Flutter 颤振:未处理的异常:对空值使用空检查运算符
我正在用颤振应用程序中的SQFLite数据库进行练习。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 添加此信息后,该信息列表将显示在submit按钮下[其中我使用Flutter 颤振:未处理的异常:对空值使用空检查运算符,flutter,listview,null,unhandled-exception,sqflite,Flutter,Listview,Null,Unhandled Exception,Sqflite,我正在用颤振应用程序中的SQFLite数据库进行练习。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 添加此信息后,该信息列表将显示在submit按钮下[其中我使用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,它会显示这样一个错误- flutter: Done Fetch [VERBOSE-2:ui_dart_state.cc(199)] Unhandled Exception: Null check
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,它会显示这样一个错误-
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>
模型类=>
class联系人{
静态常数tblContact=“触点”;
静态常数colId=“id”;
静态常量colName=“name”;
static const colMobile=“mobile”;
接触({
这个身份证,
这个名字,
这个.手机,,
});
int-id;
字符串名;
字符串移动;
映射toMap(){
Map Map={colName:name,colMobile:mobile};
如果(id!=null)映射[colId]=id;
返回图;
}
Contact.fromMap(地图地图){
id=地图[colId];
名称=地图[名称];
mobile=map[colMobile];
}
}
数据库帮助器类=>
导入'dart:io';
.....
类数据库助手{
static const_databaseName=“ContactData.db”;
静态常量_databaseVersion=1;
//
这意味着如果找不到索引,它可以返回null,因此您应该使模型属性为null
class Contact {
...
int? id;
String? name;
String? mobile;
你能确认Contact.fromMap()确实返回了一个正确的联系人模型吗?@ExtremeGeek,我按照说明操作,你能看一下吗?
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];
}
}
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();
}
}
class Contact {
...
int? id;
String? name;
String? mobile;