attachment_repository.dart

abstract class AttachmentRepository {
  Future<Either<Failure, List<Attachment>>> getData();
}

class AttachmentRepositoryImpl implements AttachmentRepository {
  AttachmentRepositoryImpl({
    required this.remoteDataSource,
  });

  final AttachmentRemoteDataSource remoteDataSource;

  @override
  Future<Either<Failure, List<Attachment>>> getData() async {
    try {
      final result = await remoteDataSource.getData();
      return right(result.toDomainList);
    } on RestApiException {
      return left(
        const Failure.serverError(),
      );
    }
  }
}

extension DTOListToDomainList on List<AttachmentDTO> {
  List<Attachment> get toDomainList => map((e) => e.toDomain()).toList();
}

failure.dart

class Failure {
  const factory Failure.serverError() = Failure();
  const factory Failure.anotherFailure() = Failure();
}