Add CreateTagHelper to RazorPage.

- The CreateTagHelper method is responsible for creating and activating TagHelpers.
- Added the support for requesting the ViewContext.
- Added tests to validate the tag helper creation mechanism.

#1104

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

@@ -11,6 +11,7 @@ using System.Threading.Tasks;
 using Microsoft.AspNet.Http;
 using Microsoft.AspNet.Mvc.Rendering;
 using Microsoft.AspNet.PageExecutionInstrumentation;
+using Microsoft.AspNet.Razor.Runtime.TagHelpers;
 using Microsoft.Framework.DependencyInjection;
 
 namespace Microsoft.AspNet.Mvc.Razor
@@ -24,6 +25,7 @@ namespace Microsoft.AspNet.Mvc.Razor
         private readonly Stack<TextWriter> _writerScopes;
         private TextWriter _originalWriter;
         private IUrlHelper _urlHelper;
+        private ITypeActivator _typeActivator;
         private bool _renderedBody;
 
         public RazorPage()
@@ -111,6 +113,38 @@ namespace Microsoft.AspNet.Mvc.Razor
         /// <inheritdoc />
         public abstract Task ExecuteAsync();
 
+        private ITypeActivator TypeActivator
+        {
+            get
+            {
+                if(_typeActivator == null)
+                {
+                    _typeActivator = ViewContext.HttpContext.RequestServices.GetService<ITypeActivator>();
+                }
+
+                return _typeActivator;
+            }
+        }
+
+        /// <summary>
+        /// Creates and activates a <see cref="ITagHelper"/>.
+        /// </summary>
+        /// <typeparam name="TTagHelper">A <see cref="ITagHelper"/> type.</typeparam>
+        /// <returns>The activated <see cref="ITagHelper"/>.</returns>
+        /// <remarks>
+        /// If the <see cref= "ITagHelper" /> implements <see cref="ICanHasViewContext"/> the 
+        /// <see cref="ICanHasViewContext.Contextualize(ViewContext)"/> method is called with <see cref="ViewContext"/>.
+        /// </remarks>
+        public TTagHelper CreateTagHelper<TTagHelper>() where TTagHelper : ITagHelper
+        {
+            var tagHelper = TypeActivator.CreateInstance<TTagHelper>(ViewContext.HttpContext.RequestServices);
+            var hasViewContext = tagHelper as ICanHasViewContext;
+
+            hasViewContext?.Contextualize(ViewContext);
+
+            return tagHelper;
+        }
+
         /// <summary>
         /// Starts a new writing scope.
         /// </summary>

test/Microsoft.AspNet.Mvc.Razor.Test/RazorPageCreateTagHelperTest.cs

@@ -0,0 +1,146 @@
+// 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;
+using System.IO;
+using System.Threading.Tasks;
+using Microsoft.AspNet.Http;
+using Microsoft.AspNet.Mvc.ModelBinding;
+using Microsoft.AspNet.Mvc.Rendering;
+using Microsoft.AspNet.Razor.Runtime.TagHelpers;
+using Microsoft.AspNet.Routing;
+using Microsoft.Framework.DependencyInjection;
+using Moq;
+using Xunit;
+
+namespace Microsoft.AspNet.Mvc.Razor
+{
+    public class RazorPageCreateTagHelperTest
+    {
+        [Fact]
+        public void CreateTagHelper_CreatesProvidedTagHelperType()
+        {
+            // Arrange
+            var instance = CreateTestRazorPage();
+
+            // Act
+            var tagHelper = instance.CreateTagHelper<NoServiceTagHelper>();
+
+            // Assert
+            Assert.NotNull(tagHelper);
+        }
+
+        [Fact]
+        public void CreateTagHelper_ActivatesProvidedTagHelperType()
+        {
+            // Arrange
+            var instance = CreateTestRazorPage();
+
+            // Act
+            var tagHelper = instance.CreateTagHelper<ServiceTagHelper>();
+
+            // Assert
+            Assert.NotNull(tagHelper.PassedInService);
+        }
+
+        [Fact]
+        public void CreateTagHelper_ContextualizesProvidedTagHelperType()
+        {
+            // Arrange
+            var instance = CreateTestRazorPage();
+
+            // Act
+            var tagHelper = instance.CreateTagHelper<ViewContextTagHelper>();
+
+            // Assert
+            Assert.NotNull(tagHelper.ViewContext);
+        }
+
+        [Fact]
+        public void CreateTagHelper_ContextualizesAndActivatesProvidedTagHelperType()
+        {
+            // Arrange
+            var instance = CreateTestRazorPage();
+
+            // Act
+            var tagHelper = instance.CreateTagHelper<ViewContextServiceTagHelper>();
+
+            // Assert
+            Assert.NotNull(tagHelper.ViewContext);
+            Assert.NotNull(tagHelper.PassedInService);
+        }
+
+        private static TestRazorPage CreateTestRazorPage()
+        {
+            var typeActivator = new TypeActivator();
+            var activator = new RazorPageActivator(typeActivator);
+            var serviceProvider = new Mock<IServiceProvider>();
+            var myService = new MyService();
+            serviceProvider.Setup(mock => mock.GetService(typeof(MyService)))
+                           .Returns(myService);
+            serviceProvider.Setup(mock => mock.GetService(typeof(ITypeActivator)))
+                           .Returns(typeActivator);
+            var httpContext = new Mock<HttpContext>();
+            httpContext.SetupGet(c => c.RequestServices)
+                       .Returns(serviceProvider.Object);
+            var routeContext = new RouteContext(httpContext.Object);
+            var actionContext = new ActionContext(routeContext, new ActionDescriptor());
+            var viewData = new ViewDataDictionary(Mock.Of<IModelMetadataProvider>());
+            var viewContext = new ViewContext(actionContext,
+                                              Mock.Of<IView>(),
+                                              viewData,
+                                              TextWriter.Null);
+
+            return new TestRazorPage
+            {
+                ViewContext = viewContext
+            };
+        }
+
+        private class TestRazorPage : RazorPage<dynamic>
+        {
+            public override Task ExecuteAsync()
+            {
+                throw new NotImplementedException();
+            }
+        }
+
+        private class NoServiceTagHelper : TagHelper
+        {
+        }
+
+        private class ServiceTagHelper : TagHelper
+        {
+            public MyService PassedInService { get; set; }
+
+            public ServiceTagHelper(MyService service)
+            {
+                PassedInService = service;
+            }
+        }
+
+        private class ViewContextTagHelper : TagHelper, ICanHasViewContext
+        {
+            public ViewContext ViewContext { get; set; }
+
+            public void Contextualize([NotNull]ViewContext viewContext)
+            {
+                ViewContext = viewContext;
+            }
+        }
+
+        private class ViewContextServiceTagHelper : ViewContextTagHelper
+        {
+            public MyService PassedInService { get; set; }
+
+            public ViewContextServiceTagHelper(MyService service)
+            {
+                PassedInService = service;
+            }
+        }
+
+        private class MyService
+        {
+        }
+    }
+}