Support WithOpenApi for registering transformers

src/OpenApi/src/Extensions/OpenApiEndpointConventionBuilderExtensions.cs

@@ -121,4 +121,17 @@ private static void AddAndConfigureOperationForEndpoint(EndpointBuilder endpoint
             }
         }
     }
+
+    /// <summary>
+    /// Adds <see cref="IOpenApiOperationTransformer"/> that targets the given endpoint.
+    /// </summary>
+    /// <typeparam name="TBuilder">The type of the endpoint convention builder.</typeparam>
+    /// <param name="builder">The endpoint convention builder.</param>
+    /// <param name="transformer">A function that transforms an <see cref="OpenApiOperation"/>.</param>
+    /// <returns>The endpoint convention builder with the transformer added.</returns>
+    public static TBuilder WithOpenApi<TBuilder>(this TBuilder builder, Func<OpenApiOperation, OpenApiOperationTransformerContext, CancellationToken, Task> transformer) where TBuilder : IEndpointConventionBuilder
+    {
+        builder.WithMetadata(new DelegateOpenApiOperationTransformer(transformer));
+        return builder;
+    }
 }

src/OpenApi/src/PublicAPI.Unshipped.txt

@@ -1 +1,2 @@
 #nullable enable
+static Microsoft.AspNetCore.Builder.OpenApiEndpointConventionBuilderExtensions.WithOpenApi<TBuilder>(this TBuilder builder, System.Func<Microsoft.OpenApi.Models.OpenApiOperation!, Microsoft.AspNetCore.OpenApi.OpenApiOperationTransformerContext!, System.Threading.CancellationToken, System.Threading.Tasks.Task!>! transformer) -> TBuilder

src/OpenApi/src/Services/OpenApiDocumentService.cs

@@ -267,6 +267,14 @@ private async Task<Dictionary<OperationType, OpenApiOperation>> GetOperationsAsy
                 var transformer = operationTransformers[i];
                 await transformer.TransformAsync(operation, operationContext, cancellationToken);
             }
+
+            var endpointOperationTransformer = description.ActionDescriptor.EndpointMetadata
+                    .OfType<DelegateOpenApiOperationTransformer>()
+                    .LastOrDefault();
+            if (endpointOperationTransformer is not null)
+            {
+                await endpointOperationTransformer.TransformAsync(operation, operationContext, cancellationToken);
+            }
         }
         return operations;
     }

src/OpenApi/test/Microsoft.AspNetCore.OpenApi.Tests/Transformers/OperationTransformerTests.cs

@@ -478,6 +478,61 @@ public async Task OperationTransformer_CanAccessTransientServiceFromContextAppli
         Assert.Equal(4, Dependency.InstantiationCount);
     }
 
+    [Fact]
+    public async Task WithOpenApi_CanApplyTransformer()
+    {
+        var builder = CreateBuilder();
+
+        builder.MapGet("/", () => { })
+            .WithOpenApi((operation, context, cancellationToken) =>
+            {
+                operation.Description = "Operation Description";
+                return Task.CompletedTask;
+            });
+
+        await VerifyOpenApiDocument(builder, document =>
+        {
+            Assert.Collection(document.Paths.OrderBy(p => p.Key),
+                path =>
+                {
+                    Assert.Equal("/", path.Key);
+                    var operation = Assert.Single(path.Value.Operations.Values);
+                    Assert.Equal("Operation Description", operation.Description);
+                });
+        });
+    }
+
+    [Fact]
+    public async Task WithOpenApi_TransformerRunsAfterOtherTransformers()
+    {
+        var builder = CreateBuilder();
+
+        builder.MapGet("/", () => { })
+            .WithOpenApi((operation, context, cancellationToken) =>
+            {
+                operation.Description = "Operation Description";
+                return Task.CompletedTask;
+            });
+
+        var options = new OpenApiOptions();
+        options.AddOperationTransformer((operation, context, cancellationToken) =>
+        {
+            operation.Description = "Operation Description 2";
+            return Task.CompletedTask;
+        });
+
+        await VerifyOpenApiDocument(builder, document =>
+        {
+            Assert.Collection(document.Paths.OrderBy(p => p.Key),
+                path =>
+                {
+                    Assert.Equal("/", path.Key);
+                    var operation = Assert.Single(path.Value.Operations.Values);
+                    Assert.Equal("Operation Description", operation.Description);
+                });
+        });
+    }
+
     private class ActivatedTransformer : IOpenApiOperationTransformer
     {
         public Task TransformAsync(OpenApiOperation operation, OpenApiOperationTransformerContext context, CancellationToken cancellationToken)