ReturnToTopDemo.kt

override fun start() {
        itemDBObservable = itemDao.getAllItems()
        compositeDisposable.clear()
        compositeDisposable += itemDBObservable
                .subscribeOn(Schedulers.io())
                .observeOn(AndroidSchedulers.mainThread())
                .scan { previousList: List<Item>, newList: List<Item> ->
                    val newItems = newList - previousList
                    if (previousList.isNotEmpty() && newItems.isNotEmpty()) {
                        showReturnToTopButton()
                    }
                    newList
                }
                .subscribeBy {
                    updateItemList(it)
                }
}

fun updateItemList(itemList: List<Item>) = itemAdapter.submitList(itemList)

fun showReturnToTopButton() {
    showReturnToTopButton = true
}