Bootstrap 4 WordPress Nav Walker

BootstrapNavWalker.php

<?php

/**
 * A custom WordPress nav walker class to implement the Bootstrap 4 navigation style in a custom theme using the WordPress built in menu manager.
 * Edited from orignal version of Edward McIntyre: https://github.com/twittem/wp-bootstrap-navwalker
 *
 * @author Eduard Milushi
 */
class BootstrapNavWalker extends Walker_Nav_Menu
{
    /**
     * @param string $output Passed by reference. Used to append additional content.
     * @param int    $depth  Depth of page. Used for padding.
     * @param object $args
     *
     * @since 3.0.0
     *
     * @see   Walker::start_lvl()
     */
    public function start_lvl(&$output, $depth = 0, $args = null): void
    {
        $indent = str_repeat("\t", $depth);
        $output .= "\n$indent<ul role=\"menu\" class=\" dropdown-menu\">\n";
    }

    /**
     * @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 object $args
     *
     * @param int    $id
     *
     * @since 3.0.0
     *
     * @see   Walker::start_el()
     */
    public function start_el(&$output, $item, $depth = 0, $args = null, $id = null): void
    {
        $indent = $depth ? str_repeat("\t", $depth) : '';
        /**
         * Dividers, Headers or Disabled
         * =============================
         * Determine whether the item is a Divider, Header, Disabled or regular
         * menu item. To prevent errors we use the strcasecmp() function to so a
         * comparison that is not case sensitive. The strcasecmp() function returns
         * a 0 if the strings are equal.
         */
        if ($depth === 1) {

            if (strcasecmp($item->attr_title, 'divider') === 0) {
                $output .= $indent.'<li role="presentation" class="divider">';
            } elseif (strcasecmp($item->title, 'divider') === 0) {
                $output .= $indent.'<li role="presentation" class="divider">';
            } elseif (strcasecmp($item->attr_title, 'dropdown-header') === 0) {
                $output .= $indent.'<li role="presentation" class="dropdown-header">'.esc_attr($item->title);
            }

        }

        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)$item->classes;
            $classes[]   = 'nav-item menu-item-'.$item->ID;
            $class_names = implode(' ', apply_filters('nav_menu_css_class', array_filter($classes), $item, $args));

            if ($args->has_children) {
                $class_names .= ' dropdown';
            }

            if (in_array('current-menu-item', $classes, true)) {
                $class_names .= ' active';
            }

            $class_names    = $class_names ? ' class="'.esc_attr($class_names).'"' : '';
            $item_id        = apply_filters('nav_menu_item_id', 'menu-item-'.$item->ID, $item, $args);
            $item_id        = $item_id ? ' id="'.esc_attr($item_id).'"' : '';
            $output         .= $indent.'<li'.$item_id.$value.$class_names.'>';
            $atts           = [];
            $atts['title']  = ! empty($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 nav-link';
                $atts['aria-haspopup'] = 'true';
            } else {
                $atts['class'] = 'nav-link';
                $atts['href']  = ! empty($item->url) ? $item->url : '';
            }

            $atts       = apply_filters('nav_menu_link_attributes', $atts, $item, $args);
            $attributes = '';
            foreach ($atts as $attr => $value) {
                if ( ! empty($value)) {
                    $value      = ($attr === 'href') ? 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 .= '</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.
     *
     * @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.
     * @since 2.5.0
     *
     * @see   Walker::start_el()
     */
    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 menu 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, EXTR_OVERWRITE);
            $fb_output = null;
            if ($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;
        }
    }
}