Flutter/Dart：将 this.variable 作为参数传递

Question

我有一个我想使用单个小部件创建的配置文件字段列表，但我是 Flutter 的新手，似乎无法解决一件事：通过参数传递变量。我已经能够使用this.variable = value 创建许多工作正常的小部件，但现在我正试图将其转换为单个小部件以免重复自己，这就是我遇到问题的地方。

我有以下代码（当然删除了所有我认为不必要的代码）。这里目前显示错误The setter 'listType' isn't defined for the class '_ProfileDataState'.

class _ProfileDataState extends State<ProfileData> {
  final _countries = DropDownLists.countries;
  String _country;
  var listType; //<-- added this per comments

  Widget profileDropDown(var list, var listType) {
    return Card(
      onTap: () async {
        AlertDialog(
          content: DropdownButtonFormField<String>(
            isExpanded: true,
            items: list.map((String value) {
              return DropdownMenuItem<String>(
                value: value,
                child: Text(value),
              );
            }).toList(),
            isDense: true,
            value: listType,
            onChanged: (value) {
              FocusScope.of(context).requestFocus(FocusNode());
                setState(() {
                  this.listType = value;
                });
              },
          )
        )
      }
    )
  }

  @override
    Widget build(BuildContext context) {
      return profileDropDown(_countries, _country),
...

Answer 1

class _ProfileDataState extends State<ProfileData> {
  String _country;
  var listType; // declare a variable

  @override
  void initState() {
    super.initState();
    listType = widget.listType; // assign it a value here
  }

  Widget profileDropDown(var listType) {
    return Card(
        onTap: () async {
          AlertDialog(
            onChanged: (value) {
              FocusScope.of(context).requestFocus(FocusNode());
              setState(() {
                this.listType = value; // works
              });
            },
          )
        }
    )
  }

  // other methods
}