private function whenArticleTitleChanged(ArticleTitleChanged $event)
 {
     $this->articles[(string) $event->articleId()]->title = $event->title();
 }
 private function whenArticleTitleChanged($data, ArticleTitleChanged $event)
 {
     $data[(string) $event->articleId()]['title'] = $event->title();
     return $data;
 }