Flutter - DropdownButton onChanged 返回 null

Question

我正在创建一个下拉列表，但是每当我单击一个值并尝试在 onChanged 部分打印它时，它都会打印 null，我做错了什么？ 谢谢。 这是我的代码：

  List<UserFontModel> _fonts = [
    UserFontModel(fontFamily: 'Regular', fontWeight: FontWeight.w400),
    UserFontModel(fontFamily: 'Bold', fontWeight: FontWeight.w700),
    UserFontModel(fontFamily: 'Medium', fontWeight: FontWeight.w500),
    UserFontModel(fontFamily: 'Light', fontWeight: FontWeight.w300),
    UserFontModel(fontFamily: 'Thin', fontWeight: FontWeight.w100),
  ];

  String _selectedFontStyle;

    new DropdownButton<String>(
                      value: _selectedFontStyle,
                      hint: Text('Style'),
                      items: _fonts.map((fonts) => DropdownMenuItem<String> (
                        child: Container(
                            width: MediaQuery.of(context).size.width * 0.2,
                          child: Text(fonts.fontFamily, style: TextStyle(fontWeight: fonts.fontWeight),),
                        ),
                      )).toList(),
                      onChanged: (String _) {
                          print(_);
                      },
                    ),

当我选择一个项目时，我会看到：

flutter: null

有人可以帮忙吗？

Answer 1

请试试这个。

List<UserFontModel> _fonts = [
    UserFontModel(fontFamily: 'Regular', fontWeight: FontWeight.w400),
    UserFontModel(fontFamily: 'Bold', fontWeight: FontWeight.w700),
    UserFontModel(fontFamily: 'Medium', fontWeight: FontWeight.w500),
    UserFontModel(fontFamily: 'Light', fontWeight: FontWeight.w300),
    UserFontModel(fontFamily: 'Thin', fontWeight: FontWeight.w100),
  ];

  String _selectedFontStyle;

DropdownButton<String>(
  hint: Text("Style"),
  value: _selectedFontStyle,
  onChanged: (String Value) {
    setState(() {
       _selectedFontStyle = Value;
      });
     },
  items: _fonts.map((fonts) {
    return  DropdownMenuItem<String>(
        value: fonts.fontFamily,
        child: new Container(
          width: MediaQuery.of(context).size.width * 0.2,
          child: Text(fonts.fontFamily, style: TextStyle(fontWeight: fonts.fontWeight),),
        )
    );
  }).toList(),
),

Answer 2

DropdownMenuItem 返回它的值字段。 所以你需要为孩子增加价值。

像这样尝试。

new DropdownButton<String>(
                        value: _selectedFontStyle,
                        hint: Text('Style'),
                        items: _fonts.map((fonts) => DropdownMenuItem<String> (
                          child: Container(
                            width: MediaQuery.of(context).size.width * 0.2,
                            child: Text(fonts.fontFamily, style: TextStyle(fontWeight: fonts.fontWeight),),
                          ),
                          value: fonts.fontFamily, // add this line
                        )).toList(),
                        onChanged: (String _) {
                          print(_);
                        },
                      ),

Answer 3

这对我有用：

    new DropdownButton<UserFontModel>(
      value: _fonts[0],
      hint: Text('Style'),
      items: _fonts
          .map<DropdownMenuItem<UserFontModel>>((UserFontModel font) {
        return DropdownMenuItem<UserFontModel>(
          value: font,
          child: Text(
            font.fontFamily,
            style: TextStyle(fontWeight: font.fontWeight),
          ),
        );
      }).toList(),
      onChanged: (UserFontModel _) {
        print(_.fontFamily);
      },
    ),