navigation-menu-default.html

{{#has_container}}
  {{#if has_container_id_class}}
<{{container}} id="{{container_id}}" class="{{container_class}}">
  {{else if has_container_id}}
<{{container}} id="{{container_id}}">
  {{else if has_container_class}}
<{{container}} class="{{container_class}}">
  {{else}}
<{{container}}>
  {{/if}}
{{/has_container}}
{{#if has_menu_id_class}}
<ul id="{{menu_id}}" class="{{menu_class}}">
{{else if has_menu_id}}
<ul id="{{menu_id}}">
{{else if has_menu_class}}
<ul class="{{menu_class}}">
{{else}}
<ul>
{{/if}}
  <li class="nav-item"><a class="nav-link btn btn-lg btn-error" href="{{wp_admin_url path='nav-menus.php'}}">Create or Assign <strong><em>{{menu_theme_location}}</em></strong></a></li>
</ul>
{{#has_container}}
</{{container}}>
{{/has_container}}

navigation-menu-element.html

<li class="nav-item {{classes}}">
    <a class="nav-link" href="{{url}}"{{# has_target}} target="{{target}}"{{/has_target}}{{# has_attr_title}}{{attr_title}}{{/ has_attr_title}}>{{title}}</a>
</li>

NavMenuWalker.php

<?php

namespace Dreamery\WP;

/* PHP */
use Exception;

/* WP Global */
use Walker;
use Dreamery\Template;

class NavMenuWalker extends Walker {
    public $tree_type = array('post_type', 'taxonomy', 'custom');

    /**
     * Database fields to use, same as the default WordPress Walker_Nav_Menu class
     */
    public $db_fields = array('parent' => 'menu_item_parent', 'id' => 'db_id');


    private $newLevel = null;

    public function start_lvl(&$output, $depth = 0, $args = array()) {
echo 'start_lvl()<br>';
        $this->newLevel = true;
        $this->elements = array();
    }

    public function end_lvl(&$output, $depth = 0, $args = array()) {
echo 'end_lvl()<br>';

        /*
         * Render level and all child elements
         */
        /*
                $output .= '<ul>';
                $output .= $this->$elements;
                $output .= '</ul>';
        */
        $this->newLevel = true;

        return $output;
    }

    public function start_el(&$output, $item, $depth = 0, $args = array(), $id = 0) {
echo 'start_el()<br>';
        if ($this->newLevel === null)
            $this->newLevel = true;

        /*
         * Build Element
         */
        $this->newLevel = false;
    }

    public function end_el(&$output, $item, $depth = 0, $args = array()) {
echo 'end_el()<br>';
        $t = new Template();
        $options = array(
            'url' => $item->url,
            'title' => $item->title,
            'target' => $item->target,
            'has_target' => (empty($item->target)) ? false : true,
            'classes' => implode(' ', $item->classes),
            'attr_title' => $item->attr_title,
            'has_attr_title' => (empty($item->attr_title)) ? false : true,
         );
        $output .= $t->render('navigation-menu-element', $options);
    }

    public function walk($elements, $max_depth) {
        $output = parent::walk($elements, $max_depth);
        return $output;
    }

    /*
     *
     * YYY:
     * Does something need to be done with before, after, link_before, link_after, or items_wrap
     * as passed to us in $args?
     */
    public static function fallback($args) {
        if (current_user_can('manage_options')) {
            $options = array_intersect_key($args, array('container' => true, 'container_id' => true,
                'container_class' => true, 'menu_id' => true, 'menu_class' => true));

            $options['has_container'] = (empty($options['container'])) ? false : true;

            $options['has_container_id_class'] = false;
            $options['has_container_id'] = false;
            $options['has_container_class'] = false;
            if (!empty($options['container_id']) && !empty($options['container_class']))
                $options['has_container_id_class'] = true;
            if (!empty($options['container_id']))
                $options['has_container_id'] = true;
            if (!empty($options['container_class']))
                $options['has_container_class'] = true;

            $options['has_menu_id_class'] = false;
            $options['has_menu_id'] = false;
            $options['has_menu_class'] = false;
            if (!empty($options['menu_id']) && !empty($options['menu_class']))
                $options['has_menu_id_class'] = true;
            if (!empty($options['menu_id']))
                $options['has_menu_id'] = true;
            if (!empty($options['menu_class']))
                $options['has_menu_class'] = true;

            $menus = get_registered_nav_menus();
            $options['menu_theme_location_ident'] = $args['theme_location'];
            $options['menu_theme_location'] = $menus[$options['menu_theme_location_ident']];

            // get compiler
            $t = new Template();
            echo $t->render('navigation-menu-default', $options);
        }
    }
}