我为在数据库中获取数据而编写的函数返回空。数据库里有数据。在颤振中

Question

widgets库捕获的异常接收者：null尝试调用：notListesiniGetir（）

图3此处代码：

[Future<int> kategoriSil(int kategoriID) async {
    var db= await _getDatabase();
    var sonuc= await db.delete("kategori",  where: 'kategoriID= ?', whereArgs: \[kategoriID\]);
    return sonuc;
  }

  Future<List<Map<String, dynamic>>> notlariGetir() async {
    var db = await _getDatabase();
    var sonuc= await db.rawQuery('select * from "not" inner join kategori on kategori.kategorID = "not".kategoriID;');
    return sonuc;
  }
  Future<List<Not>> notListesiniGetir() async{
    var notlarMapListesi = await notlariGetir();
    var notListesi = List<Not>();
    for(Map map in notlarMapListesi){
      notListesi.add(Not.fromMap(map));

    }
    return notListesi;

  }][1]

错误代码接收者：null尝试调用：notListesiniGetir（）

相关的导致错误的小部件是:Notlar file://c://flutter%20calismalari/not_sepeti/lib/main.dart:63:27抛出异常时，这是堆栈:#0object.nosuchmethod(dart:core-patch/object_patch.dart:54:5)#1_notlarstate.build（包:not_sepet/main.dart:166:30)#2 statefulelement.build（包:flutter/src/widgets/framework.dart:4802:27)#3 performRebuild（包:flutter/src/widgets/framework.dart:4857:11)...

在此输入图像描述bwg.png

Answer 1

问题不在代码中，而是在屏幕截图中（您应该将其作为代码片段添加到问题中）。

您必须等待条件snapshot.hasdata

从

If (snapshot.connectionState == ConnectionState.done)

If (snapshot.connectionState == ConnectionState.done && snapshot.hasData)

var notListesi = List<Not>()
List<Not> notListesi = <Not>[ ]; // better

DatabaseHelper databaseHelper = DatabaSeHelper();

databaseHelper = DatabaseHelper();