class InitSubscriber

class InitSubscriber implements EventSubscriberInterface {
    
    /**
     * A config object for the advagg_mod configuration.
     *
     * @var \Drupal\Core\Config\Config
     */
    protected $config;
    
    /**
     * An editable config object for the advagg configuration.
     *
     * @var \Drupal\Core\Config\Config
     */
    protected $advaggConfig;
    
    /**
     * Constructs the Subscriber object.
     *
     * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
     *   A config factory for retrieving required config objects.
     */
    public function __construct(ConfigFactoryInterface $config_factory) {
        $this->config = $config_factory->get('advagg.settings');
        $this->advaggConfig = $config_factory->getEditable('advagg.settings');
    }
    
    /**
     * {@inheritdoc}
     */
    public static function getSubscribedEvents() {
        return [
            KernelEvents::REQUEST => [
                'onEvent',
                0,
            ],
        ];
    }
    
    /**
     * Synchronize global_counter variable between sites.
     *
     * Only if using unified_multisite_dir.
     */
    public function onEvent() {
        $dir = rtrim($this->config
            ->get('unified_multisite_dir'), '/');
        if (empty($dir) || !file_exists($dir) || !is_dir($dir)) {
            return;
        }
        $counter_filename = $dir . '/_global_counter';
        $local_counter = advagg_get_global_counter();
        if (!file_exists($counter_filename)) {
            file_unmanaged_save_data($local_counter, $counter_filename, FILE_EXISTS_REPLACE);
        }
        else {
            $shared_counter = (int) file_get_contents($counter_filename);
            if ($shared_counter == $local_counter) {
                // Counters are the same, return.
                return;
            }
            elseif ($shared_counter < $local_counter) {
                // Local counter is higher, update saved file and return.
                ile_unmanaged_save_data($local_counter, $counter_filename, FILE_EXISTS_REPLACE);
                return;
            }
            elseif ($shared_counter > $local_counter) {
                // Shared counter is higher, update local copy and return.
                $this->advaggConfig
                    ->set('global_counter', $shared_counter)
                    ->save();
                return;
            }
        }
    }

}