Api.kt

class ApiClient private constructor(context: Context) {

    val apiService = createApiService(context)

    private fun createApiService(context: Context): ApiService {
        return Retrofit.Builder()
                .baseUrl(context.getString(R.string.api_base_url))
                .addConverterFactory(createConverterFactory())
                .client(createHttpClient(context))
                .build()
                .create(ApiService::class.java)
    }

    private fun createConverterFactory(): GsonConverterFactory {
        return GsonConverterFactory.create(GSON)
    }

    private fun createHttpClient(context: Context): OkHttpClient {
        val interceptor = HttpLoggingInterceptor()

        interceptor.level = HttpLoggingInterceptor.Level.BODY

        return OkHttpClient.Builder()
                .addInterceptor(interceptor)
                .build()
    }

    companion object {
	...
    }

}

interface ApiService {

    @GET("doors_state/read")
    fun readLock(): Call<LockResponse>

    @POST("doors_state/toggle")
    fun toggleLock(): Call<LockResponse>

    @FormUrlEncoded
    @POST("users/register")
    fun register(
            @Field("user_id") user_id: String,
            @Field("registration_id") registration_id: String
    ): Call<ResponseBody>

}

data class LockResponse(val status: String, val body: LockState)

data class LockState(val id: Int, val state: String, val created_at: Date) {

    companion object {

        const val OPEN = "open"
        const val CLOSED = "closed"

        fun fromJson(json: String): LockState? = try {
            ApiClient.GSON.fromJson<LockState>(json, LockState::class.java)
        } catch (e: JsonSyntaxException) {
            e.printStackTrace()
            null
        }

    }

    fun isOpen(): Boolean {
        return OPEN.equals(state)
    }

}