add_filter( 'wp_nav_menu_objects', 'submenu_limit', 10, 2 );
function submenu_limit( $items, $args ) {
if ( empty($args->menu_item_start) )
return $items;
$parent_ids = wp_filter_object_list( $items, array( 'menu_item_parent' => 0 ), 'and', 'ID' );
if( !empty( $parent_ids ) ){
$menu_item_end = ( isset( $args->menu_item_end ) ) ? (int) $args->menu_item_end : count( $parent_ids );
$total = ($menu_item_end) - ((int) $args->menu_item_start);
$range_parent_ids = array_slice( (array) $parent_ids, ($args->menu_item_start)-1, $total+1, true );
if( !empty( $range_parent_ids ) ) {
$return_items = array();
foreach ( $range_parent_ids as $parent_id ) {
$return_items[] = $parent_id;
$children = submenu_get_children_ids( $parent_id, $items );
if( !empty($children) ) {
$return_items = array_merge( $return_items, $children );
}
}
foreach( $items as $key => $item ){
if( !in_array( $item->ID, $return_items ) ){
unset( $items[$key] );
}
}
} // if !empty($range_parent_ids)
} // if !empty($parent_ids)
return $items;
}
function submenu_get_children_ids( $id, $items ) {
$ids = wp_filter_object_list( $items, array( 'menu_item_parent' => $id ), 'and', 'ID' );
foreach ( $ids as $id ) {
$ids = array_merge( $ids, submenu_get_children_ids( $id, $items ) );
}
return $ids;
}