The author uses Flutter for UI, Golang binary for the app backend, and uses channels passing serialized protobuf messages between them. Since adding a new API call requires changes in multiple places, they ended up defining a "god message" which has a oneof with all possible options:

  message CarrotAPI {
    oneof api {
      Function1API function1 = 10;
      Function2API function2 = 11;
    }
  }
  
  message Function1API {
        message Request {}
        message Response {}
        
        Request request = 1;
        Response response = 2;
  }

and then dispatch the call using switch to check the value of the api field.

This is... not really a good API design. I wonder if the problem of "multiple places to be edited" can be solved in an easier way by making a custom protoc plugin [1] which will take care of generating all the boilerplate for both Dart and Golang. This is how support for new languages and use cases is normally added to protoc.

[1]: https://protobuf.dev/reference/other/