[UITableViewDropDelegate] #drop

ShoppingListViewController.swift

extension ShoppingListViewController: UITableViewDropDelegate {
  func tableView(_ tableView: UITableView, performDropWith coordinator: UITableViewDropCoordinator) {
    guard let destinationIndexPath = coordinator.destinationIndexPath else {
      return
    }
    DispatchQueue.main.async { [weak self] in
      tableView.beginUpdates()
      coordinator.items.forEach({ (item) in
        guard let sourceIndexPath = item.sourceIndexPath,
        let `self` = self
          else {return}
        
        let row = self.shoppingList.remove(at: sourceIndexPath.row)
        self.shoppingList.insert(row, at: destinationIndexPath.row)
        tableView.moveRow(at: sourceIndexPath, to: destinationIndexPath)
      })
      tableView.endUpdates()
    }
  }
  
  func tableView(_ tableView: UITableView, canHandle session: UIDropSession) -> Bool {
    return session.canLoadObjects(ofClass: ListItem.self)
  }
  
  func tableView(_ tableView: UITableView, dropSessionDidUpdate session: UIDropSession, withDestinationIndexPath destinationIndexPath: IndexPath?) -> UITableViewDropProposal {
    return UITableViewDropProposal(operation: .move, intent: .insertAtDestinationIndexPath)
  }
}