Consider callbacks in unused schemas

[jmini]

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.4.x, 4.0.x. Default: master.
• Copied the technical committee: @OpenAPITools/generator-core-team (change to the core)

Description of the PR

Callbacks can reference Schemas. The ModelUtils methods that compute used and unused schemas should visit the callbacks.

Callbacks effort in general is tracked as #372.

[jimschubert]

Code looks good. I'm not that familiar with callbacks, so I don't know how I'd go about testing this. I'll leave the evaluation from that perspective to someone else with experience in callback usages.

[jimschubert]

I know most of the code here was just reorganized, but I just had a question. I'm wondering why we follow visitor pattern only for a few elements (visitSchema) but not others which are commented here like "Params", "RequestBody", "Responses", and "Callbacks"? It seems like this would make for smaller surface area for unit testing edge cases.

[jmini]

@jimschubert: I never forgot your comment about having a true visitor Pattern for an OpenAPI spec.

I have implemented it in OpenAPITools/empoa#14

See: https://openapitools.github.io/empoa/#visitor-pattern

[jmini]

Thank you for the review.

This PR is mainly about taking into account the Schema that can be referenced in PathItems of Callbacks. I did not change anything else.

Of course a proper/complete visitor pattern would be better. This might also be an overkill for this use case. In the ModelUtils the visitor approach is internal (private) in order to be able to share core between following methods:

List<String> getAllUsedSchemas(OpenAPI openAPI)
List<String> getUnusedSchemas(OpenAPI openAPI)
List<String> getSchemasUsedOnlyInFormParam(OpenAPI openAPI)