Custom Walker to extract current sub-menu

/**
 * Custom Walker to extract current sub-menu
 *
 * @since xx
 * @uses Walker
 */
class Custom_Walker_Nav_Sub_Menu extends Walker_Nav_Menu {

    var $found_parents = array();

    /**
     * Start the element output.
     *
     * The $args parameter holds additional values that may be used with the child
     * class methods. Includes the element output also.
     *
     * @since 2.1.0
     * @abstract
     *
     * @param string $output            Passed by reference. Used to append additional content.
     * @param object $object            The data object.
     * @param int    $depth             Depth of the item.
     * @param array  $args              An array of additional arguments.
     * @param int    $current_object_id ID of the current item.
     */
    function start_el( &$output, $item, $depth, $args ) {

        global $wp_query;

        //this only works for second level sub navigations
        $parent_item_id = 0;

        $indent = ( $depth ) ? str_repeat( "\t", $depth ) : '';

        $class_names = $value = '';

        $classes = empty( $item->classes ) ? array() : (array) $item->classes;

        $class_names = join( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item ) );
        $class_names = ' class="' . esc_attr( $class_names ) . '"';


        // Checks if the current element is in the current selection (it is menu sublevel)
        if ( strpos( $class_names, 'current-menu-item' ) || strpos( $class_names, 'current-menu-parent' ) || strpos( $class_names, 'current-menu-ancestor' ) || ( is_array( $this->found_parents ) && in_array( $item->menu_item_parent, $this->found_parents ) ) ) {

            // Keep track of all selected parents
            $this->found_parents[] = $item->ID;

            //check if the item_parent matches the current item_parent
            if ( $item->menu_item_parent != $parent_item_id ) {

                $output .= $indent . '<li id="menu-item-' . $item->ID . '"' . $value . $class_names . '>';

                $attributes = !empty( $item->attr_title ) ? ' title="' . esc_attr( $item->attr_title ) . '"' : '';
                $attributes .=!empty( $item->target ) ? ' target="' . esc_attr( $item->target ) . '"' : '';
                $attributes .=!empty( $item->xfn ) ? ' rel="' . esc_attr( $item->xfn ) . '"' : '';
                $attributes .=!empty( $item->url ) ? ' href="' . esc_attr( $item->url ) . '"' : '';


                // get user defined attributes for thumbnail images
                $attr_defaults = array( 'class' => 'nav_thumbnail ', 'alt' => esc_attr( $item->attr_title ), 'title' => esc_attr( $item->attr_title ) );
                $attr = isset( $args->thumbnail_attr ) ? $args->thumbnail_attr : '';
                $attr = wp_parse_args( $attr, $attr_defaults );
                $item_output = $args->before;
                $item_output .= '<a' . $attributes . '>';
                if ( $depth == 1 ) {
                    // thumbnail image output
                    $item_output .= ( isset( $args->thumbnail_link ) && $args->thumbnail_link ) ? '<a' . $attributes . '>' : '';
                    $item_output .= apply_filters( 'menu_item_thumbnail', ( isset( $args->thumbnail ) && $args->thumbnail ) ? get_the_post_thumbnail( $item->object_id, ( isset( $args->thumbnail_size ) ) ? $args->thumbnail_size : 'thumbnail', $attr ) : '', $item, $args, $depth );
                    $item_output .= ( isset( $args->thumbnail_link ) && $args->thumbnail_link ) ? '</a>' : '';
                }
                // menu link output
                $item_output .= $args->link_before . apply_filters( 'the_title', $item->title, $item->ID ) . $args->link_after;
                // menu description
                $item_output .= '<span class="menu-item-description">' . $item->description . '</span>';
                // close menu link anchor
                $item_output .= '</a>';
                $item_output .= $args->after;

                $output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );
            }
        }
    }

    /**
     * Ends the element output, if needed.
     *
     * The $args parameter holds additional values that may be used with the child class methods.
     *
     * @since 2.1.0
     * @abstract
     *
     * @param string $output Passed by reference. Used to append additional content.
     * @param object $object The data object.
     * @param int    $depth  Depth of the item.
     * @param array  $args   An array of additional arguments.
     */
    function end_el( &$output, $item, $depth ) {
        // Closes only the opened li
        if ( is_array( $this->found_parents ) && in_array( $item->ID, $this->found_parents ) ) {
            $output .= "</li>\n";
        }
    }

    /**
     * Starts the list before the elements are added.
     *
     * The $args parameter holds additional values that may be used with the child
     * class methods. This method is called at the start of the output list.
     *
     * @since 2.1.0
     * @abstract
     *
     * @param string $output Passed by reference. Used to append additional content.
     * @param int    $depth  Depth of the item.
     * @param array  $args   An array of additional arguments.
     */
    function start_lvl( &$output, $depth = 0 ) {
        $output .= "\n<ul class=\"sub-menu\">\n";
    }

    /**
     * Ends the list of after the elements are added.
     *
     * The $args parameter holds additional values that may be used with the child
     * class methods. This method finishes the list at the end of output of the elements.
     *
     * @since 2.1.0
     * @abstract
     *
     * @param string $output Passed by reference. Used to append additional content.
     * @param int    $depth  Depth of the item.
     * @param array  $args   An array of additional arguments.
     */
    function end_lvl( &$output, $depth ) {
        $indent = str_repeat( "\t", $depth );
        // If the sub-menu is empty, strip the opening tag, else closes it
        if ( substr( $output, -22 ) == "<ul class=\"sub-menu\">\n" ) {
            $output = substr( $output, 0, strlen( $output ) - 23 );
        } else {
            $output .= "$indent</ul>\n";
        }
    }

}