class ConfigSubscriber

class ConfigSubscriber implements EventSubscriberInterface {
    
    /**
     * The language manager.
     *
     * @var \Drupal\Core\Language\LanguageManagerInterface
     */
    protected $languageManager;
    
    /**
     * The default language.
     *
     * @var \Drupal\Core\Language\LanguageDefault
     */
    protected $languageDefault;
    
    /**
     * The configuration factory.
     *
     * @var \Drupal\Core\Config\ConfigFactoryInterface
     */
    protected $configFactory;
    
    /**
     * The language negotiator.
     *
     * @var \Drupal\language\LanguageNegotiatorInterface
     */
    protected $languageNegotiator;
    
    /**
     * The language path processor.
     *
     * @var \Drupal\language\HttpKernel\PathProcessorLanguage
     */
    protected $pathProcessorLanguage;
    
    /**
     * Constructs a new class object.
     *
     * @param \Drupal\Core\Language\LanguageManagerInterface $language_manager
     *   The language manager.
     * @param \Drupal\Core\Language\LanguageDefault $language_default
     *   The default language.
     * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
     *   The configuration factory.
     * @param \Drupal\language\LanguageNegotiatorInterface $language_negotiator
     *   The language negotiator.
     */
    public function __construct(LanguageManagerInterface $language_manager, LanguageDefault $language_default, ConfigFactoryInterface $config_factory, LanguageNegotiatorInterface $language_negotiator) {
        $this->languageManager = $language_manager;
        $this->languageDefault = $language_default;
        $this->configFactory = $config_factory;
        $this->languageNegotiator = $language_negotiator;
    }
    
    /**
     * Causes the container to be rebuilt on the next request.
     *
     * This event subscriber assumes that the new default langcode and old default
     * langcode are valid langcodes. If the schema definition of either
     * system.site:default_langcode or language.negotiation::url.prefixes changes
     * then this event must be changed to work with both the old and new schema
     * definition so this event is update safe.
     *
     * @param \Drupal\Core\Config\ConfigCrudEvent $event
     *   The configuration event.
     */
    public function onConfigSave(ConfigCrudEvent $event) {
        $saved_config = $event->getConfig();
        if ($saved_config->getName() == 'system.site' && $event->isChanged('default_langcode')) {
            $new_default_langcode = $saved_config->get('default_langcode');
            $default_language = $this->configFactory
                ->get('language.entity.' . $new_default_langcode);
            // During an import the language might not exist yet.
            if (!$default_language->isNew()) {
                $this->languageDefault
                    ->set(new Language($default_language->get()));
                $this->languageManager
                    ->reset();
                // Directly update language negotiation settings instead of calling
                // language_negotiation_url_prefixes_update() to ensure that the code
                // obeys the hook_update_N() restrictions.
                $negotiation_config = $this->configFactory
                    ->getEditable('language.negotiation');
                $negotiation_changed = FALSE;
                $url_prefixes = $negotiation_config->get('url.prefixes');
                $old_default_langcode = $saved_config->getOriginal('default_langcode');
                if (empty($url_prefixes[$old_default_langcode])) {
                    $negotiation_config->set('url.prefixes.' . $old_default_langcode, $old_default_langcode);
                    $negotiation_changed = TRUE;
                }
                if (empty($url_prefixes[$new_default_langcode])) {
                    $negotiation_config->set('url.prefixes.' . $new_default_langcode, '');
                    $negotiation_changed = TRUE;
                }
                if ($negotiation_changed) {
                    $negotiation_config->save(TRUE);
                }
            }
            // Trigger a container rebuild on the next request by invalidating it.
            ConfigurableLanguageManager::rebuildServices();
        }
        elseif ($saved_config->getName() == 'language.types' && $event->isChanged('negotiation')) {
            // If the negotiation configuration changed the language negotiator and
            // the language path processor have to be reset so that they regenerate
            // the method instances and also sort them accordingly to the new config.
            $this->languageNegotiator
                ->reset();
            if (isset($this->pathProcessorLanguage)) {
                $this->pathProcessorLanguage
                    ->reset();
            }
        }
    }
    
    /**
     * Injects the language path processors on multilingual site configuration.
     *
     * @param \Drupal\language\HttpKernel\PathProcessorLanguage $path_processor_language
     *   The language path processor.
     */
    public function setPathProcessorLanguage(PathProcessorLanguage $path_processor_language) {
        $this->pathProcessorLanguage = $path_processor_language;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function getSubscribedEvents() : array {
        $events[ConfigEvents::SAVE][] = [
            'onConfigSave',
            0,
        ];
        return $events;
    }

}