site_admin_menu.module

<?php

/**
 * @file
 * contains the implementation of hooks invoked by Drupal core.
 */
use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Menu\MenuTreeParameters;
use Drupal\Core\Routing\RouteMatchInterface;
use Drupal\Core\Link;
use Drupal\Core\Url;


/**
 *  Implements hook_toolbar_alter().
 */
function site_admin_menu_toolbar_alter(&$items) {
  $items['administration']['tray']['toolbar_administration']['#pre_render'] = array('site_admin_menu_prerender_toolbar_administration_tray');
}

/**
 * Renders the toolbar's administration tray.
 * This is a clone of core's toolbar_prerender_toolbar_administration_tray()
 * function, which uses setMaxDepth(4) instead of setTopLevelOnly()
 * @param array $element
 *   A renderable array.
 *
 * @return array
 *   The updated renderable array.
 *
 * @see toolbar_prerender_toolbar_administration_tray()
 */
function site_admin_menu_prerender_toolbar_administration_tray(array $element) {
  // machine name of the menu to use for site admins
  $site_admin_menu = 'owner';
  // permission to check against to determine whether to load the system admin menu of the custom site admin menu
  $permission = 'administer content types';
  
  $menu_tree = \Drupal::service('toolbar.menu_tree');
  $parameters = new MenuTreeParameters();
  if(\Drupal::currentUser()->hasPermission($permission)) {
    $parameters->setRoot('system.admin')->excludeRoot()->setMaxDepth(4)->onlyEnabledLinks();
    $tree = $menu_tree->load(NULL, $parameters);
  } else {
    $parameters->setMinDepth(1)->setMaxDepth(4)->onlyEnabledLinks();
    $tree = $menu_tree->load($site_admin_menu, $parameters);
  }
  $manipulators = array(
    array('callable' => 'menu.default_tree_manipulators:checkAccess'),
    array('callable' => 'menu.default_tree_manipulators:generateIndexAndSort'),
    array('callable' => 'site_admin_menu_menu_navigation_links'),
  );
  $tree = $menu_tree->transform($tree, $manipulators);
  $element['administration_menu'] = $menu_tree->build($tree);

  return $element;
}

/**
 * Adds toolbar-specific attributes to the menu link tree.
 *
 * @param \Drupal\Core\Menu\MenuLinkTreeElement[] $tree
 *   The menu link tree to manipulate.
 *
 * @return \Drupal\Core\Menu\MenuLinkTreeElement[]
 *   The manipulated menu link tree.
 */
function site_admin_menu_menu_navigation_links(array $tree) {
  foreach ($tree as $element) {
    if ($element->subtree) {
      site_admin_menu_menu_navigation_links($element->subtree);
    }
    $link = $element->link;

    // Get the non-localized title to make the icon class.
    $definition = $link->getPluginDefinition();

    $element->options['attributes']['class'][] = 'toolbar-icon';
    $element->options['attributes']['class'][] = 'toolbar-icon-' . strtolower(str_replace(array('.', ' ', '_'), array('-', '-', '-'), $definition['id']));
    $element->options['attributes']['class'][] = 'toolbar-admin-icon-' . strtolower(str_replace(array('.', ' ', '_'), array('-', '-', '-'), $link->getTitle()));
    $element->options['attributes']['title'] = $link->getDescription();
  }
  return $tree;
}