我在MyDropDownState类中创建了一个reset函数：

void reset(){
    setState((){
        _selection = null;
    });
}

这会将选择设置为null并设置下拉列表的状态,从而有效地重置下拉列表.

问题的核心是当按下AppBar上的IconButton时我必须调用此函数.我尝试了多种方法,但无法访问我创建的MyDropDown类的状态.

这是MyDropDown的代码,它的状态,简化：

class MyDropDown extends StatefulWidget {

  final Map<String,String> _itemMap;

  MyDropDown(this._itemMap);

  @override
  MyDropDownState createState() => new MyDropDownState();
}

class MyDropDownState extends State<MyDropDown> {

  String _selection;

  void reset(){
    setState((){
      _selection = null;
    });
  }

  @override
  void initState() {
    _selection = null;
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
      return new DropdownButton(
          value: _selection,//getDropItems builds dropdown items
          items: getDropItems(widget._itemMap),onChanged: (s) {
            setState(() {
              _selection = s;
            });
          },);
  }

}

在我的主页面中,我创建了一个新的MyDropDown

final MyDropDown cityDropdown = new MyDropDown(cityLookup);

然后这是AppBar(在一个脚手架内),它持有我要按下的IconButton来重置Dropdown.

appBar : new AppBar(
    title: new Text('Filter Jobs'),actions: <Widget>[
      new IconButton(
          icon: new Icon(Icons.refresh),onPressed: () {
            print('Reset dropdowns');
            //this is where I would call reset() on cityDropdown's state,if I could figure out how to get to it :/
          },),],