Walker_Nav_Menu

17class Walker_Nav_Menu extends Walker {18	/**19	 * What the class handles.20	 *21	 * @since 3.0.022	 * @var string23	 *24	 * @see Walker::$tree_type25	 */26	public $tree_type = array( 'post_type', 'taxonomy', 'custom' );27 28	/**29	 * Database fields to use.30	 *31	 * @since 3.0.032	 * @todo Decouple this.33	 * @var string[]34	 *35	 * @see Walker::$db_fields36	 */37	public $db_fields = array(38		'parent' => 'menu_item_parent',39		'id'     => 'db_id',40	);41 42	/**43	 * The URL to the privacy policy page.44	 *45	 * @since 6.8.046	 * @var string47	 */48	private $privacy_policy_url;49 50	/**51	 * Constructor.52	 *53	 * @since 6.8.054	 */55	public function __construct() {56		$this->privacy_policy_url = get_privacy_policy_url();57	}58 59	/**60	 * Starts the list before the elements are added.61	 *62	 * @since 3.0.063	 *64	 * @see Walker::start_lvl()65	 *66	 * @param string   $output Used to append additional content (passed by reference).67	 * @param int      $depth  Depth of menu item. Used for padding.68	 * @param stdClass $args   An object of wp_nav_menu() arguments.69	 */70	public function start_lvl( &$output, $depth = 0, $args = null ) {71		if ( isset( $args->item_spacing ) && 'discard' === $args->item_spacing ) {72			$t = '';73			$n = '';74		} else {75			$t = "\t";76			$n = "\n";77		}78		$indent = str_repeat( $t, $depth );79 80		// Default class.81		$classes = array( 'sub-menu' );82 83		/**84		 * Filters the CSS class(es) applied to a menu list element.85		 *86		 * @since 4.8.087		 *88		 * @param string[] $classes Array of the CSS classes that are applied to the menu `<ul>` element.89		 * @param stdClass $args    An object of `wp_nav_menu()` arguments.90		 * @param int      $depth   Depth of menu item. Used for padding.91		 */92		$class_names = implode( ' ', apply_filters( 'nav_menu_submenu_css_class', $classes, $args, $depth ) );93 94		$atts          = array();95		$atts['class'] = ! empty( $class_names ) ? $class_names : '';96