Add AddVisitor method and Configure hook (#3897)

Fixes https://github.com/microsoft/typespec/issues/3827

Also adds a Configure hook so that AddVisitor can be called directly
from the Plugin. Considered just using the ctor but that would mean we
would have virtual property calls in a ctor.

packages/http-client-csharp/generator/Microsoft.Generator.CSharp/src/CodeModelPlugin.cs

@@ -41,6 +41,14 @@ namespace Microsoft.Generator.CSharp
             _inputLibrary = new(() => new InputLibrary(Instance.Configuration.OutputDirectory));
         }
 
+        // for mocking
+        protected CodeModelPlugin()
+        {
+            // should be mocked
+            Configuration = null!;
+            _inputLibrary = new(() => null!);
+        }
+
         private Lazy<InputLibrary> _inputLibrary;
 
         // Extensibility points to be implemented by a plugin
@@ -50,5 +58,9 @@ namespace Microsoft.Generator.CSharp
         public InputLibrary InputLibrary => _inputLibrary.Value;
         public virtual TypeProviderWriter GetWriter(TypeProvider provider) => new(provider);
         public virtual IReadOnlyList<MetadataReference> AdditionalMetadataReferences => Array.Empty<MetadataReference>();
+
+        public virtual void Configure()
+        {
+        }
     }
 }

packages/http-client-csharp/generator/Microsoft.Generator.CSharp/src/OutputLibrary.cs

@@ -8,6 +8,7 @@ namespace Microsoft.Generator.CSharp
 {
     public class OutputLibrary
     {
+        private List<OutputLibraryVisitor> _visitors = new();
         public OutputLibrary()
         {
         }
@@ -57,6 +58,11 @@ namespace Microsoft.Generator.CSharp
         }
 
         // TODO - make this more additive instead of replace https://github.com/microsoft/typespec/issues/3827
-        protected internal virtual IEnumerable<OutputLibraryVisitor> GetOutputLibraryVisitors() => [];
+        protected internal virtual IEnumerable<OutputLibraryVisitor> GetOutputLibraryVisitors() => _visitors;
+
+        public void AddVisitor(OutputLibraryVisitor visitor)
+        {
+            _visitors.Add(visitor);
+        }
     }
 }

packages/http-client-csharp/generator/Microsoft.Generator.CSharp/src/StartUp/PluginHandler.cs

@@ -13,26 +13,32 @@ namespace Microsoft.Generator.CSharp
         public void LoadPlugin(CommandLineOptions options)
         {
             using DirectoryCatalog directoryCatalog = new(AppContext.BaseDirectory);
-            using (CompositionContainer container = new(directoryCatalog))
-            {
-                container.ComposeExportedValue(new GeneratorContext(Configuration.Load(options.OutputDirectory)));
-                container.ComposeParts(this);
-                bool loaded = false;
-                foreach (var plugin in Plugins!)
-                {
-                    if (plugin.Metadata.PluginName == options.PluginName)
-                    {
-                        CodeModelPlugin.Instance = plugin.Value;
-                        loaded = true;
-                        break;
-                    }
-                }
+            using CompositionContainer container = new(directoryCatalog);
 
-                if (!loaded)
+            container.ComposeExportedValue(new GeneratorContext(Configuration.Load(options.OutputDirectory)));
+            container.ComposeParts(this);
+
+            SelectPlugin(options.PluginName!);
+        }
+
+        internal void SelectPlugin(string pluginName)
+        {
+            bool loaded = false;
+            foreach (var plugin in Plugins!)
+            {
+                if (plugin.Metadata.PluginName == pluginName)
                 {
-                    throw new InvalidOperationException($"Plugin {options.PluginName} not found.");
+                    CodeModelPlugin.Instance = plugin.Value;
+                    loaded = true;
+                    CodeModelPlugin.Instance.Configure();
+                    break;
                 }
             }
+
+            if (!loaded)
+            {
+                throw new InvalidOperationException($"Plugin {pluginName} not found.");
+            }
         }
 
         [ImportMany]

packages/http-client-csharp/generator/Microsoft.Generator.CSharp/test/OutputLibraryTests.cs

@@ -2,6 +2,8 @@
 // Licensed under the MIT License.
 
 using System;
+using System.Collections.Generic;
+using System.Linq;
 using Microsoft.Generator.CSharp.Providers;
 using NUnit.Framework;
 
@@ -16,7 +18,7 @@ namespace Microsoft.Generator.CSharp.Tests
         [SetUp]
         public void Setup()
         {
-            _outputLibrary = new MockOutputLibrary();
+            _outputLibrary = new TestOutputLibrary();
         }
 
         // Tests that the BuildTypeProviders method is successfully overridden.
@@ -26,9 +28,54 @@ namespace Microsoft.Generator.CSharp.Tests
             Assert.Throws<NotImplementedException>(() => { object shouldFail = _outputLibrary.TypeProviders; });
         }
 
-        internal class MockOutputLibrary : OutputLibrary
+        [Test]
+        public void CanAddVisitors()
         {
-            public MockOutputLibrary() : base() { }
+            _outputLibrary.AddVisitor(new TestOutputLibraryVisitor());
+            Assert.AreEqual(1, _outputLibrary.GetOutputLibraryVisitors().Count());
+        }
+
+        [Test]
+        public void CanOverrideGetOutputLibraryVisitors()
+        {
+            var outputLibrary = new TestOutputLibraryOverridingVisitors(new [] { new TestOutputLibraryVisitor() });
+            Assert.AreEqual(1, outputLibrary.GetOutputLibraryVisitors().Count());
+
+            outputLibrary.AddVisitor(new TestOutputLibraryVisitor());
+            Assert.AreEqual(2, outputLibrary.GetOutputLibraryVisitors().Count());
+        }
+
+        private class TestOutputLibraryVisitor : OutputLibraryVisitor
+        {
+        }
+
+        private class TestOutputLibrary : OutputLibrary
+        {
+            protected override TypeProvider[] BuildTypeProviders()
+            {
+                throw new NotImplementedException();
+            }
+        }
+
+        private class TestOutputLibraryOverridingVisitors : OutputLibrary
+        {
+            private readonly IEnumerable<OutputLibraryVisitor>? _visitors;
+            public TestOutputLibraryOverridingVisitors(IEnumerable<OutputLibraryVisitor>? visitors = null)
+            {
+                _visitors = visitors;
+            }
+
+            protected internal override IEnumerable<OutputLibraryVisitor> GetOutputLibraryVisitors()
+            {
+                foreach (var visitor in base.GetOutputLibraryVisitors())
+                {
+                    yield return visitor;
+                }
+                foreach (var visitor in _visitors ?? [])
+                {
+                    yield return visitor;
+                }
+            }
 
             protected override TypeProvider[] BuildTypeProviders()
             {

packages/http-client-csharp/generator/Microsoft.Generator.CSharp/test/StartUp/PluginHandlerTests.cs

@@ -0,0 +1,46 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License.
+
+using System.Collections.Generic;
+using Moq;
+using NUnit.Framework;
+
+namespace Microsoft.Generator.CSharp.Tests.StartUp
+{
+    public class PluginHandlerTests
+    {
+        [Test]
+        public void SelectPluginFindsMatchingPlugin()
+        {
+            var pluginHandler = new PluginHandler();
+            var metadataMock = new Mock<IMetadata>();
+            metadataMock.SetupGet(m => m.PluginName).Returns("MockPlugin");
+            var mockPlugin = new Mock<CodeModelPlugin>();
+
+            pluginHandler.Plugins = new List<System.Lazy<CodeModelPlugin, IMetadata>>
+            {
+                new(() => mockPlugin.Object, metadataMock.Object),
+            };
+
+            Assert.DoesNotThrow(() => pluginHandler.SelectPlugin("MockPlugin"));
+            mockPlugin.Verify(p => p.Configure(), Times.Once);
+        }
+
+        [Test]
+        public void SelectPluginThrowsWhenNoMatch()
+        {
+            var pluginHandler = new PluginHandler();
+            var metadataMock = new Mock<IMetadata>();
+            metadataMock.SetupGet(m => m.PluginName).Returns("MockPlugin");
+            var mockPlugin = new Mock<CodeModelPlugin>();
+
+            pluginHandler.Plugins = new List<System.Lazy<CodeModelPlugin, IMetadata>>
+            {
+                new(() => mockPlugin.Object, metadataMock.Object),
+            };
+
+            Assert.Throws<System.InvalidOperationException>(() => pluginHandler.SelectPlugin("NonExistentPlugin"));
+            mockPlugin.Verify(p => p.Configure(), Times.Never);
+        }
+    }
+}

packages/http-client-csharp/generator/SamplePlugin/src/OutputTypes/SamplePluginOutputLibrary.cs

@@ -1,12 +0,0 @@
-// Copyright (c) Microsoft Corporation. All rights reserved.
-// Licensed under the MIT License.
-
-using System.Collections.Generic;
-using Microsoft.Generator.CSharp.ClientModel;
-namespace SamplePlugin
-{
-    public class SamplePluginOutputLibrary : ScmOutputLibrary
-    {
-        protected override IEnumerable<SamplePluginOutputLibraryVisitor> GetOutputLibraryVisitors() => [new SamplePluginOutputLibraryVisitor()];
-    }
-}

packages/http-client-csharp/generator/SamplePlugin/src/SampleCodeModelPlugin.cs

@@ -14,6 +14,9 @@ namespace SamplePlugin
     {
         public override SamplePluginTypeFactory TypeFactory { get; } = new SamplePluginTypeFactory();
 
-        public override OutputLibrary OutputLibrary { get; } = new SamplePluginOutputLibrary();
+        public override void Configure()
+        {
+            OutputLibrary.AddVisitor(new SamplePluginOutputLibraryVisitor());
+        }
     }
 }