我试着把我的逻辑变成这样：

内部调用以下代码

function start_el( &$output, $item, $depth, $args ) {
$this->sub_items = 0; // declared variable as private $sub_items = 0 above inside class 
            if ( ( $locations = get_nav_menu_locations() ) && isset( $locations[ $args->theme_location ] ) ) {
                $menu = wp_get_nav_menu_object( $locations[ $args->theme_location ] );
                $menu_items = wp_get_nav_menu_items($menu->term_id);
                foreach( $menu_items as $menu_item ) {

                        if( $menu_item->menu_item_parent == $item->ID ) {
                            $this->sub_items++; 
                        }
                }
           } 
}

我想每次$this->sub\\u items返回一级child的计算值时，我都可以在里面访问它

function start_lvl( &$output, $depth ) {
     echo $this->sub_items;
}

但不幸的是，它不能正确输出：(