retrofit

interface UsersApi {
    @GET("users" + "&format=json")
    fun fetchContents(): Call<MyResponse>
}

class UserFetchr {

    private val usersApi: UsersApi

    init {
        val retrofit: Retrofit = Retrofit.Builder()
            .baseUrl("https://reqres.in/api/")
            .addConverterFactory(GsonConverterFactory.create())
            .build()

        usersApi = retrofit.create(UsersApi::class.java)
    }

    fun fetchContents(): LiveData<List<UserItem>> {

        val responseLiveData: MutableLiveData<List<UserItem>> = MutableLiveData()
        val usersRequest: Call<MyResponse> = usersApi.fetchContents()

        usersRequest.enqueue(object : Callback<MyResponse> {

            override fun onFailure(call: Call<MyResponse>, t: Throwable) {
                Log.e(TAG, "Failde to fetch users", t)
                t.printStackTrace()
            }

            override fun onResponse(call: Call<MyResponse>,
                                    response: Response<MyResponse>) {

                val myResponse: MyResponse? = response.body()
                val userResponse: UserResponse? = myResponse?.users
                val userItems: List<UserItem> = userResponse?.userItems
                    ?: mutableListOf()
                responseLiveData.value = userItems
            }
        })
        return responseLiveData
    }
}

class MyResponse(var users: UserResponse)

class UserResponse(@SerializedName("data") var userItems: List<UserItem>)

data class UserItem(
    @SerializedName("id") var id: Int,
    @SerializedName("avatar") var urlImage: String,
    @SerializedName("first_name") var firstName: String,
    @SerializedName("last_name") var lastName: String,
    @SerializedName("email") var email: String
){}