extension ViewController: HorizontalStickyHeaderLayoutDelegate {
    private enum Const {
        static let headerSize = CGSize(width: 100, height: 38)
        static let itemSize0  = CGSize(width: 50, height: 50)
        static let itemSize1  = CGSize(width: 80, height: 80)
        static let headerLeft: CGFloat = 8
    }
    func collectionView(_ collectionView: UICollectionView, hshlSizeForItemAtIndexPath indexPath: IndexPath) -> CGSize {
        if indexPath.section % 2 == 0 {
            return Const.itemSize0
        } else {
            return Const.itemSize1
        }
    }
    func collectionView(_ collectionView: UICollectionView, hshlSizeForHeaderAtSection section: Int) -> CGSize {
        return Const.headerSize
    }
    func collectionView(_ collectionView: UICollectionView, hshlHeaderInsetsAtSection section: Int) -> UIEdgeInsets {
        return UIEdgeInsets(top: 0, left: Const.headerLeft, bottom: 20, right: 20)
    }
    func collectionView(_ collectionView: UICollectionView, hshlMinSpacingForCellsAtSection section: Int) -> CGFloat {
        return 20
    }
    func collectionView(_ collectionView: UICollectionView, hshlSectionInsetsAtSection section: Int) -> UIEdgeInsets {
        return UIEdgeInsets(top: 0, left: 0, bottom: 0, right: section == 4 ? 0 : 20)
    }
}

    // Either in UICollectionViewDelegate or this override method.
    override func didUpdateFocus(in context: UIFocusUpdateContext, with coordinator: UIFocusAnimationCoordinator) {
        layout.updatePoppingHeaderIndexPaths()
        let (pop, unpop) = self.getHeaders(poppingHeadersIndexPaths: self.layout.poppingHeaderIndexPaths)
        UIView.animate(withDuration: Const.unpopDuration, delay: 0, options: [.curveEaseOut], animations: {
            unpop.forEach { $0.unpopHeader() }
        }, completion: nil)
        coordinator.addCoordinatedAnimations({
            pop.forEach { $0.popHeader() }
        }, completion: nil)
        super.didUpdateFocus(in: context, with: coordinator)
    }

    func collectionView(_ collectionView: UICollectionView, hshlDidUpdatePoppingHeaderIndexPaths indexPaths: [IndexPath]) {
        let (pop, unpop) = self.getHeaders(poppingHeadersIndexPaths: self.layout.poppingHeaderIndexPaths)
        UIView.animate(withDuration: Const.unpopDuration, delay: 0, options: [.curveEaseOut], animations: {
            unpop.forEach { $0.unpopHeader() }
            pop.forEach { $0.popHeader() }
        }, completion: nil)
    }