TaskChecklistItemHandler::onExecuteCheckListItemMoveItem

function onExecuteCheckListItemMoveItem(BitrixMainEvent $event)
{
	$parameters = $event->getParameters();
	$selectedItemId = (int)$parameters[0];
	$insertAfterItemId = (int)$parameters[1];

	if ($selectedItemId > 0 && $insertAfterItemId > 0)
	{
		$connection = Application::getConnection();

		$items = $this->getItems($selectedItemId, $connection);

		$newSortIndex = (int)$items[$insertAfterItemId]['SORT_INDEX'] + 1;
		$oldSortIndex = (int)$items[$selectedItemId]['SORT_INDEX'];
		$newParentId = (int)$items[$insertAfterItemId]['PARENT_ID'];
		$oldParentId = (int)$items[$selectedItemId]['PARENT_ID'];

		$nextIndex = $newSortIndex + 1;
		$prevIndex = $oldSortIndex;

		$recountedSortIndexes = [];
		foreach ($items as $id => $item)
		{
			$parentId = (int)$item['PARENT_ID'];
			$sortIndex = (int)$item['SORT_INDEX'];

			if ($parentId === $newParentId && $sortIndex >= $newSortIndex)
			{
				$recountedSortIndexes[$id] = $nextIndex;
				$nextIndex++;
			}
			else if ($parentId === $oldParentId && $sortIndex > $oldSortIndex)
			{
				$recountedSortIndexes[$id] = $prevIndex;
				$prevIndex++;
			}
		}
		$recountedSortIndexes[$selectedItemId] = $newSortIndex;

		$this->updateSortIndexes($recountedSortIndexes, $connection);
		$this->updateParents($newParentId, $oldParentId, $selectedItemId);
	}
}