flutter - flutter 正确打印有 future map 的 map

Question

有人可以提供此代码帮助吗？
我必须在下拉列表中将查询结果打印为列表
错误 flutter 回给我的是:

The following NoSuchMethodError was thrown buildingDropDownFormField(dirty, dependencies: [_FormScope], state:FormFieldState#e264a): Class 'Future<List>' has noinstance method '[]'. Receiver: Instance of 'Future<List>'Tried calling: The relevant error-causing widget was:
DropDownFormField

我已经尝试过getAllBrockers和getAllBrockers2的方式

class _AutomationAddScreen extends State<AutomationAdd> {
  DataRepository _repository;
  final _formKey = GlobalKey<FormState>();
  String _broN = "";
  Future<Map> _broMap;
  Future<List> _bro;

  static Future<List> getAllBrockers2(Future<List<Brockers>> l) async {
    List q;
    List<Brockers> b = await l;
    for (var i = 0; i < b.length; i++) {
      q.add(DataList(b[i].name, b[i].id));
    }
    return q;
  }

  static Future<Map> getAllBrockers(Future<List<Brockers>> l) async {
    Map hBro;
    Map m;
    List<Brockers> b = await l;
    for (var i = 0; i < b.length; i++) {
      m = {"display": b[i].name, "value": b[i].id};
      hBro.addAll(m);
    }
    return hBro;
  }

  @override
  void initState() {
    super.initState();
    _repository = SqlRepository();
    //_broMap = getAllBrockers(_repository.getAllBrockers());
    _bro = getAllBrockers2(_repository.getAllBrockers());
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        body: Center(
          child: Form(
            key: _formKey,
            child: ListView(
              children: <Widget>[
                //SELECT BROCKER
                Padding(
                  padding: EdgeInsets.only(top: 16),
                  child: DropDownFormField(
                    titleText: 'Connessione',
                    hintText: 'Selezionane uno',
                    value: _broN,
                    //dataSource: [_broMap],
                    dataSource: [_bro],
                    textField: 'display',
                    valueField: 'value',
                    onChanged: (display) {
                      setState(() {
                        _broN = display.toString();
                      });
                    },
                  ),
                ),
              ],
            ),
          ),
        ));
  }
}

class DataList {
  String display;
  int value;

  DataList(this.display, this.value);

  @override
  String toString() {
    return '{ ${this.display}, ${this.value} }';
  }
}

Answer 1

您应该使用Future<List>或Future<Map>而不是List或Map。该错误专门提到它无法在[]上调用Future<List>。DropdownFormField无法创建其项列表，因为您提供了Future。
您可以尝试使用以下方法-

class _AutomationAddScreen extends State<AutomationAdd> {
  DataRepository _repository;
  final _formKey = GlobalKey<FormState>();
  String _broN = "";
  Map _broMap; // New
  List _bro = List<DataList>(); // New

  static Future<void> getAllBrockers2() async { // New
    final b = await _repository.getAllBrockers(); // New
    for (var i = 0; i < b.length; i++) {
      _bro.add(DataList(b[i].name, b[i].id)); // New
    }
    setState(() {}); // New
  }

  static Future<Map> getAllBrockers(Future<List<Brockers>> l) async {
    Map hBro = {}; // New
    Map m;
    List<Brockers> b = await l;
    for (var i = 0; i < b.length; i++) {
      m = {"display": b[i].name, "value": b[i].id};
      hBro.addAll(m);
    }
    return hBro;
  }

  @override
  void initState() {
    super.initState();
    _repository = SqlRepository();
    //_broMap = getAllBrockers(_repository.getAllBrockers());
    getAllBrockers2(); // New
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        body: Center(
          child: Form(
            key: _formKey,
            child: ListView(
              children: <Widget>[
                //SELECT BROCKER
                Padding(
                  padding: EdgeInsets.only(top: 16),
                  child: DropDownFormField(
                    titleText: 'Connessione',
                    hintText: 'Selezionane uno',
                    value: _broN,
                    //dataSource: [_broMap],
                    dataSource: [_bro],
                    textField: 'display',
                    valueField: 'value',
                    onChanged: (display) {
                      setState(() {
                        _broN = display.toString();
                      });
                    },
                  ),
                ),
              ],
            ),
          ),
        ));
  }
}

class DataList {
  String display;
  int value;

  DataList(this.display, this.value);

  @override
  String toString() {
    return '{ ${this.display}, ${this.value} }';
  }
}