无法更改下拉按钮值 Flutter

Question

我想从firebase获取下拉按钮的初始值；但是当我尝试在构建方法中设置 governorateDDValue = selectedUser.governorate; Dropdown 的值从 firebase 获取值，但我无法更改它

DropdownButton.gif

这是我的代码

class UserInfo extends StatefulWidget {
  static const routeName = '/UserInfoScreen';

  const UserInfo({Key? key}) : super(key: key);

  @override
  _UserInfoState createState() => _UserInfoState();
}

class _UserInfoState extends State<UserInfo> {

  late User selectedUser;
  final date = DateFormat('yyyy-MM-dd').format(DateTime.now()).toString();

  var governorateDDValue;

  @override
  Widget build(BuildContext context) {

    final userList= Provider.of<List<User>>(context);
    final userID = ModalRoute.of(context)!.settings.arguments as String;
    selectedUser =
        userList.firstWhere((user) => user.id == userID);

    // this line makes dropdown value always equal to value from firestore
    governorateDDValue = selectedUser.governorate;

    return Scaffold(
      appBar: AppBar(
        title: Text('report'),
      ),
      body: SingleChildScrollView(
        child: Container(
          child: Row(
            children: <Widget>[
                    Text('Governorate',),
                    Container(height: 5),
                    DropdownButton<String>(
                      value: governorateDDValue,
                      icon: const Icon(Icons.arrow_downward),
                      iconSize: 24,
                      elevation: 16,
                      style: const TextStyle(color: Colors.deepPurple),
                      onChanged: (String? newValue) {
                        setState(() {
                          governorateDDValue = newValue!;
                        });
                      },
                      items: Constants.governoratesOfEgypt
                          .map<DropdownMenuItem<String>>((String value) {
                        return DropdownMenuItem<String>(
                          value: value,
                          child: Text(value),
                        );
                      }).toList(),
          ),
        ],
      ),
    ),
  ),
);

} }

提前致谢

Answer 1

你不能改变它的原因是每次调用setState你的build方法。因此，您的值将始终设置为 governorateDDValue = selectedUser.governorate; 因此，要允许更改，您应该将此 governorateDDValue = selectedUser.governorate; 放在 iniState

中

或者你可以这样做，这样它只会设置一次

class UserInfo extends StatefulWidget {
  static const routeName = '/UserInfoScreen';

  const UserInfo({Key? key}) : super(key: key);

  @override
  _UserInfoState createState() => _UserInfoState();
}

class _UserInfoState extends State<UserInfo> {

  late User selectedUser;
  final date = DateFormat('yyyy-MM-dd').format(DateTime.now()).toString();
  bool initState = true; // ADD HERE
  var governorateDDValue;

  @override
  Widget build(BuildContext context) {

    final userList= Provider.of<List<User>>(context);
    final userID = ModalRoute.of(context)!.settings.arguments as String;
    selectedUser =
        userList.firstWhere((user) => user.id == userID);

    // this line makes dropdown value always equal to value from firestore
    if(initState){  // ADD HERE
    governorateDDValue = selectedUser.governorate; 
    initState = false;  // ADD HERE
    }

    return Scaffold(
      appBar: AppBar(
        title: Text('report'),
      ),
      body: SingleChildScrollView(
        child: Container(
          child: Row(
            children: <Widget>[
                    Text('Governorate',),
                    Container(height: 5),
                    DropdownButton<String>(
                      value: governorateDDValue,
                      icon: const Icon(Icons.arrow_downward),
                      iconSize: 24,
                      elevation: 16,
                      style: const TextStyle(color: Colors.deepPurple),
                      onChanged: (String? newValue) {
                        setState(() {
                          governorateDDValue = newValue!;
                        });
                      },
                      items: Constants.governoratesOfEgypt
                          .map<DropdownMenuItem<String>>((String value) {
                        return DropdownMenuItem<String>(
                          value: value,
                          child: Text(value),
                        );
                      }).toList(),
          ),
        ],
      ),
    ),
  ),
);
}

Answer 2

因为您在构建小部件中使用 governorateDDValue = selectedUser.governorate;，所以每次更改下拉菜单时都会重置其值构建小部件将重建并且下拉列表的值将保持等于来自 firebase 的值你应该在构建小部件之外使用 governorateDDValue = selectedUser.governorate; 这段代码应该可以工作

class UserInfo extends StatefulWidget {
  static const routeName = '/UserInfoScreen';

  const UserInfo({Key? key}) : super(key: key);

  @override
  _UserInfoState createState() => _UserInfoState();
}

class _UserInfoState extends State<UserInfo> {

  var loading = false;

  late User selectedUser;
  final date = DateFormat('yyyy-MM-dd').format(DateTime.now()).toString();

  var governorateDDValue;


  @override
  void initState() {
    super.initState();
    loading = true;
    print('Future.delayed outside');
    print(loading);
    Future.delayed(Duration.zero, () {
      governorateDDValue = selectedUser.governorate;
      setState(() {
        loading = false;
      });
    });
  }


  @override
  Widget build(BuildContext context) {
    final userList = Provider.of<List<User>>(context);
    final userID = ModalRoute
        .of(context)!
        .settings
        .arguments as String;
    selectedUser =
        userList.firstWhere((user) => user.id == userID);

    // this line makes dropdown value always equal to value from firestore
    governorateDDValue = selectedUser.governorate;

    return Scaffold(
      appBar: AppBar(
        title: Text('report'),
      ),
      body: SingleChildScrollView(
        child: Container(
          child: Row(
            children: <Widget>[
              Text('Governorate',),
              Container(height: 5),
              DropdownButton<String>(
                value: governorateDDValue,
                icon: const Icon(Icons.arrow_downward),
                iconSize: 24,
                elevation: 16,
                style: const TextStyle(color: Colors.deepPurple),
                onChanged: (String? newValue) {
                  setState(() {
                    governorateDDValue = newValue!;
                  });
                },
                items: Constants.governoratesOfEgypt
                    .map<DropdownMenuItem<String>>((String value) {
                  return DropdownMenuItem<String>(
                    value: value,
                    child: Text(value),
                  );
                }).toList(),
              ),
            ],
          ),
        ),
      ),
    );

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