从Sqflite获取数据

Question

我正在尝试从Sqflite获取数据，之前我已经成功使用过脚本，但是这次我的脚本不起作用，不同之处在于这次我必须将数据库放入资产文件中，然后我尝试将其移至“数据库”文件夹，然后使用它，

我的问题是第13行：

DbStudentManager._();

每当我取消注释时，就会出现错误，指出DbStudentManager类没有默认构造函数

final DbStudentManager dbmanager = new DbStudentManager();

以上代码在不同的小部件中生成错误

    import 'dart:async';
    import 'dart:io';
    import 'package:path/path.dart';
    import 'package:flutter/services.dart';
    import 'package:sqflite/sqflite.dart';
    import 'package:flutter/foundation.dart';
    
    class DbStudentManager {
      Database _database;
      static final _instance = DbStudentManager._();
      static get instance => _instance;
      Completer<Database> _completer;
      DbStudentManager._();
    
      Future<Database> get database async {
        if (_completer == null) {
          _completer = Completer();
          _openDatabase();
        }
        return _completer.future;
      }
    
      Future _openDatabase() async {
        final databasesPath = await getDatabasesPath();
        final path = join(databasesPath, 'st.db');
        final exists = await databaseExists(path);
        if (!exists) {
          print("Creating new copy from asset");
          try {
            await Directory(dirname(path)).create(recursive: true);
          } catch (_) {}
    
          // Copy from asset
          ByteData data = await rootBundle.load(join("assets", "st.db"));
          List<int> bytes =
              data.buffer.asUint8List(data.offsetInBytes, data.lengthInBytes);
    
          // Write and flush the bytes written
          await File(path).writeAsBytes(bytes, flush: true);
        } else {
          print("Opening existing database");
        }
    
        final _database = await openDatabase(path);
        _completer.complete(database);
    
        Future<List<Reslt>> getStudentList() async {
          await _openDatabase();
          final List<Map<String, dynamic>> maps = await _database
              .rawQuery('SELECT * FROM wshakan ORDER BY id LIMIT 10');
          //await _database.query('localsongs', orderBy: "id DESC");
          return List.generate(maps.length, (i) {
            return Reslt(
              id: maps[i]['id'],
              wsha: maps[i]['wsha'],
              favd: maps[i]['who'],
            );
          });
        }
      }
    
    }
    
    class Reslt {
      int id;
      String wsha;
      int favd;
      Reslt(
          {@required this.wsha,
          @required this.favd,
          this.id});
      Map<String, dynamic> toMap() {
        return {
          'wsha': wsha,
          'mana': mana,
          'favd': favd,
        };
      }
    }