颤动错误:不能将Object?';类型的值赋给';String';类型的变量。 [英] Flutter error: A value of type 'Object?' can't be assigned to a variable of type 'String'
本文介绍了颤动错误:不能将Object?';类型的值赋给';String';类型的变量。的处理方法,对大家解决问题具有一定的参考价值,需要的朋友们下面随着小编来一起学习吧!
问题描述
我正在使用下拉按钮字段,并收到此错误:
A value of type 'Object?' can't be assigned to a variable of type 'String'.
Try changing the type of the variable, or casting the right-hand type to 'String'.dart(invalid_assignment)
编码:
class SettingsForm extends StatefulWidget {
@override
_SettingsFormState createState() => _SettingsFormState();
}
class _SettingsFormState extends State<SettingsForm> {
final _formKey = GlobalKey<FormState>();
final List<String> sugars = ['0', '1', '2', '3', '4'];
final List<int> strengths = [100, 200, 300, 400, 500, 600, 700, 800, 900];
// form values
String? _currentName;
String? _currentSugars;
int? _currentStrength;
@override
Widget build(BuildContext context) {
MyUser user = Provider.of<MyUser>(context);
return StreamBuilder<UserData>(
stream: DatabaseService(uid: user.uid).userData,
builder: (context, snapshot) {
if (snapshot.hasData) {
UserData? userData = snapshot.data;
return Form(
key: _formKey,
child: Column(
children: <Widget>[
Text(
'Update your brew settings.',
style: TextStyle(fontSize: 18.0),
),
SizedBox(height: 20.0),
TextFormField(
initialValue: userData!.name,
decoration: textInputDecoration,
validator: (val) =>
val!.isEmpty ? 'Please enter a name' : null,
onChanged: (val) => setState(() => _currentName = val),
),
SizedBox(height: 10.0),
DropdownButtonFormField(
value: _currentSugars ?? userData.sugars,
decoration: textInputDecoration,
items: sugars.map((sugar) {
return DropdownMenuItem(
value: sugar,
child: Text('$sugar sugars'),
);
}).toList(),
onChanged: (val) => setState(() => _currentSugars = val), <--Error here **val** right one
),
SizedBox(height: 10.0),
Slider(
value: (_currentStrength ?? userData.strength).toDouble(),
activeColor:
Colors.brown[_currentStrength ?? userData.strength],
inactiveColor:
Colors.brown[_currentStrength ?? userData.strength],
min: 100.0,
max: 900.0,
divisions: 8,
onChanged: (val) =>
setState(() => _currentStrength = val.round()),
),
ElevatedButton(
style:
ElevatedButton.styleFrom(primary: Colors.pink[400]),
child: Text(
'Update',
style: TextStyle(color: Colors.white),
),
onPressed: () async {
if (_formKey.currentState!.validate()) {
await DatabaseService(uid: user.uid).updateUserData(
_currentSugars ?? snapshot.data!.sugars,
_currentName ?? snapshot.data!.name,
_currentStrength ?? snapshot.data!.strength);
Navigator.pop(context);
}
}),
],
),
);
} else {
return Loading();
}
});
}
}
UpdateUpdate 2 类型为‘string?’的值?不能赋给‘string’类型的变量。 请尝试更改变量的类型,或将右侧类型强制转换为"String"。
推荐答案
将泛型类型String
分配给DropdownButtonFormField:
DropdownButtonFormField<String>(
value: _currentSugars,
decoration: textInputDecoration,
items: sugars.map((sugar) {
return DropdownMenuItem(
value: sugar,
child: Text('$sugar sugars'),
);
}).toList(),
onChanged: (val) => setState(() => _currentSugars = val),
),
除非您指定类型String
,否则DART会假设它拥有的最泛型类型之一Object?
为DropdownButtonFormField
Complete演示(更新为使用空安全)
class Demo extends StatefulWidget {
Demo({Key? key}) : super(key: key);
@override
_DemoState createState() => _DemoState();
}
class _DemoState extends State<Demo> {
final sugars = ['candy', 'chocolate', 'snicker'];
String? _currentSugars = 'candy';
@override
Widget build(BuildContext context) {
return Scaffold(
body: Center(
child: DropdownButtonFormField<String>(
value: _currentSugars,
items: sugars.map((sugar) {
return DropdownMenuItem(
value: sugar,
child: Text('$sugar sugars'),
);
}).toList(),
onChanged: (val) => setState(() => _currentSugars = val),
),
),
);
}
}
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