[OpenAPI] Refactor MapOpenApi (dotnet#62194)

Author: martincostello

src/OpenApi/src/Extensions/OpenApiEndpointRouteBuilderExtensions.cs

@@ -54,21 +54,27 @@ public static IEndpointConventionBuilder MapOpenApi(this IEndpointRouteBuilder e
                     var document = await documentService.GetOpenApiDocumentAsync(context.RequestServices, context.Request, context.RequestAborted);
                     var documentOptions = options.Get(lowercasedDocumentName);
 
-                    using var writer = new Utf8BufferTextWriter();
-                    writer.SetWriter(context.Response.BodyWriter);
+                    using var textWriter = new Utf8BufferTextWriter();
+                    textWriter.SetWriter(context.Response.BodyWriter);
+
+                    string contentType;
+                    OpenApiWriterBase openApiWriter;
 
                     if (UseYaml(pattern))
                     {
-                        context.Response.ContentType = "text/plain+yaml;charset=utf-8";
-                        await context.Response.StartAsync();
-                        await document.SerializeAsync(new OpenApiYamlWriter(writer), documentOptions.OpenApiVersion, context.RequestAborted);
+                        contentType = "text/plain+yaml;charset=utf-8";
+                        openApiWriter = new OpenApiYamlWriter(textWriter);
                     }
                     else
                     {
-                        context.Response.ContentType = "application/json;charset=utf-8";
-                        await context.Response.StartAsync();
-                        await document.SerializeAsync(new OpenApiJsonWriter(writer), documentOptions.OpenApiVersion, context.RequestAborted);
+                        contentType = "application/json;charset=utf-8";
+                        openApiWriter = new OpenApiJsonWriter(textWriter);
                     }
+
+                    context.Response.ContentType = contentType;
+
+                    await context.Response.StartAsync();
+                    await document.SerializeAsync(openApiWriter, documentOptions.OpenApiVersion, context.RequestAborted);
                     await context.Response.BodyWriter.FlushAsync(context.RequestAborted);
                 }
             }).ExcludeFromDescription();