SQLite连接在Flutter中返回null

2 回答

• 0

  如果有帮助，这是我的工作代码 . 注意我在db中使用异步getter并在方法中使用db时等待 . 害怕我也是一个颤抖的新手 .

  注意我通过Future与DatabaseHelper交互 . 请参阅第二个代码块 . 我认为这需要使用FutureBuilder . 我怀疑你需要使用它来处理（略微）缓慢加载异步数据库调用 .

  class DatabaseHelper {
    final String tableName = "Gear";
  
    static Database _db;
  
    Future<Database> get db async {
      if (_db != null) return _db;
      _db = await initDb();
      await importData();
      return _db;
    }
  
    initDb() async {
      // Get a location using path_provider
      var databasesPath = await getDatabasesPath();
      String path = join(databasesPath, "gear_log.db");
  
      await deleteDatabase(path);
      var theDb = await openDatabase(path, version: 1,
          onCreate: (Database db, int version) async {
        print('creating db...');
  
        String sql = await rootBundle.loadString('assets/db/schema.txt');
        for (var s in sql.split(";")) {
          if (s.length > 5) {
            // catching any hidden characters at end of schema
            await db.execute(s + ';');
          }
        }
        // When creating the db, create the table
      });
  
      return theDb;
    }
  
    importData() async {
      final datasources =
          'observation_bodyissue.json,observation_observation.json,observation_observation_bodyIssues.json,observation_observation_shoeIssues.json,observation_shoeissue.json,observation_gearissue.json,runner_runner.json,gear_actualpair.json,gear_characteristic.json,gear_shoe.json,gear_family.json,gear_maker.json,gear_gear.json,gear_gear_characteristics.json,users_user.json'
              .split(',');
      var batch = _db.batch();
  
      for (var datasource in datasources) {
        try {
          String str = await rootBundle.loadString('assets/db/data/$datasource');
          String table = datasource.split('.')[0];
  
          if (str.length > 0 && str != '[]') {
            List<dynamic> _list = await json.decode(str);
            for (var _json in _list) {
              batch.insert(table, _json);
            }
          }
        } catch (e) {
          print(e.toString());
        }
      }
      print('added db data');
      var results = await batch.commit();
      //print(results);
      //print('imported data');
    }
  
    Future<List<Item>> getItems() async {
      var dbClient = await db;
  
      List<Map> list = await dbClient.rawQuery('SELECT *'
          'FROM "gear_actualpair" '
          'DESC, "gear_actualpair"."created" DESC');
  
      //prob need to specify 'first X' at sometime
  
      List<Item> items = new List();
      for (int i = 0; i < list.length; i++) {
        Item item = Item.fromMap(list[i]);
        if (list[i]['selected'] == 1) {
          item.observations = await getObservations(list[i]['id']);
        }
        items.add(item);
      }
  
      return items;
    }
  }
  

  与dbHelper类交互：

  Future<List<Item>> fetchItemsFromDatabase() async {
    var dbHelper = DatabaseHelper();
    Future<List<Item>> items = dbHelper.getItems();
    return items;
  }
  

  回复于 2024-05-02T22:58:12+08:00
• 1

  我有这样的共同抽象提供者：

  abstract class DbProvider<T extends DbItem> {
    static Future<String> get localPath async {
      final directory = await getApplicationDocumentsDirectory();
      return directory.path;
    }
  
    Database db;
  
    Future open() async {
      String path = await localPath;
      db = await openDatabase(join(path, file_name), version: 1, onCreate: (database, version) async {
        await database.execute(''' create table */code for creating table/* ''');
      });
    }
  
    Future _update(T item) async {
      return await db.update(getTableName(), item.toMap(), where: "$col_dbId = ?", whereArgs: [item.dbId]);
    }
  
    Future _insert(T item) async {
      return await db.insert(getTableName(), item.toMap());
    }
  
    Future delete(String dbId) async {
      return await db.delete(getTableName(), where: "$col_dbId = ?", whereArgs: [dbId]);
    }
  
    Future close() => db.close();
  
    Future remove() => db.delete(getTableName());
  
    String getTableName();
  
    Future save(T item);
  
    Future<Iterable<T>> get();
  }
  

  并做一些交易：

  void loadDbData() async {
    FavoriteProvider favoriteProvider = FavoriteProvider();
    await favoriteProvider.open();
    Iterable<DbItem> favorites = await favoriteProvider.getConferences();
    await favoriteProvider.close();
    ...
  }
  

  回复于 2024-05-02T22:58:12+08:00