Flutter error: A value of type 'Object?' can't be assigned to a variable of type 'String'

Question

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)

代码：

    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();
          }
        });
  }
}

Update Update 2 'String?' 类型的值不能分配给 'String' 类型的变量。尝试更改变量的类型，或将右侧的类型强制转换为 'String'.

Answer 1

将通用类型 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), 
                  ),

除非你指定类型 String dart 假设它拥有的最通用类型之一 Object? 作为 DropdownButtonFormField

的通用类型

完整演示（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),
        ),
      ),
    );
  }
}

Answer 2

尝试将 val 作为字符串，或任何您想变成的类型：

onChanged: (val) => setState(() => _currentSugars = val as String),

Answer 3

要修复添加 类型参数 或 cast 从动态显式

您可以通过添加类型参数来避免此错误：

void filterValues<T>(bool Function(T) filter) {}
filterValues<String>((x) => x.contains('Hello'));

您还可以使用转换：

void filterValues(bool Function(dynamic) filter) {}
filterValues((x) => (x as String).contains('Hello'));

Flutter error: A value of type 'Object?' can't be assigned to a variable of type 'String'

Flutter error: A value of type 'Object?' can't be assigned to a variable of type 'String'

typeerror

dart

flutter

完整演示（更新为使用空安全）