class ThemeNegotiator

class ThemeNegotiator implements ThemeNegotiatorInterface {
    
    /**
     * Holds an array of theme negotiator IDs, sorted by priority.
     *
     * @var string[]
     */
    protected $negotiators = [];
    
    /**
     * The access checker for themes.
     *
     * @var \Drupal\Core\Theme\ThemeAccessCheck
     */
    protected $themeAccess;
    
    /**
     * The class resolver.
     *
     * @var \Drupal\Core\DependencyInjection\ClassResolverInterface
     */
    protected $classResolver;
    
    /**
     * Constructs a new ThemeNegotiator.
     *
     * @param \Drupal\Core\Theme\ThemeAccessCheck $theme_access
     *   The access checker for themes.
     * @param \Drupal\Core\DependencyInjection\ClassResolverInterface $class_resolver
     *   The class resolver.
     * @param string[] $negotiators
     *   An array of negotiator IDs.
     */
    public function __construct(ThemeAccessCheck $theme_access, ClassResolverInterface $class_resolver, array $negotiators) {
        $this->themeAccess = $theme_access;
        $this->negotiators = $negotiators;
        $this->classResolver = $class_resolver;
    }
    
    /**
     * {@inheritdoc}
     */
    public function applies(RouteMatchInterface $route_match) {
        return TRUE;
    }
    
    /**
     * {@inheritdoc}
     */
    public function determineActiveTheme(RouteMatchInterface $route_match) {
        foreach ($this->negotiators as $negotiator_id) {
            $negotiator = $this->classResolver
                ->getInstanceFromDefinition($negotiator_id);
            if ($negotiator->applies($route_match)) {
                $theme = $negotiator->determineActiveTheme($route_match);
                if ($theme !== NULL && $this->themeAccess
                    ->checkAccess($theme)) {
                    return $theme;
                }
            }
        }
    }

}