[kotlin][moshi] add polymorphic support

[kvn-stgl]

Currently it's not possible to use Kotlin and Moshi with the oneOf-Keyword. I tried to implement the PolymorphicJsonAdapterFactory to support this feature.
The reason why I set the target branch to master is because the current strategy will fail during runtime (it's not allowed with moshi to serialize a interface directly).

I tested it with the following config:

generatorName: kotlin
outputDir: samples/client/3_0/kotlin-retrofit2-moshi-polymorphic
library: jvm-retrofit2
inputSpec: modules/openapi-generator/src/test/resources/3_0/allOf.yaml
templateDir: modules/openapi-generator/src/main/resources/kotlin-client
additionalProperties:
  serializationLibrary: moshi
  artifactId: kotlin-retrofit2-moshi-polymorphic
  moshiCodeGen: "true"

Saddly in the current petstore samples are no discriminator, that's why I didn't add a sample. My plans for the future are to directly change the interface to a sealed class. Any recommendations for the strategy therefore?

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh
  

  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master, 5.1.x, 6.0.x
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

technical committee : @jimschubert @dr4ke616 @karismann @Zomzog @andrewemery @4brunu @yutaka0m

[auto-labeler]

👍 Thanks for opening this issue!
🏷 I have applied any labels matching special text in your issue.

The team will review the labels and make any necessary changes.

[kvn-stgl]

Short note: This is also a first step to support #8059 together with Moshi.

[wing328]

     {{#moshi}}
     {{^discriminator}}@Json(name = "{{{vendorExtensions.x-base-name-literal}}}"){{/discriminator}}
     {{/moshi}}

to avoid blank line in the output.

[kvn-stgl]

Changed. Thanks for the hint 👍

[wing328]

Same as https://github.com/OpenAPITools/openapi-generator/pull/9159/files#r614518206

[kvn-stgl]

Changed. Thanks for the hint 👍

[wing328]

My plans for the future are to directly change the interface to a sealed class. Any recommendations for the strategy therefore?

Please test with https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/test/resources/3_0/java/petstore-with-fake-endpoints-models-for-testing-with-http-signature.yaml#L1927 instead.

For oneOf implementation in Kotline, the oneOf impelmentation in java (jersey2), csharp-netcore are good starting points. PM me if Slack if you need help on this.

[4brunu]

@kvn-stgl adding support for oneOf is great, thanks for creating this PR 👍
Could you please fix the merge conflicts and create a sample with a oneOf example?
Thanks

[kuFEAR]

Any updates? And I want to adopt the same logic for kotlinx.serialization

[4brunu]

Not that I'm aware of

[kuFEAR]

where I can look on reference implementation of polymorphic codegen? As I see in the PR author don't add template with inheritance of sealed class in subclasses

[kuFEAR]

I've found for https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/resources/ruby-client/partial_oneof_module.mustache

[cwahl1]

@4brunu I'm going start working on resolving the issues in this PR. Where should I submit my work when I'm done?

[4brunu]

You should fork this repo, make the changes in your fork, and then create a PR back to this repo.