DbUpdateNegotiator.php

<?php

namespace Drupal\system\Theme;

use Drupal\Core\Config\ConfigFactoryInterface;
use Drupal\Core\Extension\ThemeHandlerInterface;
use Drupal\Core\Routing\RouteMatchInterface;
use Drupal\Core\Site\Settings;
use Drupal\Core\Theme\ThemeNegotiatorInterface;

/**
 * Sets the active theme for the database update pages.
 */
class DbUpdateNegotiator implements ThemeNegotiatorInterface {
    
    /**
     * The config factory.
     *
     * @var \Drupal\Core\Config\ConfigFactoryInterface
     */
    protected $configFactory;
    
    /**
     * The theme handler.
     *
     * @var \Drupal\Core\Extension\ThemeHandlerInterface
     */
    protected $themeHandler;
    
    /**
     * Constructs a DbUpdateNegotiator.
     *
     * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
     *   The config factory.
     * @param \Drupal\Core\Extension\ThemeHandlerInterface|null $theme_handler
     *   The theme handler.
     */
    public function __construct(ConfigFactoryInterface $config_factory, ThemeHandlerInterface $theme_handler = NULL) {
        $this->configFactory = $config_factory;
        if ($theme_handler === NULL) {
            @trigger_error('Calling ' . __METHOD__ . '() without the $theme_handler argument is deprecated in drupal:9.4.0 and $theme_handler argument will be required in drupal:10.0.0. See https://www.drupal.org/node/3279699', E_USER_DEPRECATED);
            $theme_handler = \Drupal::service('theme_handler');
        }
        $this->themeHandler = $theme_handler;
    }
    
    /**
     * {@inheritdoc}
     */
    public function applies(RouteMatchInterface $route_match) {
        return $route_match->getRouteName() == 'system.db_update';
    }
    
    /**
     * {@inheritdoc}
     */
    public function determineActiveTheme(RouteMatchInterface $route_match) {
        $custom_theme = Settings::get('maintenance_theme');
        if (!$custom_theme) {
            $custom_theme = $this->themeHandler
                ->themeExists('claro') ? 'claro' : 'seven';
        }
        return $custom_theme;
    }

}