[Fixes #4087] Add AddViewComponentsAsServices() and ServiceBasedViewC…

…omponentActivator

* Added ViewComponentFeture and ViewComponentFeatureProvider to perform view component discovery.
* Changed view component discovery to use application parts.
* Changed ViewComponentDescriptorProvider to make use of Application parts.
* Added AddViewComponentsAsServices method on IMvcBuilder that performs view component
  discovery through the ApplicationPartManager and registers those view components as
  services in the service collection. Assemblies should be added to the ApplicationPartManager
  in order to discover view components in them in them.

src/Microsoft.AspNetCore.Mvc.ViewFeatures/DependencyInjection/MvcViewFeaturesMvcBuilderExtensions.cs

@@ -2,7 +2,10 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
+using System.Linq;
 using Microsoft.AspNetCore.Mvc;
+using Microsoft.AspNetCore.Mvc.ViewComponents;
+using Microsoft.Extensions.DependencyInjection.Extensions;
 
 namespace Microsoft.Extensions.DependencyInjection
 {
@@ -16,6 +19,7 @@ public static class MvcViewFeaturesMvcBuilderExtensions
         /// </summary>
         /// <param name="builder">The <see cref="IMvcBuilder"/>.</param>
         /// <param name="setupAction">The <see cref="MvcViewOptions"/> which need to be configured.</param>
+        /// <returns>The <see cref="IMvcBuilder"/>.</returns>
         public static IMvcBuilder AddViewOptions(
             this IMvcBuilder builder,
             Action<MvcViewOptions> setupAction)
@@ -33,5 +37,30 @@ public static IMvcBuilder AddViewOptions(
             builder.Services.Configure(setupAction);
             return builder;
         }
+
+        /// <summary>
+        /// Registers discovered view components as services in the <see cref="IServiceCollection"/>.
+        /// </summary>
+        /// <param name="builder">The <see cref="IMvcBuilder"/>.</param>
+        /// <returns>The <see cref="IMvcBuilder"/>.</returns>
+        public static IMvcBuilder AddViewComponentsAsServices(this IMvcBuilder builder)
+        {
+            if (builder == null)
+            {
+                throw new ArgumentNullException(nameof(builder));
+            }
+
+            var feature = new ViewComponentFeature();
+            builder.PartManager.PopulateFeature(feature);
+
+            foreach (var viewComponent in feature.ViewComponents.Select(vc => vc.AsType()))
+            {
+                builder.Services.TryAddTransient(viewComponent, viewComponent);
+            }
+
+            builder.Services.Replace(ServiceDescriptor.Singleton<IViewComponentActivator, ServiceBasedViewComponentActivator>());
+
+            return builder;
+        }
     }
 }

src/Microsoft.AspNetCore.Mvc.ViewFeatures/DependencyInjection/MvcViewFeaturesMvcCoreBuilderExtensions.cs

@@ -3,7 +3,9 @@
 
 using System;
 using System.Buffers;
+using System.Linq;
 using Microsoft.AspNetCore.Mvc;
+using Microsoft.AspNetCore.Mvc.ApplicationParts;
 using Microsoft.AspNetCore.Mvc.Controllers;
 using Microsoft.AspNetCore.Mvc.Formatters;
 using Microsoft.AspNetCore.Mvc.Rendering;
@@ -26,10 +28,19 @@ public static IMvcCoreBuilder AddViews(this IMvcCoreBuilder builder)
             }
 
             builder.AddDataAnnotations();
+            AddViewComponentApplicationPartsProviders(builder.PartManager);
             AddViewServices(builder.Services);
             return builder;
         }
 
+        private static void AddViewComponentApplicationPartsProviders(ApplicationPartManager manager)
+        {
+            if (!manager.FeatureProviders.OfType<ViewComponentFeatureProvider>().Any())
+            {
+                manager.FeatureProviders.Add(new ViewComponentFeatureProvider());
+            }
+        }
+
         public static IMvcCoreBuilder AddViews(
             this IMvcCoreBuilder builder,
             Action<MvcViewOptions> setupAction)

src/Microsoft.AspNetCore.Mvc.ViewFeatures/ViewComponents/DefaultViewComponentDescriptorProvider.cs

@@ -6,7 +6,7 @@
 using System.Linq;
 using System.Reflection;
 using System.Threading.Tasks;
-using Microsoft.AspNetCore.Mvc.Infrastructure;
+using Microsoft.AspNetCore.Mvc.ApplicationParts;
 using Microsoft.AspNetCore.Mvc.ViewFeatures;
 
 namespace Microsoft.AspNetCore.Mvc.ViewComponents
@@ -18,64 +18,47 @@ public class DefaultViewComponentDescriptorProvider : IViewComponentDescriptorPr
     {
         private const string AsyncMethodName = "InvokeAsync";
         private const string SyncMethodName = "Invoke";
-        private readonly IAssemblyProvider _assemblyProvider;
+        private readonly ApplicationPartManager _partManager;
 
         /// <summary>
         /// Creates a new <see cref="DefaultViewComponentDescriptorProvider"/>.
         /// </summary>
-        /// <param name="assemblyProvider">The <see cref="IAssemblyProvider"/>.</param>
-        public DefaultViewComponentDescriptorProvider(IAssemblyProvider assemblyProvider)
+        /// <param name="partManager">The <see cref="ApplicationPartManager"/>.</param>
+        public DefaultViewComponentDescriptorProvider(ApplicationPartManager partManager)
         {
-            _assemblyProvider = assemblyProvider;
+            if (partManager == null)
+            {
+                throw new ArgumentNullException(nameof(partManager));
+            }
+
+            _partManager = partManager;
         }
 
         /// <inheritdoc />
         public virtual IEnumerable<ViewComponentDescriptor> GetViewComponents()
         {
-            var types = GetCandidateTypes();
-
-            return types
-                .Where(IsViewComponentType)
-                .Select(CreateDescriptor);
+            return GetCandidateTypes().Select(CreateDescriptor);
         }
 
         /// <summary>
-        /// Gets the candidate <see cref="TypeInfo"/> instances. The results of this will be provided to
-        /// <see cref="IsViewComponentType"/> for filtering.
+        /// Gets the candidate <see cref="TypeInfo"/> instances provided by the <see cref="ApplicationPartManager"/>.
         /// </summary>
         /// <returns>A list of <see cref="TypeInfo"/> instances.</returns>
         protected virtual IEnumerable<TypeInfo> GetCandidateTypes()
         {
-            var assemblies = _assemblyProvider.CandidateAssemblies;
-            return assemblies.SelectMany(a => a.ExportedTypes).Select(t => t.GetTypeInfo());
-        }
-
-        /// <summary>
-        /// Determines whether or not the given <see cref="TypeInfo"/> is a view component class.
-        /// </summary>
-        /// <param name="typeInfo">The <see cref="TypeInfo"/>.</param>
-        /// <returns>
-        /// <c>true</c> if <paramref name="typeInfo"/>represents a view component class, otherwise <c>false</c>.
-        /// </returns>
-        protected virtual bool IsViewComponentType(TypeInfo typeInfo)
-        {
-            if (typeInfo == null)
-            {
-                throw new ArgumentNullException(nameof(typeInfo));
-            }
-
-            return ViewComponentConventions.IsComponent(typeInfo);
+            var feature = new ViewComponentFeature();
+            _partManager.PopulateFeature(feature);
+            return feature.ViewComponents;
         }
 
         private static ViewComponentDescriptor CreateDescriptor(TypeInfo typeInfo)
         {
-            var type = typeInfo.AsType();
             var candidate = new ViewComponentDescriptor
             {
                 FullName = ViewComponentConventions.GetComponentFullName(typeInfo),
                 ShortName = ViewComponentConventions.GetComponentName(typeInfo),
                 TypeInfo = typeInfo,
-                MethodInfo = FindMethod(type)
+                MethodInfo = FindMethod(typeInfo.AsType())
             };
 
             return candidate;

src/Microsoft.AspNetCore.Mvc.ViewFeatures/ViewComponents/ServiceBasedViewComponentActivator.cs

@@ -0,0 +1,33 @@
+// 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 Microsoft.Extensions.DependencyInjection;
+
+namespace Microsoft.AspNetCore.Mvc.ViewComponents
+{
+    /// <summary>
+    /// A <see cref="IViewComponentActivator"/> that retrieves view components as services from the request's
+    /// <see cref="IServiceProvider"/>.
+    /// </summary>
+    public class ServiceBasedViewComponentActivator : IViewComponentActivator
+    {
+        /// <inheritdoc />
+        public object Create(ViewComponentContext context)
+        {
+            if (context == null)
+            {
+                throw new ArgumentNullException(nameof(context));
+            }
+
+            var viewComponentType = context.ViewComponentDescriptor.TypeInfo.AsType();
+
+            return context.ViewContext.HttpContext.RequestServices.GetRequiredService(viewComponentType);
+        }
+
+        /// <inheritdoc />
+        public virtual void Release(ViewComponentContext context, object viewComponent)
+        {
+        }
+    }
+}

src/Microsoft.AspNetCore.Mvc.ViewFeatures/ViewComponents/ViewComponentFeature.cs

@@ -0,0 +1,24 @@
+// 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.Collections.Generic;
+using System.Reflection;
+using Microsoft.AspNetCore.Mvc.ApplicationParts;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace Microsoft.AspNetCore.Mvc.ViewComponents
+{
+    /// <summary>
+    /// The list of view component types in an MVC application.The <see cref="ViewComponentFeature"/> can be populated
+    /// using the <see cref="ApplicationPartManager"/> that is available during startup at <see cref="IMvcBuilder.PartManager"/>
+    /// and <see cref="IMvcCoreBuilder.PartManager"/> or at a later stage by requiring the <see cref="ApplicationPartManager"/>
+    /// as a dependency in a component.
+    /// </summary>
+    public class ViewComponentFeature
+    {
+        /// <summary>
+        /// Gets the list of view component types in an MVC application.
+        /// </summary>
+        public IList<TypeInfo> ViewComponents { get; } = new List<TypeInfo>();
+    }
+}

src/Microsoft.AspNetCore.Mvc.ViewFeatures/ViewComponents/ViewComponentFeatureProvider.cs

@@ -0,0 +1,38 @@
+// 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.Collections.Generic;
+using System.Linq;
+using Microsoft.AspNetCore.Mvc.ApplicationParts;
+
+namespace Microsoft.AspNetCore.Mvc.ViewComponents
+{
+    /// <summary>
+    /// Discovers view components from a list of <see cref="ApplicationPart"/> instances.
+    /// </summary>
+    public class ViewComponentFeatureProvider : IApplicationFeatureProvider<ViewComponentFeature>
+    {
+        /// <inheritdoc />
+        public void PopulateFeature(IEnumerable<ApplicationPart> parts, ViewComponentFeature feature)
+        {
+            if (parts == null)
+            {
+                throw new ArgumentNullException(nameof(parts));
+            }
+
+            if (feature == null)
+            {
+                throw new ArgumentNullException(nameof(feature));
+            }
+
+            foreach (var type in parts.OfType<IApplicationPartTypeProvider>().SelectMany(p => p.Types))
+            {
+                if (ViewComponentConventions.IsComponent(type) && ! feature.ViewComponents.Contains(type))
+                {
+                    feature.ViewComponents.Add(type);
+                }
+            }
+        }
+    }
+}

test/Microsoft.AspNetCore.Mvc.Core.Test/Controllers/ControllerFeatureProviderTest.cs

@@ -280,7 +280,7 @@ public void AncestorTypeDoesNotHaveControllerAttribute_IsNotController()
         }
 
         [Fact]
-        public void GetFeature_OnlyRunsOnParts_ThatImplementIExportTypes()
+        public void GetFeature_OnlyRunsOnParts_ThatImplementIApplicationPartTypeProvider()
         {
             // Arrange
             var otherPart = new Mock<ApplicationPart>();

test/Microsoft.AspNetCore.Mvc.FunctionalTests/MvcTestFixture.cs

@@ -10,6 +10,7 @@
 using Microsoft.AspNetCore.Mvc.ApplicationParts;
 using Microsoft.AspNetCore.Mvc.Controllers;
 using Microsoft.AspNetCore.Mvc.Infrastructure;
+using Microsoft.AspNetCore.Mvc.ViewComponents;
 using Microsoft.AspNetCore.TestHost;
 using Microsoft.AspNetCore.Testing;
 using Microsoft.Extensions.DependencyInjection;
@@ -72,7 +73,10 @@ protected virtual void InitializeServices(IServiceCollection services)
 
             var manager = new ApplicationPartManager();
             manager.ApplicationParts.Add(new AssemblyPart(startupAssembly));
+
             manager.FeatureProviders.Add(new ControllerFeatureProvider());
+            manager.FeatureProviders.Add(new ViewComponentFeatureProvider());
+
             services.AddSingleton(manager);
         }
 

test/Microsoft.AspNetCore.Mvc.FunctionalTests/ViewComponentFromServicesTests.cs

@@ -0,0 +1,35 @@
+// 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.Net.Http;
+using System.Threading.Tasks;
+using Xunit;
+
+namespace Microsoft.AspNetCore.Mvc.FunctionalTests
+{
+    public class ViewComponentFromServicesTest : IClassFixture<MvcTestFixture<ControllersFromServicesWebSite.Startup>>
+    {
+        public ViewComponentFromServicesTest(MvcTestFixture<ControllersFromServicesWebSite.Startup> fixture)
+        {
+            Client = fixture.Client;
+        }
+
+        public HttpClient Client { get; }
+
+        [Fact]
+        public async Task ViewComponentsWithConstructorInjectionAreCreatedAndActivated()
+        {
+            // Arrange
+            var expected = "Value = 3";
+            var request = new HttpRequestMessage(HttpMethod.Get, "http://localhost/another/InServicesViewComponent");
+
+            // Act
+            var response = await Client.SendAsync(request);
+            var responseText = await response.Content.ReadAsStringAsync();
+
+            // Assert
+            Assert.Equal(expected, responseText);
+        }
+    }
+}

test/Microsoft.AspNetCore.Mvc.Test/MvcServiceCollectionExtensionsTest.cs

@@ -10,6 +10,7 @@
 using Microsoft.AspNetCore.Mvc.ActionConstraints;
 using Microsoft.AspNetCore.Mvc.ApiExplorer;
 using Microsoft.AspNetCore.Mvc.ApplicationModels;
+using Microsoft.AspNetCore.Mvc.ApplicationParts;
 using Microsoft.AspNetCore.Mvc.Controllers;
 using Microsoft.AspNetCore.Mvc.Cors.Internal;
 using Microsoft.AspNetCore.Mvc.DataAnnotations.Internal;
@@ -18,6 +19,7 @@
 using Microsoft.AspNetCore.Mvc.Internal;
 using Microsoft.AspNetCore.Mvc.Razor;
 using Microsoft.AspNetCore.Mvc.Razor.Internal;
+using Microsoft.AspNetCore.Mvc.ViewComponents;
 using Microsoft.AspNetCore.Mvc.ViewFeatures;
 using Microsoft.AspNetCore.Mvc.ViewFeatures.Internal;
 using Microsoft.AspNetCore.Routing;
@@ -122,6 +124,48 @@ public void AddMvcServicesTwice_DoesNotAddDuplicates()
             }
         }
 
+        [Fact]
+        public void AddMvcTwice_DoesNotAddApplicationFeatureProvidersTwice()
+        {
+            // Arrange
+            var services = new ServiceCollection();
+            var providers = new IApplicationFeatureProvider[]
+            {
+                new ControllerFeatureProvider(),
+                new ViewComponentFeatureProvider()
+            };
+
+            // Act
+            services.AddMvc();
+            services.AddMvc();
+
+            // Assert
+            var descriptor = Assert.Single(services, d => d.ServiceType == typeof(ApplicationPartManager));
+            Assert.Equal(ServiceLifetime.Singleton, descriptor.Lifetime);
+            Assert.NotNull(descriptor.ImplementationInstance);
+            var manager = Assert.IsType<ApplicationPartManager>(descriptor.ImplementationInstance);
+
+            Assert.Equal(2, manager.FeatureProviders.Count);
+            Assert.IsType<ControllerFeatureProvider>(manager.FeatureProviders[0]);
+            Assert.IsType<ViewComponentFeatureProvider>(manager.FeatureProviders[1]);
+        }
+
+        [Fact]
+        public void AddMvcCore_ReusesExistingApplicationPartManagerInstance_IfFoundOnServiceCollection()
+        {
+            // Arrange
+            var services = new ServiceCollection();
+            var manager = new ApplicationPartManager();
+            services.AddSingleton(manager);
+
+            // Act
+            services.AddMvc();
+
+            // Assert
+            var descriptor = Assert.Single(services, d => d.ServiceType == typeof(ApplicationPartManager));
+            Assert.Same(manager, descriptor.ImplementationInstance);
+        }
+
         private IEnumerable<Type> SingleRegistrationServiceTypes
         {
             get