flutter_db_init.dart

class GrammarManager {
  static const DB_NAME = "grammar.db";

  Database _grammarDb;
  var _path;

  Future<bool> isDBExisting() async {
    final _databasesPath = await getDatabasesPath();
    if (!Directory(_databasesPath).existsSync()) {
      Directory(_databasesPath).createSync();
    }

    _path = join(_databasesPath, DB_NAME);
    return await File(_path).exists();
  }

  Future<Database> initDB() async {
    if (_grammarDb != null) return _grammarDb;

    var exists = await isDBExisting();
    if(!exists) {
      print("Creating new copy from asset");
      ByteData data = await rootBundle.load(join("assets", DB_NAME));
      List<int> bytes = data.buffer.asUint8List(data.offsetInBytes, data.lengthInBytes);
      await File(_path).writeAsBytes(bytes);
    }

    _grammarDb = await openDatabase(_path, readOnly: true);
    return _grammarDb;
  }
}