通过Wordpress Walker类创建下拉菜单

Question

所以我有一个侧边栏菜单。但是它不显示下拉菜单。相反，子类别显示在主类别的正下方。我是js和Wordpress的新手。

这是我自定义的Walker课程：

    <?php

/**
 * Location:
 *  wp-includes/nav-menu-template.php
 *  source: Walker_Nav_Menu
 */
class Crb_Main_Menu_Walker extends Walker_Nav_Menu {
    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;

        $args = apply_filters( 'nav_menu_item_args', $args, $item, $depth );

        $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 : '';
        //
        $item_img = carbon_get_post_meta( $item->ID, 'img' );
        $item_desc = carbon_get_post_meta( $item->ID, 'desc' );
        // --- END ---

        $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 . '"';
            }
        }

        $title = apply_filters( 'the_title', $item->title, $item->ID );
        $title = apply_filters( 'nav_menu_item_title', $title, $item, $args, $depth );

        $item_output = $args->before;
        $item_output .= '<a' . $attributes . '><figure>' . wp_get_attachment_image( $item_img, 'full' ) . '</figure><span>';
        $item_output .= $args->link_before . $title . $args->link_after;
        $item_output .= '</span><small>' . $item_desc .'</small></a>';
        $item_output .= $args->after;

        $output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );
    }
} // Walker_Nav_Menu

这是在主页上进行的课堂通话：

<?php wp_nav_menu( array(
                        'theme_location' => 'menu_3',
                        'menu_class'     => 'ul-reset',
                        'walker'         => new Crb_Main_Menu_Walker(),
                    ) ); ?>

请帮助我。如何制作下拉菜单？