这让我抓狂。以下是解决方案，希望有人觉得它有用，或者建议一个更好、更短的解决方案：

class sstmyp_Walker_Nav_Menu extends Walker_Nav_Menu {

    function display_element($element, &$children_elements, $max_depth, $depth=0, $args, &$output) {

        $id_field = $this->db_fields[\'id\'];

        // add custom class and carets to menu has children
        if(in_array("menu-item-has-children", $element->classes)) {
            $element->classes[] = \'dropdown\';
            $element->title .= \' <i class="fa fa-angle-down"></i>\';
        }

        // add class active
        if(in_array("current-menu-item", $element->classes)) {
            $element->classes[] = \'active\';
        }

        parent::display_element($element, $children_elements, $max_depth, $depth, $args, $output);
    }       

    function start_lvl(&$output, $depth = 1, $args = Array() ) {
        $indent = str_repeat("\\t", $depth);
        $output .= "\\n$indent<ul class=\\"dropdown-menu\\">\\n"; 
    }

    public function start_el( &$output, $item, $depth = 0, $args = array(), $id = 0 ) {
        $indent = ( $depth ) ? str_repeat( "\\t", $depth ) : \'\';

        $classes = empty( $item->classes ) ? array() : (array) $item->classes;
        $classes[] = \'menu-item-\' . $item->ID;

        $class_names = join( \' \', apply_filters( \'nav_menu_css_class\', array_filter( $classes ), $item, $args, $depth ) );
        $class_names = $class_names ? \' class="\' . esc_attr( $class_names ) . \'"\' : \'\';

        $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        : \'\';
        //add attribute
        if(in_array("dropdown-toggle", $classes)) {
            $atts[\'data-toggle\'] = "dropdown";
            $atts[\'class\'] = "dropdown-toggle";
        }
        $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 . \'"\';
            }
        }

        $item_output = $args->before;
        $item_output .= \'<a\'. $attributes .\'>\';
         // This filter is documented in wp-includes/post-template.php
        $item_output .= $args->link_before . apply_filters( \'the_title\', $item->title, $item->ID ) . $args->link_after;
        $item_output .= \'</a>\';
        $item_output .= $args->after;
        $output .= apply_filters( \'walker_nav_menu_start_el\', $item_output, $item, $depth, $args );
    }       

}