apply_filters( 'nav_menu_link_attributes', array $atts, WP_Post $item, stdClass $args, int $depth )

function nav_menu_link_attributes_fun ($atts, $item, $args) {
  $menu_item_parent = $item->menu_item_parent;
  if ($menu_item_parent == 0) {
    $atts['class'] = 'nav-link';
  }
  return $atts;
}
// 过滤菜单项a元素的class属性
add_filter('nav_menu_link_attributes', 'nav_menu_link_attributes_fun', 10, 3);