DropdownButton 无法更新它的值（空）

Question

The full page code is very long but my DropdownButton widget code like this.

The problems are,

first: I can't update my selectedCity, it doesn't get an update. Also, the print function calls null, since my cityList data is like [new york, paris, london] etc...

second: flutter doesn't change focus from any TextField to DropdownButton fully. I mean, clicked TextField, then DropdownButton but focus reverts to that TextField after the button click. It is default action of Flutter?

  List<dynamic> _cityList;
  String _selectedCity;

  @override
  Widget build(BuildContext context) {
    return DropdownButton(
      value: _selectedCity,
      style: TextStyle(
        fontSize: 11,
        color: textColor,
      ),
      items: _cityList.map((city) {
        return DropdownMenuItem<String>(
          child: Padding(
            padding: const EdgeInsets.only(left: 4),
            child: Text(city),
          ),
        );
      }).toList(),
      onChanged: (String value) {
        setState(() {
          _selectedCity = value;
          print(_selectedCity);
        });
      },
      isExpanded: true,
    );
  }

Edit: The solution of resetting FocusNode after selecting an item from DropdownMenuItem is adding this line inside of setstate like this:

this: FocusScope.of(context).requestFocus(new FocusNode());

to here: onChanged:(){setSate((){here}}

Answer 1

对于焦点问题，您应该使用带有下拉列表的focusNodes ，另一个带有文本字段https://docs.flutter.io/flutter/widgets/FocusNode-class.html 。

Answer 2

I hope it will help you. I have modified your code a little bit

 List<dynamic> _cityList;
 String _selectedCity;

It will show the Dropdown Button and when you click on it and select any value showing in the print

 @override
  Widget build(BuildContext context) {
    // This method is rerun every time setState is called, for instance as done
    // by the _incrementCounter method above.
    //
    // The Flutter framework has been optimized to make rerunning build methods
    // fast, so that you can just rebuild anything that needs updating rather
    // than having to individually change instances of widgets.
    return Scaffold(
      body: ListView(
        children: [
          Column(
            children: <Widget>[
              DropdownButton<String>(
                items: _cityList.map((dynamic value) {
                  return DropdownMenuItem<String>(
                    value: value,
                    child: new Text(value),
                  );
                }).toList(),
                onChanged: (value) {
                  setState(() {
                    _selectedCity = value;
                    print(_selectedCity);
                  });
                },
              ),
            ],
          ),
        ],
      ),
    );
  }