未处理的异常：LateInitializationError：字段“_id@21016470”尚未初始化

Question

当我运行我的项目编译器给我这个语句“LateInitializationError: Field 'Db' has”时，我无法解决这个问题。 当我执行 dabase_client.dart 中的函数，即 saveItem() 函数时，它正在创建问题，函数编译器给我指示，在某些 dart 文件中存在代码问题。我将为您提供我的整个项目代码，以便您可以轻松找出造成问题的确切问题。

ma​​in.dart

import 'package:flutter/material.dart';
import 'package:no_to_do_app/ui/home.dart';

void main() => runApp(new MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      debugShowCheckedModeBanner: false,
      title: 'NotoDo',
      home: new Home(),
    );
  }
}

home.dart

import 'package:flutter/material.dart';
import 'package:no_to_do_app/ui/notodo_screen.dart';
class Home extends StatelessWidget {
  const Home({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: Text('NoToDo'),
        backgroundColor: Colors.black54,
      ),
      body: new NoToDoScreen(

      )
    );
  }
}

database_client.dart

import 'dart:async';
import 'dart:io';
import 'package:flutter/cupertino.dart';
import 'package:no_to_do_app/model/nodo_item.dart';
import 'package:path_provider/path_provider.dart';
import 'package:sqflite/sqflite.dart';
import 'package:path/path.dart';

class DatabaseHelper
{
  static final DatabaseHelper _instance =  DatabaseHelper.private();
  DatabaseHelper.private();
  factory DatabaseHelper() => _instance;

  final String tableName = "nodoTbl";
  final String columnId = "id";
  final String columnItemName = "itemName";
  final String columnDateCreated = "dateCreated";

  static late Database _db;

  Future<Database> get database async
  {
    /*if(_db!=null)
      {
        return _db;
      }*/
    _db =await initDb();
    return _db;
  }

  DatabaseHelper.internal();

  initDb() async
  {
    Directory documentDirectory = await getApplicationDocumentsDirectory();
    String path = join(documentDirectory.path, "notodo_db.db");
    var ourDb= await openDatabase(path , version: 1 , onCreate: _onCreate);
    return ourDb;
  }

  void _onCreate(Database db, int version) async
  {
   await db.execute(
     "CREATE TABLE $tableName(id INTEGER PRIMARY KEY , $columnItemName TEXT ,$columnDateCreated TEXT)"
   );
   debugPrint('Table is Created');
  }

  Future<int> saveItem(NoDoItem item) async
  {
    var dbClient = await database;
    int res =await dbClient.insert(tableName, item.toMap());
    debugPrint(res.toString());
    return res;
  }

  Future<List> getItems() async
  {
    var dbClient = await database;
    var result = await dbClient.rawQuery("SELECT * FROM $tableName ORDER BY $columnItemName ASC");
    return result.toList();
  }

  Future<int?> getCount() async
  {
    var dbClient = await database;
    return Sqflite.firstIntValue(await dbClient.rawQuery(
        "SELECT COUNT(*) FROM $tableName"
    ));
  }

  Future<NoDoItem?> getItem(int id) async{
    var dbClient = await database;
    var result = await dbClient.rawQuery("SELECT * FROM $tableName WHERE id=$id");
    //if(result.length==0) return null;
    if(result.isEmpty) return null;
    return  NoDoItem.fromMap(result.first);
  }

  Future<int> deleteItem(int id) async
  {
    var dbClient =await database;
    return await dbClient.delete(tableName,where: "$columnId = ?", whereArgs: [id]);
  }

  Future<int> updateItem(NoDoItem item) async
  {
   var dbClient =await database;
   return await dbClient.update(tableName, item.toMap(),
   where: "$columnId=?", whereArgs: [item.id]);
  }

  Future close() async
  {
    var dbClient =await database;
    return dbClient.close();
  }
}

nodo_item.dart

import 'package:flutter/material.dart';


class NoDoItem extends StatelessWidget {
  late String _itemName;
  late String _dateCreated;
  late int _id;

  NoDoItem( this._itemName , this._dateCreated, {Key? key}) : super(key: key);

  NoDoItem.map(dynamic obj, {Key? key}) : super(key: key)
  {
    _itemName = obj['ItemName'];
    _dateCreated = obj['DateCreated'];
    _id = obj['id'];
  }

  String get itemName => _itemName;
  String get dateCreated => _dateCreated;
  int get id => _id;

  Map<String , dynamic>toMap()
  {
    var map = <String , dynamic>{};
    map['ItemName'] = _itemName;
    map['DateCreated'] = _dateCreated;
    /*if(_id !=null)
      {
        map['id'] = _id;
      }*/
    map['id'] = _id;
    return map;
  }

  NoDoItem.fromMap(Map<String , dynamic>map, {Key? key}) : super(key: key)
  {
    _itemName = map['ItemName'];
    _dateCreated = map['DateCreated'];
    _id = map['id'];
  }

  @override
  Widget build(BuildContext context) {
    return Container(
      margin: const EdgeInsets.all(8.0),
      child: Row(
        crossAxisAlignment: CrossAxisAlignment.start,
        children: [
          Text(_itemName ,
          style: const TextStyle(
            color: Colors.white,
            fontSize: 16.5,
            fontWeight: FontWeight.bold
          ),),
          Container(
            margin: const EdgeInsets.only(top: 5.0),
            child:  Text('Created on: $_dateCreated',
              style: const TextStyle(
                color: Colors.white70,
                fontStyle: FontStyle.italic,
                fontSize: 13.4
              ),),
          )
        ],
      )
    );
  }
}

notodo_screen.dart

import 'package:flutter/material.dart';
import 'package:no_to_do_app/model/nodo_item.dart';
import 'package:no_to_do_app/util/database_client.dart';

class NoToDoScreen extends StatefulWidget {
  const NoToDoScreen({Key? key}) : super(key: key);

  @override
  _NoToDoScreenState createState() => _NoToDoScreenState();
}

class _NoToDoScreenState extends State<NoToDoScreen> {
  final TextEditingController _textEditingController = TextEditingController();
  var db = DatabaseHelper();

  void _hndleSubmitted(String text) async
  {
    _textEditingController.clear();
    NoDoItem noDoItem = NoDoItem(text,DateTime.now().toIso8601String());
    int savedItemId = await db.saveItem(noDoItem);

    debugPrint("Item saved ID: $savedItemId");
  }

  @override
  Widget build(BuildContext context) {
    return  Scaffold(
      backgroundColor: Colors.black87,
      body:  Column(),
      floatingActionButton:  FloatingActionButton(
          tooltip: 'Add Item',
          backgroundColor: Colors.redAccent,
          child:  const ListTile(
              title:  Icon(Icons.add)
          ),
          onPressed: _showFormDialog),

    );

  }

  void _showFormDialog() {
    var alert = AlertDialog(
      content: Row(
        children: [
          Expanded(
            child: TextField(
              controller: _textEditingController,
              autofocus: true,
              decoration: const InputDecoration(
                labelText: 'item',
                hintText: "eg. Don't buy Stuff",
                icon: Icon(Icons.add_circle_outline_outlined)
              ),
            ))
        ],
      ),
          actions:[
            TextButton(
              style: ButtonStyle(
        foregroundColor: MaterialStateProperty.all<Color>(Colors.blue),
    ),
    onPressed: ()
    {
    _hndleSubmitted(_textEditingController.text);
    _textEditingController.clear();
    },
    child: const Text("Save"),
            ),

            TextButton(
              style: ButtonStyle(
            foregroundColor: MaterialStateProperty.all<Color>(Colors.blue),
            ),
            onPressed:() => Navigator.pop(context),
            child: const Text("Cancel"),)
          ]
    );
    showDialog(context: context,
    builder:(_)
    {
      return alert;
    });
  }
}

编译器显示错误

E/flutter ( 3777): [ERROR:flutter/lib/ui/ui_dart_state.cc(209)] Unhandled Exception: LateInitializationError: Field '_id@21016470' has not been initialized.
E/flutter ( 3777): #0      NoDoItem._id (package:no_to_do_app/model/nodo_item.dart)
E/flutter ( 3777): #1      NoDoItem.toMap (package:no_to_do_app/model/nodo_item.dart:31:17)
E/flutter ( 3777): #2      DatabaseHelper.saveItem (package:no_to_do_app/util/database_client.dart:54:53)
E/flutter ( 3777): <asynchronous suspension>
E/flutter ( 3777): #3      _NoToDoScreenState._hndleSubmitted (package:no_to_do_app/ui/notodo_screen.dart:20:23)
E/flutter ( 3777): <asynchronous suspension>
E/flutter ( 3777): 

Answer 1

后期值需要初始值，你需要初始化它。你的代码没有这个。试试这个：

late String param = '';
late int paramId = 0;

Answer 2

好的，我已经解决了这个问题，我没有初始化代码中使用的后期字段，然后没有初始化

late String _itemName;
late String _dateCreated;
late int _itemId;

正确的方法是

late String _itemName = "";
late String _dateCreated = "";

并且 itemId 是主键，它不应该被初始化，因为数据库会自动为我们将值放入其中，因为它是主键 我们应该通过放置 ?即

  int? _itemId;