RxJava refresh token

import io.reactivex.Observable
import io.reactivex.ObservableSource
import io.reactivex.ObservableTransformer
import io.reactivex.functions.Predicate
import retrofit2.Response

enum class HttpCode(val code: Int) {
    UNAUTHORIZED(401)
}

class RefreshTokenTransformer<T : Response<*>>(private val apiService: ApiService) : ObservableTransformer<T, T> {

    override fun apply(stream: Observable<T>): ObservableSource<T> {
        return stream.flatMap { response ->
            if (response.code() == HttpCode.UNAUTHORIZED.code) {
                val token = UserManager.getInstance().getToken()

                synchronized(/* any singleton object */) {
                    val currentToken = UserManager.getInstance().getToken()

                    if (currentToken.equals(token)) {
                        return@synchronized apiService.refreshToken()
                                .flatMapObservable { tokenResponse: Response<TokenResponse> ->
                                    /* here you have fresh token */
                                    return@flatMapObservable stream
                                }
                    } else {
                        return@synchronized stream
                    }
                }

            } else {
                Observable.just(response)
            }
        }
    }
}