Apply all transformers in order instead of last-one-wins

Author: captainsafia

src/OpenApi/src/Services/OpenApiDocumentService.cs

@@ -271,10 +271,9 @@ private async Task<Dictionary<OperationType, OpenApiOperation>> GetOperationsAsy
 
             // Apply any endpoint-specific operation transformers registered via
             // the AddOpenApiOperationTransformer extension method.
-            var endpointOperationTransformer = description.ActionDescriptor.EndpointMetadata
-                .OfType<DelegateOpenApiOperationTransformer>()
-                .LastOrDefault();
-            if (endpointOperationTransformer is not null)
+            var endpointOperationTransformers = description.ActionDescriptor.EndpointMetadata
+                .OfType<DelegateOpenApiOperationTransformer>();
+            foreach (var endpointOperationTransformer in endpointOperationTransformers)
             {
                 await endpointOperationTransformer.TransformAsync(operation, operationContext, cancellationToken);
             }

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

@@ -479,7 +479,7 @@ public async Task OperationTransformer_CanAccessTransientServiceFromContextAppli
     }
 
     [Fact]
-    public async Task WithOpenApi_CanApplyTransformer()
+    public async Task AddOpenApiOperationTransformer_CanApplyTransformer()
     {
         var builder = CreateBuilder();
 
@@ -503,7 +503,7 @@ await VerifyOpenApiDocument(builder, document =>
     }
 
     [Fact]
-    public async Task WithOpenApi_TransformerRunsAfterOtherTransformers()
+    public async Task AddOpenApiOperationTransformer_TransformerRunsAfterOtherTransformers()
     {
         var builder = CreateBuilder();
 
@@ -533,6 +533,44 @@ await VerifyOpenApiDocument(builder, document =>
         });
     }
 
+    [Fact]
+    public async Task AddOpenApiOperationTransformer_SupportsMultipleTransformers()
+    {
+        var builder = CreateBuilder();
+
+        builder.MapGet("/", () => { })
+            .AddOpenApiOperationTransformer((operation, context, cancellationToken) =>
+            {
+                operation.Description = "Operation Description";
+                return Task.CompletedTask;
+            })
+            .AddOpenApiOperationTransformer((operation, context, cancellationToken) =>
+            {
+                operation.Description += " 2";
+                operation.Deprecated = true;
+                return Task.CompletedTask;
+            })
+            .AddOpenApiOperationTransformer((operation, context, cancellationToken) =>
+            {
+                operation.Description += " 3";
+                operation.OperationId = "OperationId";
+                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 2 3", operation.Description);
+                    Assert.True(operation.Deprecated);
+                    Assert.Equal("OperationId", operation.OperationId);
+                });
+        });
+    }
+
     private class ActivatedTransformer : IOpenApiOperationTransformer
     {
         public Task TransformAsync(OpenApiOperation operation, OpenApiOperationTransformerContext context, CancellationToken cancellationToken)