Making a Retrofit API call with RxJava with a Kotlin-esque API

ApiKotlin.kt

authApi.getTokens()
  .subscribe { response : NetworkResponse<AccessToken, Error> -> 
    when (response) {
        is NetworkResponse.Success<AccessToken> -> {
            // A 2XX response that's guaranteed to have a body of type AccessToken.
        }
        is NetworkResponse.ServerError<Error> -> { 
            // A non-2XX response that may have an Error as its error body.
        }
        is NetworkResponse.NetworkError -> { 
            // A request that didn't result in a response from the server.
        }
    }
  }