Flutter sqLite-选择一个值并将其传递给另一个文件中的变量

Question

我想从sqlite数据库表中选择索引，然后将值传递/分配给变量。我有一个model.dart文件，其中包含index的声明，从DatabaseHelper.dart文件中的数据库中获取并将其传递到Selection.dart文件中。

Model.dart

class Selected {
  int id;
  int index;

  Selected();
}

我已经设置好databaseHelper.dart

DatabaseHelper.dart

这是我要从中调用其值的文件。

class DatabaseHelper {
  static DatabaseHelper _databaseHelper; // Singleton databaseHelper
  static Database _database; // Singleton database

  // Named constructor to create instance of the DatabaseHelper
  DatabaseHelper._createInstance();

  factory DatabaseHelper() {
    if (_databaseHelper == null) {
      _databaseHelper = DatabaseHelper
          ._createInstance(); // this is executed only once , singleton object
    }

    return _databaseHelper;
  }

  // Declarations
  String selectedTable = 'selected__table';
  String colID = 'id';
  String colIndex = 'index';

  // Getting the database
  Future<Database> get database async {
    if (_database == null) {
      // Checking if the database is empty
      _database =
          await initializeDatabase(); // if database is empty call the database initialization function initializeDatabase()
    }

    return _database;
  }

  // Function for initializing the database if the database is empty and providing the directory for the database
  Future<Database> initializeDatabase() async {
    // Get the directory path for both android and ios to store database
    Directory directory = await getApplicationDocumentsDirectory();
    String path = join(directory.path, 'selected.db'); // database directory declaration

    var selectedDatabase =
        await openDatabase(path, version: 1, onCreate: _createDB);

    return selectedDatabase;
  }

  // Creating database table
  void _createDB(Database db, int newVersion) async {
    await db.execute('CREATE TABLE $selectedTable($colID INTEGER PRIMARY KEY AUTOINCREMENT, $colIndex INTEGER)');
    insertSelected(db);
  }

  // Fetch operation in the database
  Future getSelected() async {
    Database db = await this.database;
    int id = 1;

    var selectedIndex = db.rawQuery('SELECT index FROM $selectedTable WHERE id = $id');

    return selectedIndex;
  }

}

Selection.dart

在这个文件中，我想从databaseHelper.dart文件返回selectedIndex并将其分配给selectedLanguageIndex