class FieldFile

class FieldFile extends ProcessPluginBase implements ContainerFactoryPluginInterface {
    
    /**
     * The current migration.
     */
    protected $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
        // fatals), 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 [];
        }
    }

}