/**
* Returns an iterator which can be used to get ids of all children of the given item
* The iteration is performed from parent to child
*
* @param string $id The id or alias of the layout item
*
* @return HierarchyIterator
*
* @throws Exception\InvalidArgumentException if the id is empty
* @throws Exception\ItemNotFoundException if the layout item does not exist
*/
public function getHierarchyIterator($id)
{
$id = $this->validateAndResolveId($id);
$children = $this->hierarchy->get($this->getProperty($id, self::PATH, true));
return new HierarchyIterator($id, $children);
}