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.