class EventSubscriber

class EventSubscriber implements EventSubscriberInterface {
    
    /**
     * The active config storage.
     *
     * @var \Drupal\Core\Config\StorageInterface
     */
    protected $active;
    
    /**
     * The sync config storage.
     *
     * @var \Drupal\Core\Config\StorageInterface
     */
    protected $sync;
    
    /**
     * The Drupal state.
     *
     * @var \Drupal\Core\State\StateInterface
     */
    protected $state;
    
    /**
     * EventSubscriber constructor.
     *
     * @param \Drupal\Core\Config\StorageInterface $active
     *   The active config storage.
     * @param \Drupal\Core\Config\StorageInterface $sync
     *   The sync config storage.
     * @param \Drupal\Core\State\StateInterface $state
     *   The Drupal state.
     */
    public function __construct(StorageInterface $active, StorageInterface $sync, StateInterface $state) {
        $this->active = $active;
        $this->sync = $sync;
        $this->state = $state;
    }
    
    /**
     * The storage is transformed for importing.
     *
     * In this transformation we ignore the site name from the sync storage and
     * set it always to the currently active site name with an additional string
     * so that there will always be something to import.
     * Do not do this outside of tests.
     *
     * @param \Drupal\Core\Config\StorageTransformEvent $event
     *   The config storage transform event.
     */
    public function onImportTransform(StorageTransformEvent $event) {
        $storage = $event->getStorage();
        $site = $storage->read('system.site');
        // Only change something if the sync storage has data.
        if (!empty($site)) {
            // Add "Arrr" to the site name. Because pirates!
            // The site name which is in the sync directory will be ignored.
            $current = $this->active
                ->read('system.site');
            $site['name'] = $current['name'] . ' Arrr';
            $storage->write('system.site', $site);
        }
    }
    
    /**
     * The storage is transformed for exporting.
     *
     * In this transformation we ignore the site slogan from the site if the sync
     * storage has it. Just export it again with an additional string so that
     * there will always be something new exported.
     * Do not do this outside of tests.
     *
     * @param \Drupal\Core\Config\StorageTransformEvent $event
     *   The config storage transform event.
     */
    public function onExportTransform(StorageTransformEvent $event) {
        $sync = $this->sync
            ->read('system.site');
        // Only change something if the sync storage has data.
        if (!empty($sync)) {
            $storage = $event->getStorage();
            $site = $storage->read('system.site');
            // Add "Arrr" to the site slogan. Because pirates!
            // The active slogan will be ignored.
            $site['slogan'] = $sync['slogan'] . ' Arrr';
            $site['mail'] = $this->state
                ->get('config_transform_test_mail', '');
            $storage->write('system.site', $site);
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public static function getSubscribedEvents() : array {
        $events[ConfigEvents::STORAGE_TRANSFORM_IMPORT][] = [
            'onImportTransform',
        ];
        $events[ConfigEvents::STORAGE_TRANSFORM_EXPORT][] = [
            'onExportTransform',
        ];
        return $events;
    }

}