MYTHEME.theme.php

<?php

/**
 * Override or insert variables into the page template.
 */
function MYTHEEME_preprocess_page(&$vars, $hook) {

  // Build the main menu links tree.
  if (!empty($vars['main_menu'])) {
    // Build links.
    $config = Drupal::config('menu_ui.settings');
    $menu_enabled = Drupal::moduleHandler()->moduleExists('menu_ui');
    // When menu module is not enabled, we need hardcoded default values.
    $main_links_source = $menu_enabled ? $config->get('main_links') : 'main';
    // Allowed max depth.
    // @see MenuTreeStorage::MAX_DEPTH
    $level = 9;
    $menu_tree = \Drupal::menuTree();
    $parameters = $menu_tree->getCurrentRouteMenuTreeParameters($main_links_source);
    $parameters->setMaxDepth($level);
    $tree = $menu_tree->load($main_links_source, $parameters);
    $manipulators = array(
      array('callable' => 'menu.default_tree_manipulators:checkAccess'),
      array('callable' => 'menu.default_tree_manipulators:generateIndexAndSort'),
    );
    $tree = $menu_tree->transform($tree, $manipulators);

    $main_menu = MYTHEME_build_menu_tree($tee)
    $vars['main_menu'] = $main_menu;
    $vars['main_menu']['#attributes']['id'] = 'main-menu-links';
    $vars['main_menu']['#attributes']['class'] = array('links', 'clearfix');
    $vars['main_menu']['#theme_wrappers'] = array('menu_tree__primary');
  }
  
  function MYTHEME_build_menu_tree(array $tree){  
    $build = array();

    foreach ($tree as $data) {
      $class = array();
      /** @var \Drupal\Core\Menu\MenuLinkInterface $link */
      $link = $data->link;
      // Generally we only deal with visible links, but just in case.
      if (!$link->isEnabled()) {
        continue;
      }
      // Set a class for the <li>-tag. Only set 'expanded' class if the link
      // also has visible children within the current tree.
      if ($data->hasChildren && !empty($data->subtree)) {
        $class[] = 'expanded';
        $class[] = 'li-level-' . $level // @TODO: find a way to get level correct.
      }
      elseif ($data->hasChildren) {
        $class[] = 'collapsed';
        $class[] = 'ul-level-' . $level // @TODO: find a way to get level correct.
      }
      else {
        $class[] = 'leaf';
        $class[] = 'li-level-' . $level // @TODO: find a way to get level correct.
      }
      // Set a class if the link is in the active trail.
      if ($data->inActiveTrail) {
        $class[] = 'active-trail';
      }

      // Allow menu-specific theme overrides.
      $element['#theme'] = 'menu_link__' . strtr($link->getMenuName(), '-', '_');
      $element['#attributes']['class'] = $class;
      $element['#title'] = $link->getTitle();
      $element['#url'] = $link->getUrlObject();
      $element['#below'] = $data->subtree ? $this->build($data->subtree) : array();
      if (isset($data->options)) {
        $element['#url']->setOptions(NestedArray::mergeDeep($element['#url']->getOptions(), $data->options));
      }
      $element['#original_link'] = $link;
      // Index using the link's unique ID.
      $build[$link->getPluginId()] = $element;
    }
    if ($build) {
      // Make sure drupal_render() does not re-order the links.
      $build['#sorted'] = TRUE;
      // Get the menu name from the last link.
      $menu_name = $link->getMenuName();
      // Add the theme wrapper for outer markup.
      // Allow menu-specific theme overrides.
      $build['#theme_wrappers'][] = 'menu_tree__' . strtr($menu_name, '-', '_');
      // Set cache tag.
      $build['#cache']['tags']['menu'][$menu_name] = $menu_name;
    }

        return $build;    
  }