class-my-bs-walker-nav-menu

<?php
if ( ! defined( 'ABSPATH' ) ) {
    exit; // Exit if accessed directly.
}

/**
 * Custom walker for WordPress custom menus.
 *
 * @link https://wordpress.stackexchange.com/q/298663/137402
 *
 * @see Walker_Nav_Menu
 */
class My_BS_Walker_Nav_Menu extends Walker_Nav_Menu {
    /**
     * Starts the list before the elements are added.
     *
     * @see Walker_Nav_Menu::start_lvl()
     */
    public function start_lvl( &$output, $depth = 0, $args = array() ) {
        // Default class.
        $classes = array( 'menu-vertical' );

        /**
         * Filters the CSS class(es) applied to a menu list element.
         */
        $class_names = join( ' ', apply_filters( 'nav_menu_submenu_css_class', $classes, $args, $depth ) );
        $class_names = $class_names ? ' class="' . esc_attr( $class_names ) . '"' : '';

        $output .= '' .
            '<div class="dropdown__container">' .
                '<div class="container">' .
                    '<div class="row">' .
                        '<div class="dropdown__content col-lg-2">' .
                            "<ul{$class_names}>";
    }

    /**
     * Ends the list of after the elements are added.
     *
     * @see Walker_Nav_Menu::end_lvl()
     */
    public function end_lvl( &$output, $depth = 0, $args = array() ) {
        $output .= '' .
                            '</ul>' . // End .menu-vertical
                        '</div>' .    // End .dropdown__content
                    '</div>' .        // End .row
                '</div>' .            // End .container
            '</div>';                 // End .dropdown__container
    }

    /**
     * Starts the element output.
     *
     * @see Walker_Nav_Menu::start_el()
     */
    public function start_el( &$output, $item, $depth = 0, $args = array(), $id = 0 ) {
        $item->item_spacing = 'discard';

        $classes = empty( $item->classes ) ? array() : (array) $item->classes;
        if ( in_array( 'menu-item-has-children', $classes ) ) {
            $depth2 = intval( $args->depth );
            // Displays a link to the page and not a "dropdown trigger" text.
            if ( $depth2 > 0 && (int) $depth + 1 === $depth2 ) {
                return parent::start_el( $output, $item, $depth, $args, $id );
            }
        } else {
            // Displays a link to the page and not a "dropdown trigger" text.
            return parent::start_el( $output, $item, $depth, $args, $id );
        }

        $classes[] = 'dropdown';
        $classes[] = 'menu-item-' . $item->ID;

        /**
         * Filters the CSS class(es) applied to a menu item's list item element.
         */
        $class_names = join( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item, $args, $depth ) );
        $class_names = $class_names ? ' class="' . esc_attr( $class_names ) . '"' : '';

        $output .= '<li' . $class_names .'>';

        /** 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.
         */
        $title = apply_filters( 'nav_menu_item_title', $title, $item, $args, $depth );

        $item_output = '<span class="dropdown__trigger">';
        $item_output .= $title;
        $item_output .= '</span>';

        $output .= $item_output;
    }

    /**
     * Ends the element output, if needed.
     *
     * @see Walker_Nav_Menu::end_el()
     */
    public function end_el( &$output, $item, $depth = 0, $args = array() ) {
        $output .= '</li>';
    }

} // My_BS_Walker_Nav_Menu