Service.kt

fun <T> callback(success: (Response<T>) -> Unit, failure: (t: Throwable) -> Unit): Callback<T>? {
            return object : Callback<T> {
                override fun onResponse(call: Call<T>, response: retrofit2.Response<T>) = success(response)
                override fun onFailure(call: Call<T>, t: Throwable) = failure(t)
            }
        }

fun parseResponse(listener: (AppResponse?, Throwable?) -> Unit): Callback<AppResponse>? {
    return callback(
            { response -> if (response.body() != null) listener(response.body(), null) else listener(null, Throwable(response.errorBody().toString())) },
            { throwable -> listener(null, throwable) }
    )
}

fun saveCheckIn(checkIn: CheckIn, listener: (AppResponse?, Throwable?) -> Unit) = checkInService.saveCheckIn(checkIn).enqueue(parseResponse(listener))