Flutter Error : The left operand can't be null, so the right operand is never executed. Try removing the operator and the right operand

Question

我正在尝试构建我的 flutter 项目，但在使用三元运算符时出现错误。我正在使用三元运算符，因为它会检查第一条语句是否为空，如果为空则它将使用第二条语句。这对我的代码有帮助，但出现错误，它指出：

左操作数不能为空，所以右操作数永远不会执行。尝试删除运算符和正确的操作数。

示例：

value: initial_value ?? current_value

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
  late String _currentName;
  late String _currentSugars;
  late 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, <--Error : **userData.sugars**
                    decoration: textInputDecoration,
                    items: sugars.map((sugar) {
                      return DropdownMenuItem(
                        value: sugar,
                        child: Text('$sugar sugars'),
                      );
                    }).toList(),
                    onChanged: (val) => setState(() => _currentSugars = val),
                  ),
                  SizedBox(height: 10.0),
                  Slider(
                    value: (_currentStrength ?? userData.strength).toDouble(), <--Error : **userData.strength**
                    activeColor:
                        Colors.brown[_currentStrength ?? userData.strength], <--Error : **userData.strength**
                    inactiveColor:
                        Colors.brown[_currentStrength ?? userData.strength], <--Error : **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, <--Error : **snapshot.data!.sugars**
                              _currentName ?? snapshot.data!.name, <--Error : **snapshot.data!.name**
                              _currentStrength ?? snapshot.data!.strength); <--Error : **snapshot.data!.strength**
                          Navigator.pop(context);
                        }
                      }),
                ],
              ),
            );
          } else {
            return Loading();
          }
        });
  }
}

'MyUser' 数据 class :

class UserData {

  final String uid;
  final String name;
  final String sugars;
  final int strength;

  UserData({ required this.uid, required this.sugars, required this.strength, required this.name });

Screenshot Update

Answer 1

您已将 _currentSugars 变量声明为不可为 null 的字符串。因此，您明确声明它不能包含空值。通过向其添加 late 关键字，编译器无法在编译时确保空安全约束。

根据late-variables

The late modifier means “enforce this variable’s constraints at runtime instead of at compile time”

所以不要将其声明为：

late String _currentSugars;

声明为：

String? _currentSugars;

Flutter Error : The left operand can't be null, so the right operand is never executed. Try removing the operator and the right operand

Flutter Error : The left operand can't be null, so the right operand is never executed. Try removing the operator and the right operand

mobile

android

flutter

flutter-layout