flutter - I/flutter (29011) : NoSuchMethodError: The method 'insert' was called on null. SQFLITE flutter

Question

当我从 api 获取数据时，我从 ui 调用数据库帮助程序类，并且我必须将这些数据转储到数据库中以在应用程序中脱机使用它。我收到这个错误。我在哪里收到此错误以及为什么数据库未初始化并允许我对其执行操作。
错误:

I/flutter ( 5420): NoSuchMethodError: The method 'insert' was called on null.
I/flutter ( 5420): Receiver: null
I/flutter ( 5420): Tried calling: insert("Deliveries", _LinkedHashMap len:6, conflictAlgorithm: Instance of 'ConflictAlgorithm')
E/flutter ( 5420): [ERROR:flutter/lib/ui/ui_dart_state.cc(157)] Unhandled Exception: NoSuchMethodError: The method 'query' was called on null.
E/flutter ( 5420): Receiver: null
E/flutter ( 5420): Tried calling: query("Deliveries")
E/flutter ( 5420): #0      Object.noSuchMethod (dart:core-patch/object_patch.dart:53:5)
E/flutter ( 5420): #1      TodoHelper.getAllTask (package:delivery_app_odoo_flutter/utils/db_todays_deliveries_model.dart:110:55)
E/flutter ( 5420): #2      _TodayDeliveriesState.inertInDb (package:delivery_app_odoo_flutter/ui/deliveries_page/today_deliveries_page.dart:49:22)
E/flutter ( 5420): <asynchronous suspension>
E/flutter ( 5420): #3      SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:1102:15)
E/flutter ( 5420): #4      SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:1049:9)
E/flutter ( 5420): #5      SchedulerBinding._handleDrawFrame (package:flutter/src/scheduler/binding.dart:957:5)
E/flutter ( 5420): #6      _rootRun (dart:async/zone.dart:1126:13)
E/flutter ( 5420): #7      _CustomZone.run (dart:async/zone.dart:1023:19)
E/flutter ( 5420): #8      _CustomZone.runGuarded (dart:async/zone.dart:925:7)
E/flutter ( 5420): #9      _invoke (dart:ui/hooks.dart:259:10)
E/flutter ( 5420): #10     _drawFrame (dart:ui/hooks.dart:217:3)
E/flutter ( 5420): 

型号:
toMap 方法中的 allitems 的转换逻辑是否正确？因为我们必须将其转换为字符串以存储到本地数据库中。

class TodayDeliveriesModel extends Equatable{
  String address;
  String CustomerName;
  String orderNumber;
  String total;
  List<ItemsModel> allItems;
  bool pending;

  TodayDeliveriesModel({this.address, this.CustomerName,this.orderNumber, this.allItems, this.total, this.pending});

  Map<String, dynamic> toMap(){
    return {
      Column_address : this.address,
      Column_CustomerName:this.CustomerName,
      Column_orderNumber:this.orderNumber,
      Column_total:this.total,
      Column_allItems:this.allItems?.map((e) =>
      e == null ? null : ItemsModel(quantity: e.quantity,name: e.name))
          ?.toList(),
      Column_pending:this.pending
    };
  }


  @override
  List<TodayDeliveriesModel> get props => deliveriesList;
}

从 UI 调用(有状态小部件):

  @override
  void initState() {
    // TODO: implement initState
    super.initState();
    TodoHelper.dba.initDatabase();
    todayDeliveries=TodayDeliveriesModel();
    listProp = todayDeliveries.props;
//    todoHelper=TodoHelper();
    insertDataInDB();
  }

  insertDataInDB()async{
    await TodoHelper.dba.insertTask(listProp);
    WidgetsBinding.instance.addPostFrameCallback(getFromDb);
  }

  void getFromDb(Duration timeStamp) async{
    await TodoHelper.dba.getAllTask().then((val){
      setState(() {
        val.forEach((v){
          dbList.add(v);
        });
      });
    });
  }

数据库助手类:

  class TodoHelper{
  Database _db;

  TodoHelper._();

  static final TodoHelper dba = TodoHelper._();


  Future<Database> get database async {
    if (_db != null) return _db;
    // if _database is null we instantiate it
    _db = await initDatabase();
    return _db;
  }


  Future initDatabase() async{
    return await openDatabase(
        join(await getDatabasesPath(), "test.db"),
        onCreate: (db, version)async{
//          return _createDb(db);
          await db.execute("CREATE TABLE $tableName($Column_id INTEGER PRIMARY KEY AUTOINCREMENT, $Column_address TEXT, $Column_CustomerName TEXT, $Column_orderNumber TEXT, $Column_total TEXT,$Column_allItems TEXT,$Column_pending TEXT)");
        },
        version: 1
    );
  }

  Future<void> insertTask(List<TodayDeliveriesModel> task) async{
    final Database = await _db;
    try{
      task.forEach((val){
        Database.insert(tableName, val.toMap(), conflictAlgorithm: ConflictAlgorithm.replace);
      });

    }catch(_){
      print(_);
    }
  }


  Future<List<TodayDeliveriesModel>> getAllTask () async{
    final List<Map<String, dynamic>> tasks = await _db.query(tableName);

    return List.generate(tasks.length, (i){
      return TodayDeliveriesModel(address: tasks[i][Column_address],total: tasks[i][Column_total],allItems: tasks[i][Column_allItems],
          CustomerName:tasks[i][Column_CustomerName] ,orderNumber: tasks[i][Column_orderNumber],pending: tasks[i][Column_pending] );
    });
  }


}

Answer 1

initDatabase()是 async但在 ToDoHelper它没有被等待。您在这里遇到的问题是 initDatabase()在不能为 async 的构造函数中或使用 await .我有一个类似的类来处理我的应用程序数据库，它等待需要数据库，然后打开数据库并将其保存到 _db变量或返回 _db如果数据库先前已打开，则为变量。这是有效的，因为它确保在尝试访问数据库之前返回正确的 future 。
如果您删除 initDatabase()从构造函数中替换Database db;使用以下私有(private)变量和异步 getter，数据库应该按预期工作。

  static Database _db;

  // If there is no database in place then initialise one
  Future<Database> get db async {
    if (_db != null) {
      return _db;
    }

    // Initialise db
    _db = await initDatabase();
    return _db;
  }

你还要确保你总是这样做 await db而不仅仅是 db .所以在上面的例子中，db.insert(...) 周围的代码可能成为

final database = await db;
task.forEach((val){
  database.insert(tableName, val.toMap(), conflictAlgorithm: ConflictAlgorithm.replace);
});