ASCollectionViewDelegate: support fast index scrolling?

[quantamrhino]

I have a class that inherits from ASCollectionViewDelegate
with implementations for
func indexTitles(for collectionView: UICollectionView) -> [String]?
and func collectionView(_ collectionView: UICollectionView, indexPathForIndexTitle title: String, at index: Int) -> IndexPath

But they never get called.
Not sure what I am doing wrong.