Using generics with Retrofit repositories. A work in progress.

RetrofitRepo.kt

package com.example.domain.api

import retrofit2.Call
import retrofit2.Callback
import retrofit2.Response
import retrofit2.Retrofit
import retrofit2.converter.gson.GsonConverterFactory

open class RetrofitRepo<out S>(clazz: Class<S>) {
    val service: S = Api.service(clazz)

    fun <T> enqueue(call: Call<T>, listener: ApiListener<T>) {
        call.enqueue(object : Callback<T> {
            override fun onResponse(call: Call<T>, response: Response<T>) {
                val data = response.body()

                if (response.isSuccessful) {
                    if (data != null) {
                        println("SUCCESS")
                        listener.onDataLoaded(data)
                    }
                } else {
                    // error response, no access to resource?
                    println("FAIL: Response Unsuccessful")
                }
            }

            override fun onFailure(call: Call<T>, error: Throwable) {
                // something went completely south (like no internet connection)
                // handle failure
                println("FAIL: Call Failed")
                println(error.message)
            }
        })
    }

    fun <T> execute(call: Call<T>): T? {
        var data: T? = null

        try {
            val result = call.execute()
            println("SUCCESS")
            if (result?.body() != null)
                data = result.body()!!
        } catch (e: Exception) {
            println("FAIL: Call Failed")
            println(e.message)
        }

        return data
    }

    object Api {
        fun <T> service(clazz: Class<T>): T {
            return  Retrofit.Builder()
                    .baseUrl("https://api.spacexdata.com/")
                    .addConverterFactory(GsonConverterFactory.create())
                    .build().create(clazz)
        }
    }
}

RocketRepo.kt

package com.example.domain.rocket.api

import com.example.domain.api.ApiListener
import com.example.domain.api.RetrofitRepo
import com.example.domain.rocket.Rocket

class RocketRepo : RetrofitRepo<RocketService>(RocketService::class.java) {

    fun loadRocket(id: String, listener: ApiListener<Rocket>) {
        enqueue(service.loadRocket(id), listener)
    }

    fun getRocket(id: String): Rocket? {
        return execute(service.loadRocket(id))
    }

    fun loadRockets(listener: ApiListener<Array<Rocket>>) {
        enqueue(service.loadRockets(), listener)
    }

    fun getRockets(): Array<Rocket>? {
        return execute(service.loadRockets())
    }

}

Thank you for saving my time, i finally understood how to do it!