData">A <see cref="ViewDataDictionary"/> to pass into the partial view.</param>
+        /// <returns>A task that represents when rendering has completed.</returns>
+        Task RenderPartialAsync([NotNull] string partialViewName, object model, ViewDataDictionary viewData);
     }
 }

src/Microsoft.AspNet.Mvc.Rendering/Properties/Resources.Designer.cs

@@ -138,22 +138,6 @@ namespace Microsoft.AspNet.Mvc.Rendering
             return string.Format(CultureInfo.CurrentCulture, GetString("ViewData_WrongTModelType"), p0, p1);
         }
 
-        /// <summary>
-        /// The view '{0}' was not found. The following locations were searched:{1}.
-        /// </summary>
-        internal static string ViewEngine_ViewNotFound
-        {
-            get { return GetString("ViewEngine_ViewNotFound"); }
-        }
-
-        /// <summary>
-        /// The view '{0}' was not found. The following locations were searched:{1}.
-        /// </summary>
-        internal static string FormatViewEngine_ViewNotFound(object p0, object p1)
-        {
-            return string.Format(CultureInfo.CurrentCulture, GetString("ViewEngine_ViewNotFound"), p0, p1);
-        }
-
         private static string GetString(string name, params string[] formatterNames)
         {
             var value = _resourceManager.GetString(name);

src/Microsoft.AspNet.Mvc.Rendering/View/ViewContext.cs

@@ -13,6 +13,10 @@ namespace Microsoft.AspNet.Mvc.Rendering
         private readonly FormContext _defaultFormContext = new FormContext();
 
         private FormContext _formContext;
+        
+        public ViewContext([NotNull] ViewContext viewContext)
+            : this(viewContext.ServiceProvider, viewContext.HttpContext, viewContext.ViewEngineContext)
+        { }
 
         public ViewContext(IServiceProvider serviceProvider, HttpContext httpContext,
             IDictionary<string, object> viewEngineContext)