class CollectionViewController: UICollectionViewController {
    func loadForVisibleItems() {
        let pathsArray = collectionView.indexPathsForVisibleItems
        let allPendingOperations = Set(pendingOperations.downloadsInProgress.keys)
        var toBeCancelled = allPendingOperations

        let visiblePaths = Set(pathsArray)
        toBeCancelled.subtract(visiblePaths)

        var toBeStarted = visiblePaths
        toBeStarted.subtract(allPendingOperations)

        for indexPath in toBeCancelled {
            if let pendingDownload = pendingOperations.downloadsInProgress[indexPath] {
                pendingDownload.cancel()
            }
            pendingOperations.downloadsInProgress.removeValue(forKey: indexPath)
        }

        for indexPath in toBeStarted {
            let recordToProcess = images[indexPath.row]
            startDownload(for: recordToProcess, at: indexPath)
        }
    }

    override func scrollViewDidEndDragging(_ scrollView: UIScrollView, willDecelerate decelerate: Bool) {
        if !decelerate {
            loadForVisibleItems()
            resumeAllOperations()
        }
    }

    override func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {
        loadForVisibleItems()
        resumeAllOperations()
    }
}