code: public function process(ActionInterface $action, EntryCollection $collection): void
      {
          $post = $action->getComponent('directComplement')->getData();
          $parent = $post->getParent();
  
          // Spread to authors of posts in the thread.
          $posts = array_merge([$parent], $parent->getChildren()->toArray());
          $done = [];
          foreach ($posts as $check) {
              // Load the author, spread if they have not been spread to yet.
              $author = $this->user_provider->loadUserByUsername($check->getAuthor());
              if (!$author instanceof UserInterface
                  || isset($done[$author->getUsername()])) {
                  continue;
              }
  
              $done[$author->getUsername()] = true;
              $collection->add(new EntryUnaware(get_class($author), $author->getId()));
          }
      }
docs: {@inheritdoc}
  
  @param ActionInterface $action     The action to spread.
  @param EntryCollection $collection The current spreads.
  
  @return void