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Bootstrap Dropdown-Navigationspillen funktionieren nicht mit wp_nav_menu ()

Ich verwende Bootstrap nav Pillen mit Dropdown-Elementen als WordPress-Menü. Das Menü ohne Dropdown-Menü sieht gut aus, aber wenn ich verschachtelte ul als Dropdown-Menü hinzufüge, funktioniert das Menü nicht wie erwartet.

HTML

<nav class="navbar navbar-default pull-right" role="navigation">
    <div class="container-fluid">
        <div class="navbar-header">
            <button type="button" class="navbar-toggle" data-toggle="collapse" data-target="#bs-example-navbar-collapse-1"> <span class="sr-only">Toggle navigation</span> <span class="icon-bar"></span> <span class="icon-bar"></span> <span class="icon-bar"></span> </button>
        </div>
        <div class="collapse navbar-collapse" id="bs-example-navbar-collapse-1">
            <ul class="nav nav-pills head-menu">
                <li class="current"><a href="index.html">Home</a> </li>
                <li><a href="authors.html">Authors</a></li>
                <li><a href="catalogue.html">Catalogue</a></li>
                <li><a class="dropdown-toggle" data-toggle="dropdown" href="#" role="button" aria-expanded="false"> Featured <span class="caret"></span> </a>
                    <ul class="dropdown-menu" role="menu">
                        <li><a href="featured-videos.html">Featured Videos</a></li>
                        <li><a href="featured-author.html">Featured Author</a></li>
                    </ul>
                </li>
                <li><a href="media.html">Media</a></li>
                <li><a href="team.html">Team</a></li>
                <li><a href="contact.html">Contact</a></li>
            </ul>
        </div>
    </div>
</nav>

PHP

<nav class="navbar navbar-default pull-right" role="navigation">
    <div class="container-fluid">
        <div class="navbar-header">
            <button type="button" class="navbar-toggle" data-toggle="collapse" data-target="#bs-example-navbar-collapse-1"> <span class="sr-only">Toggle navigation</span> <span class="icon-bar"></span> <span class="icon-bar"></span> <span class="icon-bar"></span> </button>
        </div>
        <div class="collapse navbar-collapse" id="bs-example-navbar-collapse-1">
            <?php wp_nav_menu( array( 'theme_location' => 'main-menu', 'items_wrap' => '<ul class="nav nav-pills head-menu">%3$s</ul>') ); ?>
        </div>
    </div>
</nav>

Alles ist gut, wenn es keine Dropdown-Elemente gibt. Wie kann das Problem behoben werden?

4
DCK

Ich hatte ein ähnliches Problem, als ich ein Bootstrap-Navigationsmenü erstellte und Dropdown-Links gleichzeitig angezeigt wurden. Ich habe die folgenden Schritte ausgeführt, um das Problem zu beheben:

  1. laden Sie den Navwalker von Github Repo herunter: https://github.com/wp-bootstrap/wp-bootstrap-navwalker

  2. lege diese Datei in den Stammordner des Themes und in die Datei functions.php userequire_once get_template_directory() . '/wp-bootstrap-navwalker.php';

  3. zum Schluss müssen Sie folgende Parameter hinzufügen:

    $defaults = array(
      'container' => false,
      'theme_location' => 'primary-menu',
      'menu_class' => 'nav navbar-nav navbar-right',
      'fallback_cb' => 'WP_Bootstrap_Navwalker::fallback',
      'walker' => new WP_Bootstrap_Navwalker(),
    );
    wp_nav_menu($defaults);
    

insbesondere die letzten beiden Parameter für Ihre Funktion wp_nav_menu().

Ich hoffe, es hilft

1
maverick
<?php

/**
 * Bootstrap NavWalker
 * Class Name: Bootstrap_NavWalker
 * Author: Bishal Napit
 * Author URI: https://napitwptech.com/
 * GitHub URI: https://github.com/mebishalnapit/bootstrap-navwalker/
 * Description: A custom WordPress nav walker class to implement the Bootstrap 4 navigation style in a custom WordPress
 * Bootstrap based theme using the WordPress built in menu manager.
 * License: GNU General Public License v3 or later
 * License URI: http://www.gnu.org/licenses/gpl-3.0.html
 */
class Bootstrap_NavWalker extends Walker_Nav_Menu {

    // Create the $current_menu_id_bootstrap variable for generating the current menu id
    protected $current_menu_id_bootstrap;

    /**
     * Starts the list before the elements are added.
     *
     * @since 3.0.0
     * @see   Walker::start_lvl()
     *
     * @param string   $output Passed by reference. Used to append additional content.
     * @param int      $depth  Depth of menu item. Used for padding.
     * @param stdClass $args   An object of wp_nav_menu() arguments.
     */
    public function start_lvl( &$output, $depth = 0, $args = array() ) {
        // Use the current menu id generated via start_el()
        $current_menu_id = $this->current_menu_id_bootstrap;

        // Assign the dynamic id for use inside the dropdown menu, ie, sub-menu for Bootstrap
        $id = 'aria-labelledby="navbar-dropdown-menu-link-' . $current_menu_id->ID . '"';

        if ( isset( $args->item_spacing ) && 'discard' === $args->item_spacing ) {
            $t = '';
            $n = '';
        } else {
            $t = "\t";
            $n = "\n";
        }

        $indent = str_repeat( $t, $depth );

        /**
         * Add the classes for the dropdown menu in WordPress
         *
         * 1. For WordPress default: '.sub-menu'
         * 2. For Bootstrap Sub-Menu: '.dropdown-menu'
         */
        $classes = array( 'sub-menu', 'dropdown-menu' );

        /**
         * Filters the CSS class(es) applied to a menu list element.
         *
         * @since 4.8.0
         *
         * @param array    $classes The CSS classes that are applied to the menu `<ul>` element.
         * @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_submenu_css_class', $classes, $args, $depth ) );
        $class_names = $class_names ? ' class="' . esc_attr( $class_names ) . '"' : '';

        /**
         * Change <ul> to <div> for Bootstrap Navigation
         * Add the current menu id for the sub-menu toggle feature for Bootstrap
         */
        $output .= "{$n}{$indent}<div $class_names $id>{$n}";
    }

    /**
     * Ends the list of after the elements are added.
     *
     * @since 3.0.0
     * @see   Walker::end_lvl()
     *
     * @param string   $output Passed by reference. Used to append additional content.
     * @param int      $depth  Depth of menu item. Used for padding.
     * @param stdClass $args   An object of wp_nav_menu() arguments.
     */
    public function end_lvl( &$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 );

        /**
         * Change </ul> to </div> for Bootstrap Navigation
         */
        $output .= "$indent</div>{$n}";
    }

    /**
     * Starts the element output.
     *
     * @since 3.0.0
     * @since 4.4.0 The {@see 'nav_menu_item_args'} filter was added.
     * @see   Walker::start_el()
     *
     * @param string   $output Passed by reference. Used to append additional content.
     * @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.
     * @param int      $id     Current item ID.
     */
    public function start_el( &$output, $item, $depth = 0, $args = array(), $id = 0 ) {
        // Find the current menu item id to be used for start_lvl()
        $this->current_menu_id_bootstrap = $item;

        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;

        /**
         * Add class '.nav-item' inside <li> tag for Bootstrap
         */
        $classes[] = 'nav-item';

        /**
         * Add class '.active' inside <li> tag for Bootstrap active menu as well as for the parent menu, which have the active sub-menu
         */
        if ( in_array( 'current-menu-item', $classes ) || in_array( 'current-menu-parent', $classes ) ) {
            $classes[] = 'active';
        }

        /**
         * Add class '.dropdown' inside <li> tag for Bootstrap dropdown menu, ie, <li> having sub-menu
         */
        if ( in_array( 'menu-item-has-children', $classes ) ) {
            $classes[] = 'dropdown';
        }

        /**
         * 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 ) . '"' : '';

        /**
         * <li> is required for parent menu only in Bootstrap
         */
        if ( $depth === 0 ) {
            $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 '.nav-link' class for <a> in parent menu for Bootstrap
         */
        if ( $depth === 0 ) {
            $atts['class'] = 'nav-link';
        }

        /**
         * Add the attributes for <a> in parent menu
         *
         * 1. Add '.dropdown-toggle' class for <a> in parent menu if it has sub-menu as required for Bootstrap
         * 2. Add '.dropdown' as 'data-toggle' attribute in <a> in parent menu if it has sub-menu as required for Bootstrap
         * 3. Add the current menu id attribute to indicate the exact menu to toggle for set in sub-menu div
         * 4. Add the attribute of 'true' for 'aria-haspopup' in parent menu to indicate it has sub-menus
         * 5. Add the attribute of 'false' for 'aria-expanded' in parent menu to indicate the sub-menus is hidden by default
         * 6. Add the '#' link in the <a> tag in the parent menu if it has sub-menu as required for Bootstrap
         */
        if ( $depth === 0 && in_array( 'menu-item-has-children', $classes ) ) {
            $atts['class']         .= ' dropdown-toggle';
            $atts['data-toggle']   = 'dropdown';
            $atts['id']            = 'navbar-dropdown-menu-link-' . $item->ID;
            $atts['aria-haspopup'] = "true";
            $atts['aria-expanded'] = "false";
            $atts['href']          = '#';
        }

        /**
         * Add the attributes for <a> in sub-menu
         * 1. Add '.dropdown-item' class for <a> inside sub-menu for Bootstrap
         * 2. Add the current menu id attribute if you want to style the menu differently
         */
        if ( $depth > 0 ) {
            $atts['class'] = 'dropdown-item';
            $atts['id']    = 'menu-item-' . $item->ID;
        }

        /**
         * Add '.active' class inside <a> in sub-menu for Bootstrap
         */
        if ( in_array( 'current-menu-item', $item->classes ) ) {
            $atts['class'] .= ' active';
        }

        /**
         * Add '.disabled' class for <a> in menu for Bootstrap disabled class
         */
        if ( in_array( 'disabled', $item->classes ) ) {
            $atts['class'] .= ' disabled';
        }

        /**
         * 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 );

                /**
                 * If '.disabled' class is added to the menu, add the url of '#' in it
                 */
                if ( in_array( 'disabled', $item->classes ) ) {
                    $value = ( 'href' === $attr ) ? esc_url( '#' ) : 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 );

        $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 );
    }

    /**
     * Ends the element output, if needed.
     *
     * @since 3.0.0
     * @see   Walker::end_el()
     *
     * @param string   $output Passed by reference. Used to append additional content.
     * @param WP_Post  $item   Page data object. Not used.
     * @param int      $depth  Depth of page. Not Used.
     * @param stdClass $args   An object of wp_nav_menu() arguments.
     */
    public function end_el( &$output, $item, $depth = 0, $args = array() ) {
        if ( isset( $args->item_spacing ) && 'discard' === $args->item_spacing ) {
            $t = '';
            $n = '';
        } else {
            $t = "\t";
            $n = "\n";
        }

        /**
         * <li> is required for parent menu only in Bootstrap
         */
        if ( $depth === 0 ) {
            $output .= "</li>{$n}";
        }
    }

    /**
     * Fallback menu
     * If you assign the fallback menu for your custom menu setup via wp_nav_menu function, then, this function will be
     * rendered if no menu is assigned to that menu location. You need to assign it via 'fallback_cb' array. Also, this
     * will be only rendered to the logged in users pointing them to the menu manager url.
     *
     * @param $args Arrays passed from the wp_nav_menu function
     */
    public static function fallback( $args ) {
        if ( current_user_can( 'edit_theme_options' ) ) {
            $container       = $args['container'];
            $container_id    = $args['container_id'];
            $container_class = $args['container_class'];
            $menu_class      = $args['menu_class'];
            $menu_id         = $args['menu_id'];

            // If there is container render it
            if ( $container ) {
                echo '<' . esc_attr( $container );

                // If container id is set render it
                if ( $container_id ) {
                    echo ' id="' . esc_attr( $container_id ) . '"';
                }

                // If container class is set render it
                if ( $container_class ) {
                    echo ' class="' . esc_attr( $container_class ) . '"';
                }

                echo '>';
            }

            // Default wrapper for menu is <ul>
            echo '<ul';

            // If menu id has been set render it
            if ( $menu_id ) {
                echo ' id="' . esc_attr( $menu_id ) . '"';
            }

            // If menu class has been set render it
            if ( $menu_class ) {
                echo ' class="' . esc_attr( $menu_class ) . '"';
            }

            // Close <ul> div wrapper for menu
            echo '>';

            // Display the link to Add New Menu
            echo '<li class="nav-item active"><a class="nav-link" href="' . esc_url( admin_url( 'nav-menus.php' ) ) . '">';
            esc_html_e( 'Add a menu', 'theme-textdomain' );
            echo '</a></li>';

            // Close the main <ul>
            echo '</ul>';

            // Close the main container div
            if ( $container ) {
                echo '</' . esc_attr( $container ) . '>';
            }

        }
    }

}
0
<?php
/**
 * Class Name: wp_bootstrap_navwalker
 * GitHub URI: https://github.com/twittem/wp-bootstrap-navwalker
 * Description: A custom WordPress nav walker class to implement the Bootstrap 3 navigation style in a custom theme using the WordPress built in menu manager.
 * Version: 2.0.4
 * Author: Edward McIntyre - @twittem
 * License: GPL-2.0+
 * License URI: http://www.gnu.org/licenses/gpl-2.0.txt
 */
class wp_bootstrap_navwalker extends Walker_Nav_Menu {
    /**
     * @see Walker::start_lvl()
     * @since 3.0.0
     *
     * @param string $output Passed by reference. Used to append additional content.
     * @param int $depth Depth of page. Used for padding.
     */
    public function start_lvl( &$output, $depth = 0, $args = array() ) {
        $indent = str_repeat( "\t", $depth );
        $output .= "\n$indent<ul role=\"menu\" class=\" category-list\">\n";
    }
    /**
     * @see Walker::start_el()
     * @since 3.0.0
     *
     * @param string $output Passed by reference. Used to append additional content.
     * @param object $item Menu item data object.
     * @param int $depth Depth of menu item. Used for padding.
     * @param int $current_page Menu item ID.
     * @param object $args
     */
    public function start_el( &$output, $item, $depth = 0, $args = array(), $id = 0 ) {
        $indent = ( $depth ) ? str_repeat( "\t", $depth ) : '';
        /**
         * Dividers, Headers or Disabled
         * =============================
         * Determine whether the item is a Divider, Header, Disabled or regular
         * menu item. To prevent errors we use the strcasecmp() function to so a
         * comparison that is not case sensitive. The strcasecmp() function returns
         * a 0 if the strings are equal.
         */
        if ( strcasecmp( $item->attr_title, 'divider' ) == 0 && $depth === 1 ) {
            $output .= $indent . '<li role="presentation" class="divider">';
        } else if ( strcasecmp( $item->title, 'divider') == 0 && $depth === 1 ) {
            $output .= $indent . '<li role="presentation" class="divider">';
        } else if ( strcasecmp( $item->attr_title, 'dropdown-header') == 0 && $depth === 1 ) {
            $output .= $indent . '<li role="presentation" class="dropdown-header">' . esc_attr( $item->title );
        } else if ( strcasecmp($item->attr_title, 'disabled' ) == 0 ) {
            $output .= $indent . '<li role="presentation" class="disabled"><a href="#">' . esc_attr( $item->title ) . '</a>';
        } else {
            $class_names = $value = '';
            $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 ) );
            if ( $args->has_children )
                $class_names .= ' cd-library';
            if ( in_array( 'current-menu-item', $classes ) )
                $class_names .= ' active';
            $class_names = $class_names ? ' class="' . esc_attr( $class_names ) . '"' : '';
            $id = apply_filters( 'nav_menu_item_id', 'menu-item-'. $item->ID, $item, $args );
            $id = $id ? ' id="' . esc_attr( $id ) . '"' : '';
            $output .= $indent . '<li' . $id . $value . $class_names .'>';
            $atts = array();
            $atts['title']  = ! empty( $item->title )   ? $item->title  : '';
            $atts['target'] = ! empty( $item->target )  ? $item->target : '';
            $atts['rel']    = ! empty( $item->xfn )     ? $item->xfn    : '';
            // If item has_children add atts to a.
            if ( $args->has_children && $depth === 0 ) {
                $atts['href']           = '#';
                $atts['data-toggle']    = 'dropdown';
                $atts['class']          = 'dropdown-toggle';
                $atts['aria-haspopup']  = 'true';
            } else {
                $atts['href'] = ! empty( $item->url ) ? $item->url : '';
            }
            $atts = apply_filters( 'nav_menu_link_attributes', $atts, $item, $args );
            $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;
            /*
             * Glyphicons
             * ===========
             * Since the the menu item is NOT a Divider or Header we check the see
             * if there is a value in the attr_title property. If the attr_title
             * property is NOT null we apply it as the class name for the glyphicon.
             */
            if ( ! empty( $item->attr_title ) )
                $item_output .= '<a'. $attributes .'><span class="glyphicon ' . esc_attr( $item->attr_title ) . '"></span>&nbsp;';
            else
                $item_output .= '<a'. $attributes .'>';
            $item_output .= $args->link_before . apply_filters( 'the_title', $item->title, $item->ID ) . $args->link_after;
            $item_output .= ( $args->has_children && 0 === $depth ) ? ' <span class="caret"></span></a>' : '</a>';
            $item_output .= $args->after;
            $output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );
        }
    }
    /**
     * Traverse elements to create list from elements.
     *
     * Display one element if the element doesn't have any children otherwise,
     * display the element and its children. Will only traverse up to the max
     * depth and no ignore elements under that depth.
     *
     * This method shouldn't be called directly, use the walk() method instead.
     *
     * @see Walker::start_el()
     * @since 2.5.0
     *
     * @param object $element Data object
     * @param array $children_elements List of elements to continue traversing.
     * @param int $max_depth Max depth to traverse.
     * @param int $depth Depth of current element.
     * @param array $args
     * @param string $output Passed by reference. Used to append additional content.
     * @return null Null on failure with no changes to parameters.
     */
    public function display_element( $element, &$children_elements, $max_depth, $depth, $args, &$output ) {
        if ( ! $element )
            return;
        $id_field = $this->db_fields['id'];
        // Display this element.
        if ( is_object( $args[0] ) )
           $args[0]->has_children = ! empty( $children_elements[ $element->$id_field ] );
        parent::display_element( $element, $children_elements, $max_depth, $depth, $args, $output );
    }
    /**
     * Menu Fallback
     * =============
     * If this function is assigned to the wp_nav_menu's fallback_cb variable
     * and a manu has not been assigned to the theme location in the WordPress
     * menu manager the function with display nothing to a non-logged in user,
     * and will add a link to the WordPress menu manager if logged in as an admin.
     *
     * @param array $args passed from the wp_nav_menu function.
     *
     */
    public static function fallback( $args ) {
        if ( current_user_can( 'manage_options' ) ) {
            extract( $args );
            $fb_output = null;
            if ( $container ) {
                $fb_output = '<' . $container;
                if ( $container_id )
                    $fb_output .= ' id="' . $container_id . '"';
                if ( $container_class )
                    $fb_output .= ' class="' . $container_class . '"';
                $fb_output .= '>';
            }
            $fb_output .= '<ul';
            if ( $menu_id )
                $fb_output .= ' id="' . $menu_id . '"';
            if ( $menu_class )
                $fb_output .= ' class="' . $menu_class . '"';
            $fb_output .= '>';
            $fb_output .= '<li><a href="' . admin_url( 'nav-menus.php' ) . '">Add a menu</a></li>';
            $fb_output .= '</ul>';
            if ( $container )
                $fb_output .= '</' . $container . '>';
            echo $fb_output;
        }
    }
}

//Use theme
 <?php wp_nav_menu( array(
                'menu'              => 'main-nav',
                'theme_location'    => 'main-nav',
                'depth'             => 2,
                'container'         => false,
                'menu_class'        => 'main-navigation',
                'fallback_cb'       => 'wp_bootstrap_navwalker::fallback',
                'walker'            => new wp_bootstrap_navwalker())
            ); ?>
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