Get location of cell for UICollectionView previewing - peek/pop

collectionViewCellLocation.swift

// I was getting bad results for collectionView.indexPathForItem(at: location)
// Ends up the scroll view content offset needs to be taken into consideration.

func previewingContext(_ previewingContext: UIViewControllerPreviewing, viewControllerForLocation location: CGPoint) -> UIViewController? {
    guard let collectionView = collectionView else { return nil }

  // The magic sauce
    let cellLocation = CGPoint(x: location.x + collectionView.contentOffset.x,
                               y: location.y + collectionView.contentOffset.y)

    guard let indexPath = collectionView.indexPathForItem(at: cellLocation) else { return nil }
    guard let cell = collectionView.cellForItem(at: indexPath) else { return nil }
}