Rest Client for caching

RestClient.kt

package com.thorangs.retrofitcachingtest

import android.content.Context
import com.google.gson.GsonBuilder
import okhttp3.Cache
import okhttp3.OkHttpClient
import retrofit2.Retrofit
import retrofit2.adapter.rxjava2.RxJava2CallAdapterFactory
import retrofit2.converter.gson.GsonConverterFactory


/**
 * Created by Laxman Bhattarai on 11/18/17.
 * erluxman@gmail.com
 * https://github.com/erluxman
 * https://twitter.com/erluxman
 */
class RestClient private constructor(applicationContext: Context) {
    private val gson = GsonBuilder()
            .setLenient()
            .create()!!

    private var cacheSize: Long = 20 * 1024 * 1024 // 10 MB
    private var cache = Cache(applicationContext.cacheDir, cacheSize)

    private val httpClient = OkHttpClient.Builder()
            .addInterceptor { chain ->
                var request = chain.request()
                if (!isOnline(applicationContext)) {
                    val maxStale = 60 * 60 *24
                    request = request?.newBuilder()
                            ?.header("Cache-Control", "public, only-if-cached, max-stale=$maxStale")
                            ?.build()

                }
                chain.proceed(request)
            }
            .cache(cache)
            .build()


    private val retrofit = Retrofit.Builder()
            .baseUrl(baseUrl)
            .addConverterFactory(GsonConverterFactory.create(gson))
            .client(httpClient)
            .addCallAdapterFactory(RxJava2CallAdapterFactory.create())
            .build()
    val restAPIService: RESTApis = retrofit.create(RESTApis::class.java)

    companion object {
        fun instance(applicationContext: Context) = RestClient(applicationContext)
    }

}