class FieldFile

class FieldFile extends ProcessPluginBase implements ContainerFactoryPluginInterface {
    
    /**
     * The migration process plugin, configured for lookups in d6_file.
     *
     * @var \Drupal\migrate\Plugin\MigrateProcessInterface
     *
     * @deprecated in drupal:8.8.x and is removed from drupal:9.0.0. Use
     *   the migrate.lookup service instead.
     *
     * @see https://www.drupal.org/node/3047268
     */
    protected $migrationPlugin;
    
    /**
     * 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.
     */
    // @codingStandardsIgnoreLine
    public function __construct(array $configuration, $plugin_id, $plugin_definition, MigrationInterface $migration, $migrate_lookup) {
        parent::__construct($configuration, $plugin_id, $plugin_definition);
        if ($migrate_lookup instanceof MigrateProcessInterface) {
            @trigger_error('Passing a migration process plugin as the fourth argument to ' . __METHOD__ . ' is deprecated in drupal:8.8.0 and will throw an error in drupal:9.0.0. Pass the migrate.lookup service instead. See https://www.drupal.org/node/3047268', E_USER_DEPRECATED);
            $this->migrationPlugin = $migrate_lookup;
            $migrate_lookup = \Drupal::service('migrate.lookup');
        }
        elseif (!$migrate_lookup instanceof MigrateLookupInterface) {
            throw new \InvalidArgumentException("The fifth argument to " . __METHOD__ . " must be an instance of MigrateLookupInterface.");
        }
        $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.
        // This BC layer is included because if the plugin constructor was called
        // in the legacy way with a migration_lookup process plugin, it may have
        // been preconfigured with a different migration to look up against. While
        // this is unlikely, for maximum BC we will continue to use the plugin to do
        // the lookup if it is provided, and support for this will be removed in
        // Drupal 9.
        if ($this->migrationPlugin) {
            $fid = $this->migrationPlugin
                ->transform($value['fid'], $migrate_executable, $row, $destination_property);
        }
        else {
            $lookup_result = $this->migrateLookup
                ->lookup('d6_file', [
                $value['fid'],
            ]);
            if ($lookup_result) {
                $fid = $lookup_result[0]['fid'];
            }
        }
        if (!empty($fid)) {
            return [
                'target_id' => $fid,
                'display' => $value['list'],
                'description' => isset($options['description']) ? $options['description'] : '',
                'alt' => isset($options['alt']) ? $options['alt'] : '',
                'title' => isset($options['title']) ? $options['title'] : '',
            ];
        }
        else {
            return [];
        }
    }

}