class FieldFile

class FieldFile extends ProcessPluginBase implements ContainerFactoryPluginInterface {
  
  /**
   * The current migration.
   */
  protected MigrationInterface $migration;
  
  /**
   * The migrate lookup service.
   *
   * @var \Drupal\migrate\MigrateLookupInterface
   */
  protected $migrateLookup;
  
  /**
   * Constructs a FieldFile plugin instance.
   *
   * @param array $configuration
   *   The plugin configuration.
   * @param string $plugin_id
   *   The plugin ID.
   * @param mixed $plugin_definition
   *   The plugin definition.
   * @param \Drupal\migrate\Plugin\MigrationInterface $migration
   *   The current migration.
   * @param \Drupal\migrate\MigrateLookupInterface $migrate_lookup
   *   The migrate lookup service.
   */
  public function __construct(array $configuration, $plugin_id, $plugin_definition, MigrationInterface $migration, MigrateLookupInterface $migrate_lookup) {
    parent::__construct($configuration, $plugin_id, $plugin_definition);
    $this->migration = $migration;
    $this->migrateLookup = $migrate_lookup;
  }
  
  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition, ?MigrationInterface $migration = NULL) {
    return new static($configuration, $plugin_id, $plugin_definition, $migration, $container->get('migrate.lookup'));
  }
  
  /**
   * {@inheritdoc}
   */
  public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
    $options = unserialize($value['data']);
    // Try to look up the ID of the migrated file. If one cannot be found, it
    // means the file referenced by the current field item did not migrate for
    // some reason -- file migration is notoriously brittle -- and we do NOT
    // want to send invalid file references into the field system (it causes
    // fatal errors), so return an empty item instead.
    $lookup_result = $this->migrateLookup
      ->lookup('d6_file', [
      $value['fid'],
    ]);
    if ($lookup_result) {
      return [
        'target_id' => $lookup_result[0]['fid'],
        'display' => $value['list'],
        'description' => $options['description'] ?? '',
        'alt' => $options['alt'] ?? '',
        'title' => $options['title'] ?? '',
      ];
    }
    else {
      return [];
    }
  }

}