虽然我知道Bootstrap nav menu class, 我决定自己写Walker, 因此，我可以更好地控制打印内容，如果需要，我对许可证没有任何问题。
THE PROBLEM
乍一看，我的班级几乎在我想要的所有方面都在努力。只有一个问题：出于可访问性的目的（正如Bootstrap的示例所建议的），我想添加
aria-labelledby 属性设置为包含子元素的下拉菜单。我在这方面遇到了困难，因为我不清楚首先真正执行的是什么函数。我的代码：
这个start_lvl() 功能：
public function start_lvl( &$output, $depth = 0, $args = array() ) {
if ( isset( $args->item_spacing ) && \'discard\' === $args->item_spacing ) {
$t = \'\';
$n = \'\';
} else {
$t = "\\t";
$n = "\\n";
}
$indent = str_repeat( $t, $depth );
$output .= "{$n}{$indent}<ul class=\\"dropdown-menu\\" aria-labelledby=\\"\\">{$n}";
}
The
start_el() 功能（基于原始
Walker_Nav_Menu; 检查接近尾端的地方，查看我在哪里修改了它）
public function start_el( &$output, $item, $depth = 0, $args = array(), $id = 0 ) {
if ( isset( $args->item_spacing ) && \'discard\' === $args->item_spacing ) {
$t = \'\';
$n = \'\';
} else {
$t = "\\t";
$n = "\\n";
}
$indent = ( $depth ) ? str_repeat( $t, $depth ) : \'\';
$classes = empty( $item->classes ) ? array() : (array) $item->classes;
$classes[] = \'menu-item-\' . $item->ID;
/**
* Filters the arguments for a single nav menu item.
*
* @since 4.4.0
*
* @param stdClass $args An object of wp_nav_menu() arguments.
* @param WP_Post $item Menu item data object.
* @param int $depth Depth of menu item. Used for padding.
*/
$args = apply_filters( \'nav_menu_item_args\', $args, $item, $depth );
/**
* Filters the CSS class(es) applied to a menu item\'s list item element.
*
* @since 3.0.0
* @since 4.1.0 The `$depth` parameter was added.
*
* @param array $classes The CSS classes that are applied to the menu item\'s `<li>` element.
* @param WP_Post $item The current menu item.
* @param stdClass $args An object of wp_nav_menu() arguments.
* @param int $depth Depth of menu item. Used for padding.
*/
$class_names = join( \' \', apply_filters( \'nav_menu_css_class\', array_filter( $classes ), $item, $args, $depth ) );
$class_names = $class_names ? \' class="\' . esc_attr( $class_names ) . \'"\' : \'\';
/**
* Filters the ID applied to a menu item\'s list item element.
*
* @since 3.0.1
* @since 4.1.0 The `$depth` parameter was added.
*
* @param string $menu_id The ID that is applied to the menu item\'s `<li>` element.
* @param WP_Post $item The current menu item.
* @param stdClass $args An object of wp_nav_menu() arguments.
* @param int $depth Depth of menu item. Used for padding.
*/
$id = apply_filters( \'nav_menu_item_id\', \'menu-item-\'. $item->ID, $item, $args, $depth );
$id = $id ? \' id="\' . esc_attr( $id ) . \'"\' : \'\';
$output .= $indent . \'<li\' . $id . $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 ) ? $item->url : \'\';
/**
* Filters the HTML attributes applied to a menu item\'s anchor element.
*
* @since 3.6.0
* @since 4.1.0 The `$depth` parameter was added.
*
* @param array $atts {
* The HTML attributes applied to the menu item\'s `<a>` element, empty strings are ignored.
*
* @type string $title Title attribute.
* @type string $target Target attribute.
* @type string $rel The rel attribute.
* @type string $href The href attribute.
* }
* @param WP_Post $item The current menu item.
* @param stdClass $args An object of wp_nav_menu() arguments.
* @param int $depth Depth of menu item. Used for padding.
*/
$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 . \'"\';
}
}
/** This filter is documented in wp-includes/post-template.php */
$title = apply_filters( \'the_title\', $item->title, $item->ID );
/**
* Filters a menu item\'s title.
*
* @since 4.4.0
*
* @param string $title The menu item\'s title.
* @param WP_Post $item The current menu item.
* @param stdClass $args An object of wp_nav_menu() arguments.
* @param int $depth Depth of menu item. Used for padding.
*/
$title = apply_filters( \'nav_menu_item_title\', $title, $item, $args, $depth );
if (array_search(\'menu-item-has-children\', $classes, true)) {
$attributes = \' class="btn btn-default dropdown-toggle" type="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="true" id = "link-\'.$item->ID.\'"\';
$title = $title. \' <span class="caret"></span>\';
}
$item_output = $args->before;
$item_output .= \'<a\'. $attributes .\'>\';
$item_output .= $args->link_before . $title . $args->link_after;
$item_output .= \'</a>\';
$item_output .= $args->after;
/**
* Filters a menu item\'s starting output.
*
* The menu item\'s starting output only includes `$args->before`, the opening `<a>`,
* the menu item\'s title, the closing `</a>`, and `$args->after`. Currently, there is
* no filter for modifying the opening and closing `<li>` for a menu item.
*
* @since 3.0.0
*
* @param string $item_output The menu item\'s starting HTML output.
* @param WP_Post $item Menu item data object.
* @param int $depth Depth of menu item. Used for padding.
* @param stdClass $args An object of wp_nav_menu() arguments.
*/
$output .= apply_filters( \'walker_nav_menu_start_el\', $item_output, $item, $depth, $args );
}
其他功能未触及。事实上，我需要放一些东西
start_el(), 但在里面
start_lvl(). 顺便说一句，据我所知，即使是已知的nav walker类也没有此功能。我如何实现它？非常感谢。
