Documentation on prefetching.

[johndpope]

extension TheatreVideoVC: UICollectionViewDataSourcePrefetching {
    // TikTok eager download - fetch 5 movies as soon as possible on app start
    // get the next X videos /  start downloading them...
    func collectionView(_ collectionView: UICollectionView, prefetchItemsAt indexPaths: [IndexPath]) {

if indexPaths.count >= 1 {
                    let indexPath = indexPaths[0]
                    let idx0 = IndexPath(row: indexPath.row, section: 0)
                    let idx1 = IndexPath(row: indexPath.row+1, section: 0)
                    let idx2 = IndexPath(row: indexPath.row+2, section: 0)
                    let idx3 = IndexPath(row: indexPath.row+3, section: 0)
                    let idx4 = IndexPath(row: indexPath.row+4, section: 0)
                    let prefetchIndexPaths = [idx0, idx1, idx2, idx3, idx4]
                    for idx in prefetchIndexPaths {
                        self.prefetchItem(idx: idx)
                    }
                } 
    }

private func prefetchItem(idx: IndexPath) {
        print("INFO:prefetchItem \(idx)")
        if self.feedCollectionView != nil {
            if self.feedCollectionView.isValid(indexPath: idx) {
                if let video = self.theatreDataSource?.itemIdentifier(for: idx) as? Video {
                     Celestial.shared.startDownload(for: video.url)
                }
            }
        }
    }

I used this mistakenly Celestial.shared.prefetchResources(at: [url]) - and performance was crap. Was prepared to throw in bin - but when I switched to simple Celestial.shared.startDownload(for: video.url) - it's all good AND smooth. I think it's the sync calls in prefetch that cause scrolling to come unstuck.