Flutter-DropdownButton onChanged返回null

如何解决Flutter-DropdownButton onChanged返回null

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部分中打印该值时，它都会打印null，这是我做错了什么？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，我会显示：

flutter: null

有人可以帮忙吗？

解决方法

请尝试这个。

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(),

,

DropdownMenuItem返回它的值字段。因此，您需要为孩子增加价值。

尝试这样。

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

,

这对我有用：

    new DropdownButton<UserFontModel>(
      value: _fonts[0],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);
      },