FavoritesProvider.kt

class FavoritesProvider : IFavoritesProvider {

    val database = FirebaseDatabase.getInstance()
    val gson = Gson()
    val favoritesCache = FavoritesMemoryCache()

  override fun saveBusStopFavoritesList(favorites: List<BusStopFavorite>) {
        //save in cache
        favoritesCache.saveBusStopFavoritesList(favorites)
        //save in remote
        val json = gson.toJson(favorites)
        database.getReference(getBusStopFavoritesPath(getCurrentUid())).setValue(json)
  }
  
  override fun retrieveBusStopFavoritesListSingle(): Single<List<BusStopFavorite>> {

        val cachedFavorites = favoritesCache.retrieveBusStopFavoritesList()

        if (cachedFavorites != null) {
            return Single.just(cachedFavorites)
                .subscribeOn(Schedulers.io())
                .observeOn(AndroidSchedulers.mainThread())
        } else {
            return Single.create<LinkedList<String>> { emitter ->

                database.getReference(getBusStopFavoritesPath(getCurrentUid()))
                    .addListenerForSingleValueEvent(object : ValueEventListener {
                        override fun onCancelled(error: DatabaseError) {
                            emitter.onError(error.toException())
                        }

                        override fun onDataChange(datasnapshot: DataSnapshot) {
                            val json = datasnapshot.value as String?

                            if (json != null) {
                                val listOfFavoritesType = object : TypeToken<List<BusStopFavorite>>() {}.type
                                val list = gson.fromJson<List<BusStopFavorite>>(json, listOfFavoritesType)
                                //save in cache
                                favoritesCache.saveBusStopFavoritesList(list)
                                emitter.onSuccess(list)
                            } else {
                                emitter.onSuccess(List())
                            }
                        }
                    })

            }.subscribeOn(Schedulers.io())
                .observeOn(AndroidSchedulers.mainThread())
        }

    }
  
  
}