How to support drag and drop in UICollectionView iOS

Issue #411

See DragAndDrop example

class ViewController: UIViewController, UICollectionViewDropDelegate, UICollectionViewDragDelegate {

    // MARK: - UICollectionViewDragDelegate

    func collectionView(_ collectionView: UICollectionView, itemsForBeginning session: UIDragSession, at indexPath: IndexPath) -> [UIDragItem] {
        let controller = leftController

        let provider = NSItemProvider(
            object: controller.imageForCell(indexPath: indexPath)
        )

        let dragItem = UIDragItem(itemProvider: provider)
        dragItem.localObject = indexPath
        return [dragItem]
    }

    // MARK: - UICollectionViewDropDelegate

    func collectionView(_ collectionView: UICollectionView, performDropWith coordinator: UICollectionViewDropCoordinator) {

        let destinationIndexPath: IndexPath
        if let indexPath = coordinator.destinationIndexPath {
            destinationIndexPath = indexPath
        } else {
            destinationIndexPath = IndexPath(row: 0, section: 0)
        }

        let controller = rightController

        let dragItemIndexPath = coordinator.items.last?.dragItem.localObject as! IndexPath
        let draggedItem = leftController.items[dragItemIndexPath.item]

        // remove
        leftController.items.remove(at: dragItemIndexPath.item)
        leftController.collectionView.deleteItems(at: [dragItemIndexPath])

        // insert
        controller.items.insert(draggedItem, at: destinationIndexPath.item)
        controller.collectionView.insertItems(at: [destinationIndexPath])
    }
}