如果菜单有子菜单，如何将<i>项添加到wp_nav_menu中

class Custom_Menu_Walker extends Walker_Nav_Menu {
    function start_lvl(&$output, $depth = 0, $args = array()) {
        $indent = str_repeat("\t", $depth);
        $output .= "\n$indent<ul class=\"sub-menu\">\n";
    }

    function start_el(&$output, $item, $depth = 0, $args = array(), $id = 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) . '"';

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

        $atts = array();
        $atts['title'] = !empty($item->attr_title) ? $item->attr_title : '';
        $atts['target'] = !empty($item->target) ? $item->target : '';
        $atts['rel'] = !empty($item->xfn) ? $item->xfn : '';
        $atts['href'] = !empty($item->url) ? $item->url : '';

        // 添加 <i> 标签到子菜单项
        if ($depth > 0) {
            $atts['class'] = 'menu-item-icon';
        }

        $atts = apply_filters('nav_menu_link_attributes', $atts, $item, $args, $depth);

        $attributes = '';
        foreach ($atts as $attr => $value) {
            if (!empty($value)) {
                $value = ( 'href' === $attr ) ? esc_url($value) : esc_attr($value);
                $attributes .= ' ' . $attr . '="' . $value . '"';
            }
        }

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

        // 在子菜单项前添加 <i> 标签
        if ($depth > 0) {
            $item_output = '<i class="your-icon-class"></i>' . $item_output;
        }

        $item_output .= '</a>';
        $item_output .= $args->after;

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

function custom_nav_menu($args = array()) {
    $args['walker'] = new Custom_Menu_Walker();
    wp_nav_menu($args);
}

custom_nav_menu(array(
    'theme_location' => 'primary',
    'menu_class' => 'nav-menu'
));