提示不适用于 Flutter 中的 DropdownButtonFormField

Question

我在 Flutter 中有一个带有 textformfield 和 dropdownbuttonformfield 的表单。运行我的应用程序提示对我不起作用。 提示未显示在下拉按钮表单字段中。它显示 kkk 作为初始值，而不显示 选择城市。

我在这里使用 StatefulWidget。

帮我解决这个问题。 预先感谢。

class _AddTripState extends State<AddTrip> {
  var formKey = GlobalKey<FormState>();
  TextEditingController nameController = TextEditingController();  
  TextEditingController numberController = TextEditingController();
  TextEditingController datecontroller = TextEditingController();
  final format = DateFormat("yyyy-MM-dd");
  DateTime _dateTime;
  List<String> name = ['kkk', 'rrr'];
  String _dropdownvalue = 'kkk';

@override
Widget build(BuildContext context) {
  return Scaffold(
  appBar: AppBar(
    title: Text("Add Trip"),
  ),
  body: Form(
      key: formKey,
      child: ListView(
        children: <Widget>[
          Text(
            'Name',
            textAlign: TextAlign.left,
          ),
          TextFormField(
            decoration: _inputDecoration(),
            keyboardType: TextInputType.text,
            controller: nameController,
            validator: textValidator,
          ),
          Text(
            'Number',
            textAlign: TextAlign.left,
          ),
          TextFormField(
            decoration: _inputDecoration(),
            controller: numberController,
            keyboardType: TextInputType.number,
            validator: numberValidator,
          ),
          Text(
            'Date',
            textAlign: TextAlign.left,
          ),
          TextFormField(
            readOnly: true,
            controller: datecontroller,
            validator: dateValidator,
            decoration: InputDecoration(
              border: OutlineInputBorder(),
              errorBorder: OutlineInputBorder(
                  borderSide: BorderSide(color: Colors.amber)),
              errorStyle: TextStyle(color: Colors.amber),
              suffixIcon: GestureDetector(
                child: Icon(
                  Icons.date_range,
                ),
                onTap: () {
                  showDatePicker(
                          context: context,
                          initialDate: DateTime.now(),
                          firstDate: DateTime(2018),
                          lastDate: DateTime(2020))
                      .then((value) {
                    setState(() {
                      datecontroller.text =
                          DateFormat("yyyy-MM-dd").format(value);
                    });
                  });
                },
              ),
            ),
          ),
          DropdownButtonFormField<String>(
            hint: Text('Select City'),
            validator: _cityValidator,
            decoration: InputDecoration(border: OutlineInputBorder()),
            items: name.map((value) {
              return DropdownMenuItem<String>(
                  child: Text(value), value: value);
            }).toList(),
            value: _dropdownvalue,
            onChanged: (newValue) {
              setState(() {
                _dropdownvalue = newValue;
              });
            },
          ),
          RaisedButton(
            onPressed: submit,
            child: Text('Add Trip'),
          )
        ],
      )),
);
}

我哪里错了？

Answer 1

如果

value

为空，则会显示提示。

所以在你的代码中 - 将 -

String _dropdownvalue = 'kkk';

更改为 -

String _dropdownvalue;

Answer 2

您需要使用 InputDecoration 来实现此目的：

 decoration: InputDecoration(labelText:'Select City'),

Answer 3

使 dropdownValue 可为空并设置提示文本以显示默认初始值。

DropdownButton<String>(
            value: dropdownValue,
            style: const TextStyle(color: Colors.black),
            underline: Container(
              height: 2,
              color: Colors.black,
            ),
            items: list.map<DropdownMenuItem<String>>((String value) {
              return DropdownMenuItem<String>(
                value: value,
                child: Text(value),
              );
            }).toList(),
            hint: const Text(
              "Select Item",
              style: TextStyle(
                  color: Colors.black,
                  fontSize: 16,
                  fontWeight: FontWeight.w600),
            ),
            onChanged: (String? value) {
              // This is called when the user selects an item.
              setState(() {
                dropdownValue = value!;
              });
            },
          )

列表在哪里

const List<String> list = <String>['One', 'Two', 'Three', 'Four'];

并在build()上面定义dropdownValue可为空

String? dropdownValue; // This is IMP