AdminNegotiator.php

<?php

namespace Drupal\user\Theme;

use Drupal\Core\Config\ConfigFactoryInterface;
use Drupal\Core\Entity\EntityTypeManagerInterface;
use Drupal\Core\Routing\AdminContext;
use Drupal\Core\Routing\RouteMatchInterface;
use Drupal\Core\Session\AccountInterface;
use Drupal\Core\Theme\ThemeNegotiatorInterface;

/**
 * Sets the active theme on admin pages.
 */
class AdminNegotiator implements ThemeNegotiatorInterface {
    
    /**
     * The current user.
     *
     * @var \Drupal\Core\Session\AccountInterface
     */
    protected $user;
    
    /**
     * The config factory.
     *
     * @var \Drupal\Core\Config\ConfigFactoryInterface
     */
    protected $configFactory;
    
    /**
     * The entity type manager.
     *
     * @var \Drupal\Core\Entity\EntityTypeManagerInterface
     */
    protected $entityTypeManager;
    
    /**
     * The route admin context to determine whether a route is an admin one.
     *
     * @var \Drupal\Core\Routing\AdminContext
     */
    protected $adminContext;
    
    /**
     * Creates a new AdminNegotiator instance.
     *
     * @param \Drupal\Core\Session\AccountInterface $user
     *   The current user.
     * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
     *   The config factory.
     * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager
     *   The entity type manager.
     * @param \Drupal\Core\Routing\AdminContext $admin_context
     *   The route admin context to determine whether the route is an admin one.
     */
    public function __construct(AccountInterface $user, ConfigFactoryInterface $config_factory, EntityTypeManagerInterface $entity_type_manager, AdminContext $admin_context) {
        $this->user = $user;
        $this->configFactory = $config_factory;
        $this->entityTypeManager = $entity_type_manager;
        $this->adminContext = $admin_context;
    }
    
    /**
     * {@inheritdoc}
     */
    public function applies(RouteMatchInterface $route_match) {
        return $this->entityTypeManager
            ->hasHandler('user_role', 'storage') && $this->user
            ->hasPermission('view the administration theme') && $this->adminContext
            ->isAdminRoute($route_match->getRouteObject());
    }
    
    /**
     * {@inheritdoc}
     */
    public function determineActiveTheme(RouteMatchInterface $route_match) {
        return $this->configFactory
            ->get('system.theme')
            ->get('admin') ?: NULL;
    }

}