Flutter: 未处理的异常：在空值上使用了空检查运算符

Question

我正在使用Flutter App练习SQFLite数据库。我想创建一个数据库，用户可以在其中添加朋友的姓名和手机号码。

在添加这些信息后，这些信息列表将显示在提交按钮下[我正在使用listview.builder()]。

一切都很好。数据正在添加到数据库中(我也手动检查了.db文件)。但是当我刷新列表时，它会显示如下错误-

flutter: Done Fetch
[VERBOSE-2:ui_dart_state.cc(199)] Unhandled Exception: Null check operator used on a null value
#0      QueryResultSet.columnIndex (package:sqflite_common/src/collection_utils.dart:114:32)
#1      QueryRow.[] (package:sqflite_common/src/collection_utils.dart:135:40)
#2      new Contact.fromMap (package:sample/model/contact.dart:25:15)
#3      DatabaseHelper.fetchContacts.<anonymous closure> (package:sample/utils/database_helper.dart:63:39)
#4      MappedListIterable.elementAt (dart:_internal/iterable.dart:412:31)
#5      ListIterator.moveNext (dart:_internal/iterable.dart:341:26)
#6      new _GrowableList._ofEfficientLengthIterable (dart:core-patch/growable_array.dart:188:27)
#7      new _GrowableList.of (dart:core-patch/growable_array.dart:150:28)
#8      new List.of (dart:core-patch/array_patch.dart:50:28)
#9      ListIterable.toList (dart:_internal/iterable.dart:212:44)
#10     DatabaseHelper.fetchContacts (package:sample/utils/database_helper.dart:63:51)
<asynchronous suspension>

数据获取函数也运行得很好（控制台显示print("Done Fetch");的输出）。我认为刷新函数有问题。

请有人帮助我。问题在哪里，我该如何解决？

用户界面=>

import 'package:flutter/material.dart';
.....

class FlutterLocalDatabase extends StatefulWidget {
  @override
  _FlutterLocalDatabaseState createState() => _FlutterLocalDatabaseState();
}

class _FlutterLocalDatabaseState extends State<FlutterLocalDatabase> {
 ....
  Contact _contact = Contact();
  List<Contact> contactList = [];

  DatabaseHelper _dbHelper;
  @override
  void initState() {
    super.initState();
    setState(() {
      _dbHelper = DatabaseHelper.instance;
    });

    refreshContactList();
  }

  @override
  Widget build(BuildContext context) {
    double _width = MediaQuery.of(context).size.width;
    double _height = MediaQuery.of(context).size.height;
    return Scaffold(
      appBar: AppBar(
        title: Text("Flutter SQFLite Database"),
      ),
      body: GestureDetector(
        ......
        .....
                ElevatedButton(
                    onPressed: () async {
                      print("${_contact.name}, ${_contact.mobile}");
                      if (validateAndSave(_formKey)) {
                        FocusScope.of(context).requestFocus(new FocusNode());
                        if (_contact.id == null) {
                          await _dbHelper.insertContact(_contact).then((value) {
                            print(value);
                            _resetForm();
                            refreshContactList();
                          });
                        }

                        print(contactList);
                      }
                    },
                    child: Text("Submit")),
                SizedBox(
                  height: 20,
                ),
                Expanded(
                    child: Container(
                  child: ListView.builder(
                      itemCount: contactList.length,
                      itemBuilder: (buildContext, index) {
                        var contact = contactList[index];
                        return Card(
                          child: ListTile(
                            leading: Icon(
                              Icons.account_circle,
                              size: 40,
                              color: Colors.teal,
                            ),
                            title: Text(
                              contact.name,
                              style: TextStyle(
                                color: Colors.teal,
                              ),
                            ),
                            subtitle: Text(contact.mobile,
                                style: TextStyle(
                                  color: Colors.teal,
                                )),
                            trailing: Text("${contact.id}",
                                style: TextStyle(
                                  color: Colors.teal,
                                )),
                          ),
                        );
                      }),
                ))
              ],
            ),
          ),
        ),
      ),
    );
  }

  refreshContactList() async {
    List<Contact> x = await _dbHelper.fetchContacts();
    print("Done Refresh");
    setState(() {
      contactList = x;
    });
  }

  _resetForm() {
    setState(() {
      _formKey.currentState.reset();
      _ctrlName.clear();
      _ctrlMobile.clear();
      _contact.id = null;
    });
  }
}

模型类 =>

class Contact {
  static const tblContact = "contacts";
  static const colId = "id";
  static const colName = "name";
  static const colMobile = "mobile";

  Contact({
    this.id,
    this.name,
    this.mobile,
  });

  int id;
  String name;
  String mobile;

  Map<String, dynamic> toMap() {
    Map map = <String, dynamic>{colName: name, colMobile: mobile};
    if (id != null) map[colId] = id;
    return map;
  }

  Contact.fromMap(Map<String, dynamic> map) {
    id = map[colId];
    name = map[name];
    mobile = map[colMobile];
  }
}

数据库辅助类 =>

import 'dart:io';
.....
class DatabaseHelper {
  static const _databaseName = "ContactData.db";
  static const _databaseVersion = 1;

//<====== Singleton Class
  DatabaseHelper._();
  static final DatabaseHelper instance = DatabaseHelper._();

  Database _database;
  Future<Database> get database async {
    if (_database != null) {
      return _database;
    } else {
      _database = await _initDatabase();
      return _database;
    }
  }

//CREATE DATABASE
  _initDatabase() async {
    Directory dataDirectory = await getApplicationDocumentsDirectory();
    String dbPath = join(dataDirectory.path, _databaseName);
    print(dbPath);
    return await openDatabase(dbPath,
        version: _databaseVersion, onCreate: _onCreate);
  }

  //CREATE TABLE
  _onCreate(Database db, int version) async {
    db.execute(''' 
CREATE TABLE ${Contact.tblContact}(
  ${Contact.colId} INTEGER PRIMARY KEY AUTOINCREMENT,
  ${Contact.colName} STRING NOT NULL,
  ${Contact.colMobile} STRING NOT NULL
);
''');
    print("Done on Create");
  }

  //<===================  ADD DATA

  Future<int> insertContact(Contact contact) async {
    Database db = await database;
    print("Done on Insert");
    return await db.insert(Contact.tblContact, contact.toMap());
  }

  //<==================== Read Data
  Future<List<Contact>> fetchContacts() async {
    Database db = await database;
    List<Map> contacts = await db.query(Contact.tblContact);
    print("Done Fetch");
    return contacts.length == 0
        ? []
        : contacts.map((x) => Contact.fromMap(x)).toList();
  }

}

- Abir Ahsan

---

你能确认 Contact.fromMap() 是否确实返回一个正确的 Contact 模型吗？ - Extreme Geek

@ExtremeGeek，我按照那些指示操作了，你能看一下吗？https://www.codaffection.com/flutter-article/complete-sqlite-crud-operations-in-flutter/ - Abir Ahsan

Answer 1

地图索引运算符是可空的
这意味着如果未找到索引，它可以返回null，因此您应该使您的模型属性可为空。

class Contact {
  ...

  int? id;
  String? name;
  String? mobile;

- JoshKisb