Question

当我从api获取数据时，我正在从ui调用数据库帮助程序类，并且该数据必须转储到数据库中才能在应用程序中脱机使用。我收到此错误。我在哪里收到此错误，以及为什么未初始化数据库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：

对于toMap方法中的项目，转换逻辑是否正确？因为我们必须将其转换为字符串才能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（有状态的小部件）：

  @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);
        });
      });
    });
  }

数据库帮助程序类：

  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中没有等待它。您在这里遇到的问题是initDatabase()在构造函数中，不能为async或不能使用await。我有一个类似的类来处理我的apps数据库，该数据库等待需要DB，然后打开数据库并将其保存到_db变量中，或者如果以前曾使用过数据库，则返回_db变量开了之所以有效，是因为它可以确保在尝试访问数据库之前已返回正确的期货。

如果从构造函数中删除initDatabase()，并用以下私有变量和异步getter替换Database db;，则数据库应能按预期工作。

  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;
  }

您还将确保始终执行await db而不只是db。因此，在上面的示例中，db.insert(...)周围的代码可能变为

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

I / flutter（29011）：NoSuchMethodError：方法“插入”在null上被调用。 SQFLITE颤振

1 个答案: