wp_nav_menu walker (sub menus of current level 0 item)

class Kanec_Walker_Nav_Menu extends Walker {
    /**
     * @see Walker::$tree_type
     * @since 3.0.0
     * @var string
     */
    var $tree_type = array( 'post_type', 'taxonomy', 'custom' );

    /**
     * @see Walker::$db_fields
     * @since 3.0.0
     * @todo Decouple this.
     * @var array
     */
    var $db_fields = array( 'parent' => 'menu_item_parent', 'id' => 'db_id' );

    /**
     * @see Walker::start_lvl()
     * @since 3.0.0
     *
     * @param string $output Passed by reference. Used to append additional content.
     * @param int $depth Depth of page. Used for padding.
     */
    function start_lvl(&$output, $depth) {
        $indent = str_repeat("\t", $depth);
        $output .= "\n$indent<ul class=\"sub-menu\">\n";
    }

    /**
     * @see Walker::end_lvl()
     * @since 3.0.0
     *
     * @param string $output Passed by reference. Used to append additional content.
     * @param int $depth Depth of page. Used for padding.
     */
    function end_lvl(&$output, $depth) {
        global $current_branch;
        if ($depth == 0) $current_branch = false;
        $indent = str_repeat("\t", $depth);
        $output .= "$indent</ul>\n";
    }

    /**
     * @see Walker::start_el()
     * @since 3.0.0
     *
     * @param string $output Passed by reference. Used to append additional content.
     * @param object $item Menu item data object.
     * @param int $depth Depth of menu item. Used for padding.
     * @param int $current_page Menu item ID.
     * @param object $args
     */
    function start_el(&$output, $item, $depth, $args) {
        global $wp_query;
        global $current_branch;

        // Is this menu item in the current branch?
        if(in_array('current-menu-ancestor',$item->classes) ||
        in_array('current-menu-parent',$item->classes) ||
        in_array('current-menu-item',$item->classes)) {
            $current_branch = true;
        }

        if($current_branch && $depth > 0) {
            $indent = ( $depth ) ? str_repeat( "\t", $depth ) : '';

            $class_names = $value = '';

            $classes = empty( $item->classes ) ? array() : (array) $item->classes;
            $classes[] = 'menu-item-' . $item->ID;

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

            $id = apply_filters( 'nav_menu_item_id', 'menu-item-'. $item->ID, $item, $args );
            $id = strlen( $id ) ? ' id="' . esc_attr( $id ) . '"' : '';

            $output .= $indent . '<li' . $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        ) .'"' : '';

            $item_output = $args->before;
            $item_output .= '<a'. $attributes .'>';
            $item_output .= $args->link_before . apply_filters( 'the_title', $item->title, $item->ID ) . $args->link_after;
            $item_output .= '</a>';
            $item_output .= $args->after;

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

    }

    /**
     * @see Walker::end_el()
     * @since 3.0.0
     *
     * @param string $output Passed by reference. Used to append additional content.
     * @param object $item Page data object. Not used.
     * @param int $depth Depth of page. Not Used.
     */
    function end_el(&$output, $item, $depth) {
        global $current_branch;
        if($current_branch && $depth > 0) $output .= "</li>\n";
        if($depth == 0) $current_branch = 0;
    }
}