我有这个 StatefulWidget，它应该根据条件从 MySQL 数据库检索数据并将这些检索到的值添加到数组中，以便我可以使用数组值显示在 ListView 上。我的问题是，有时当数组 (_fileValues) 和 _custUsername 变量为空或未更新时，不会引发异常。
这是我的有状态小部件 (main.dart)
String? _custUsername;
List<String> _fileValues = <String>[];
class cakeLogin extends StatefulWidget {
@override
cakeLoginPageMain createState() => cakeLoginPageMain();
}
class cakeLoginPageMain extends State<cakeLogin> {
String? _CustEmailInit;
BuildContext? loginContext;
List<String> _fileNameStores = <String>[];
void callData() {
// @ Get username based on user email and assign it to _custUsername variable
UsernameGetter().Connection(_CustEmailInit!).then((UsernameValue) {
_custUsername = UsernameValue!;
});
// @ Get item names based on the user username (_custUsername) and add them into
// _fileNameStores
NameGetter().Connection(_custUsername).then((NamesValues) {
for(var _nameItems in NamesValues) {
setState(() {
_fileNameStores.add(_nameItems);
});
}
});
// @ Remove duplicates value and add them into main list (_fileValues)
var _removeDuplicates = _fileNameStores.toSet().toList();
for(var item in _removeNamesDupes) {
setState(() {
_fileValues.add(item);
});
}
}
@override
Widget build(BuildContext context) {
final _emailController = TextEditingController();
return Scaffold(
appBar: AppBar(
title: Text("Login to App",
style: TextStyle(
fontSize: 18,
color: setupFontCol,
fontWeight: FontWeight.bold
)),
backgroundColor: setupThemeBar,
centerTitle: true,
shape: RoundedRectangleBorder(
borderRadius: BorderRadius.circular(20),
)
),
backgroundColor: setupMainTheme,
body: Padding(
padding: EdgeInsets.all(20),
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
TextFormField(
controller: _emailController,
style: TextStyle(color: Color.fromARGB(255, 214, 213, 213)),
decoration: InputDecoration(
border: OutlineInputBorder(
borderRadius: BorderRadius.circular(10),
),
filled: true,
hintStyle: TextStyle(color: Color.fromARGB(255, 197, 197, 197)),
hintText: "Enter your email",
fillColor: setupThemeBar
),
),
SizedBox(height: 18),
SizedBox(height: 25),
SizedBox(
height: 45,
width: 500,
child: ElevatedButton(
style:
ElevatedButton.styleFrom(
primary: setupFontCol,
onPrimary: Colors.white,
shape: RoundedRectangleBorder(
borderRadius: BorderRadius.circular(10),
)
),
onPressed: () {
var _CustEmail = _emailController.text;
if(_CustEmail.contains("@gmail.com") || _CustEmail.contains("@email.com")) {
if(_CustEmail != "") {
_CustEmailInit = _CustEmail;
loginContext = context;
_isFromLogin = true;
_fileValues.clear();
callData();
} else {
alertDialog("Please enter your email address",context);
}
} else {
alertDialog("Email address is not valid",context);
}
},
//color: setupFontCol,
child: Text("Login",
style: TextStyle(
color: Colors.white,
fontWeight: FontWeight.normal,
fontSize: 14,
))
),
),
],
),
),
);
}
}
UsernameGetter 类（此类将根据文本字段输入中的电子邮件从数据库表中检索用户用户名）
当我从 UsernameGetter 类打印 _usernameDetected 类时，它返回返回字符串的更新版本，但是当我将 _custUsername 分配给 UsernameValue 从 main.dart 时，它不会更新，有时会返回 null值。
import 'package:mysql_client/mysql_client.dart';
import 'dart:convert';
import 'dart:ffi';
import 'dart:typed_data';
import 'package:signuppage/main.dart' as main;
import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';
class UsernameGetter {
Future<String> Connection(String _custEmail) async {
final conn = await MySQLConnection.createConnection(
host: "",
port: ,
userName: "",
password: "",
databaseName: "",
);
await conn.connect();
var _getUsernameQue = await conn.execute("SELECT CUST_USERNAME FROM information WHERE CUST_EMAIL = :email",
{
"email": _custEmail
});
String? _usernameDetected;
for(final _rowsOfEmail in _getUsernameQue.rows) {
_usernameDetected = _rowsOfEmail.assoc()['CUST_USERNAME']!;
}
return await _usernameDetected!;
}
}
NameGetter 类（检索存储在文本列中的用户项名称）
对于 NameGetter 类，与 UsernameGetter 发生相同的情况。当我从 main.dart 检索值并将值添加到 _fileValues 列表中时，有时该值为空，有时该值未更新。
import 'package:mysql_client/mysql_client.dart';
import 'dart:convert';
import 'dart:ffi';
import 'dart:typed_data';
import 'package:signuppage/main.dart' as main;
import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';
class NameGetter {
Future<List<String>> Connection(String _custUsername) async {
final conn = await MySQLConnection.createConnection(
host: "",
port: ,
userName: "",
password: "",
databaseName: "",
);
await conn.connect();
var _getItemNames= await conn.execute("SELECT ITEM_NAMES FROM item_details WHERE CUST_USERNAME = :username",
{
"username": _custUsername
});
List<String>? _itemNamesValues= <String>[];
for(var _rowsOfItemNames in _getItemNames.rows) {
var _retrieveNames = await _rowsOfItemNames.assoc()['ITEM_NAMES']!;
_itemNamesValues.add(_retrieveNames);
}
return await _itemNamesValues;
}
}
我不确定我的错误在哪里，但我很确定我犯了很多错误，我需要有人帮助我解决。特别是关于为什么列表和变量没有更新/有时是空的。
参考链接：
Flutter ListView 未更新
我在 flutter 中的列表没有更新代码中的问题
为什么变量值在flutter中不刷新？
问题
callData()方法的精简版本如下所示：var foo; void callData() { someAsyncFunction().then( ... foo = value ... ); anotherAsyncFunction(foo).then( ... ); }如前文所述，
anotherAsyncFunction(foo)在someAsyncFunction()之后、then()之前 立即调用> 分配foo的值。您可以通过有策略地插入
print()语句来亲自观察此行为。解决方案
在底层，
Future.then()和async/await是同一件事。然而，当使用async/await时，人们通常更容易推断异步代码的行为，因为这样它读起来就像顺序执行的代码。虽然您可以使用其中任何一个，但我建议您转向
async/await以使代码在阅读时更容易理解。以下是如何重构
callData()的一个示例：这样写，很明显
custUsername是在调用NameGetter().Connection(custUsername)之前分配的。请务必通读异步编程：futures、async、await。