我有一个函数wp_nav_menu 显示当前查看页面的同级页面以及所有子页面。我需要对此进行修改，以仅显示特定页面的子页面，但我真的不明白这是如何工作的。我想做的是在wp_nav_menu 函数，并仅显示该页的子级。

这是我当前的函数：。

// add sub_menu options to wp_nav_menu
function my_wp_nav_menu_objects_sub_menu($sorted_menu_items, $args) {
    if (isset($args->sub_menu)) {
        $root_id = 0;
        foreach ($sorted_menu_items as $menu_item) {
            if ($menu_item->current) {
                $root_id = ($menu_item->menu_item_parent) ? $menu_item->menu_item_parent : $menu_item->ID;
                break;
            }
        }
        if (!isset($args->direct_parent)) {
            $prev_root_id = $root_id;
            while ($prev_root_id != 0) {
                foreach ($sorted_menu_items as $menu_item) {
                    if ($menu_item->ID == $prev_root_id) {
                        $prev_root_id = $menu_item->menu_item_parent;
                        if ($prev_root_id != 0) $root_id = $menu_item->menu_item_parent;
                        break;
                    } 
                }
            }
        }
        $menu_item_parents = array();
        foreach ($sorted_menu_items as $key => $item) {
            if ($item->ID == $root_id) $menu_item_parents[] = $item->ID;

            if (in_array($item->menu_item_parent, $menu_item_parents)) {
                $menu_item_parents[] = $item->ID;
            } else if (!( isset($args->show_parent) && in_array($item->ID, $menu_item_parents))) {
                unset( $sorted_menu_items[$key]);
            }
        }
        return $sorted_menu_items;
    } else {
        return $sorted_menu_items;
    }
}
add_filter("wp_nav_menu_objects", "my_wp_nav_menu_objects_sub_menu", 10, 2);

wp_nav_menu(array(
    "container"      => "nav",
    "depth"          => 2,
    "items_wrap"     => "<ul>%3\\$s</ul>",
    "sub_menu"       => true,
    "theme_location" => "primary",
));