Flutter：自定义下拉按钮功能不显示所选值

Question

我正在尝试创建一个返回下拉菜单的自定义函数。该函数接受 3 个参数：menuTitle、selectedValue 和字符串列表。

List<String> cities= ['Rome', 'London', 'Paris'];
String selectedCity;
String title='Select a city';

  Widget _buildDropdown(
      String menuTitle, String selectedItem, List<String> list) {
    return Container(
      width: MediaQuery.of(context).size.width * 0.8,
      margin: EdgeInsets.all(10.0),
      decoration: BoxDecoration(
          border: Border.all(color: Colors.grey[300]),
          borderRadius: BorderRadius.circular(10.0)),
      child: DropdownButtonHideUnderline(
        child: ButtonTheme(
          alignedDropdown: true,
          child: DropdownButton<String>(
            isExpanded: true,
            hint: Text(
              menuTitle.toUpperCase(),
              style: GoogleFonts.lato(
                  textStyle: TextStyle(
                color: Colors.grey[500],
                fontWeight: FontWeight.bold,
              )),
            ),
            value: selectedItem,
            items: list.map((String val) {
              return DropdownMenuItem(
                value: val,
                child: Text(
                  val.toUpperCase(),
                  style: GoogleFonts.lato(
                      textStyle: TextStyle(
                    color: Colors.grey[500],
                  )),
                ),
              );
            }).toList(),
            onChanged: (String value) {
              setState(() {
                selectedItem = value;
              });
            },
          ),
        ),
      ),
    );
  }

下拉菜单显示所有可用选项，但选择一个后，我只能看到提示文本，而看不到所选值。我将不胜感激。

Answer 1

为了让您的下拉列表反映您的 selectedValue，它应该是状态的一部分。所以按照下面的代码所示。我已经测试了这段代码，它对我有用。

class _MyAppState extends State<MyApp> {

  List<String> cities= ['Rome', 'London', 'Paris'];
//Initialize your selectedItem to selectedCity
//default selectedCity
  String selectedCity='Rome';
  String title='Select a city';



  @override
  Widget build(BuildContext context) {
    Color color=Colors.blueAccent;
    int x=color.value;

    return MaterialApp(

      home: Scaffold(
        body: Column(
          children: <Widget>[_buildDropdown("Title","",cities)],
        ),
      ),
    );
  }

  Widget _buildDropdown(
      String menuTitle, String selectedItem, List<String> list) {

    return Container(
      margin: EdgeInsets.all(10.0),
      decoration: BoxDecoration(
          border: Border.all(color: Colors.grey[300]),
          borderRadius: BorderRadius.circular(10.0)),
      child: DropdownButtonHideUnderline(
        child: ButtonTheme(
          alignedDropdown: true,
          child: DropdownButton<String>(
            isExpanded: true,
            hint: Text(
              menuTitle.toUpperCase(),
            ),
            value: selectedCity,
            items: list.map((String val) {
              return DropdownMenuItem(
                value: val,
                child: Text(
                  val.toUpperCase(),
                ),
              );
            }).toList(),
            onChanged: (String value) {
              setState(() {
                selectedCity = value;
              });
            },
          ),
        ),
      ),
    );
  }
}