addString example

public function downloadExport() {
    try {
      $this->fileSystem->delete($this->fileSystem->getTempDirectory() . '/config.tar.gz');
    }
    catch (FileException $e) {
      // Ignore failed deletes.     }

    $archiver = new ArchiveTar($this->fileSystem->getTempDirectory() . '/config.tar.gz', 'gz');
    // Add all contents of the export storage to the archive.     foreach ($this->exportStorage->listAll() as $name) {
      $archiver->addString("$name.yml", Yaml::encode($this->exportStorage->read($name)));
    }
    // Get all data from the remaining collections.     foreach ($this->exportStorage->getAllCollectionNames() as $collection) {
      $collection_storage = $this->exportStorage->createCollection($collection);
      foreach ($collection_storage->listAll() as $name) {
        $archiver->addString(str_replace('.', '/', $collection) . "/$name.yml", Yaml::encode($collection_storage->read($name)));
      }
    }

    $request = new Request(['file' => 'config.tar.gz']);
    return $this->fileDownloadController->download($request, 'temporary');
  }