For medium post https://medium.com/p/b94f656fb05d/edit

SampleAuthenticator.kt

import okhttp3.Authenticator
class MyAppAuthenticator(private val preference: SharedPreferences): Authenticator {
	// This callback is called when api request failed in 401.
	override fun authenticate(route: Route?, response: Response?): Request? {
		val currentRefreshToken = preference.getString("REFRESH_TOKEN_KEY", "")
		val (newAccessToken, newRefreshToken) = requestNewTokensToServer(currentRefreshToken).blockingGet()  // TODO error handling
		preference.edit().putString("ACCESS_TOKEN_KEY",  newAccessToken).apply()
		preference.edit().putString("REFRESH_TOKEN_KEY",  newRefreshToken).apply()
		return attachNewTokenOnRequest(response.request(), newAccessToken)
	}
}