Question

好的，我在使用FutureProvider时会遇到这个问题。

我已经在MaterialApp上方创建了FutureProvider，所以应该正确识别它？

我的小部件树有点像这样：

MyApp >> Home >> CardVehicle

这是我的main.dart代码，在其中创建对象提供程序：

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  final Service service = Service();

  @override
  Widget build(BuildContext context) {
    return FutureProvider(
      create: (_) => service.fetchCarYear(),
      catchError: (_, error) => print(error),
      child: MaterialApp(
        title: 'KPM Demo',
        theme: ThemeData(
          primarySwatch: Colors.amber,
          visualDensity: VisualDensity.adaptivePlatformDensity,
        ),
        home: Home(),
      ),
    );
  }
}

我的虚拟服务类：

class Service {
  Future<CarYear> fetchCarYear() async {
    CarYear carYear = CarYear();

    final response = await Future.delayed(
      Duration(milliseconds: 500),
      () => jsonEncode({
        "data": [
          {"year": "2020"},
          {"year": "2019"},
          {"year": "2018"}
        ]
      }),
    );

    carYear = CarYear.fromJson(jsonDecode(response));

    return carYear;
  }
}

在这里放置我的提供者：

class CardVehicle extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    CarYear carYear = Provider.of<CarYear>(context);

    return Container(
      padding: EdgeInsets.all(20),
      decoration: BoxDecoration(
        borderRadius: BorderRadius.circular(10),
        color: Colors.white,
      ),
      child: Column(
        children: <Widget>[
          DropdownButton(
            isExpanded: true,
            icon: Icon(Icons.keyboard_arrow_down),
            items: carYear.data
                .map((item) => DropdownMenuItem(child: Text(item.year)))
                .toList() ?? null,
            onChanged: null,
          ),
        ],
      ),
    );
  }
}

我在某个地方犯了错误吗？请帮忙！

编辑：这是我的CarYear课：

class CarYear {
  List<Data> data;

  CarYear({this.data});

  CarYear.fromJson(Map<String, dynamic> json) {
    if (json['data'] != null) {
      data = new List<Data>();
      json['data'].forEach((v) {
        data.add(new Data.fromJson(v));
      });
    }
  }

  Map<String, dynamic> toJson() {
    final Map<String, dynamic> data = new Map<String, dynamic>();
    if (this.data != null) {
      data['data'] = this.data.map((v) => v.toJson()).toList();
    }
    return data;
  }
}

class Data {
  String year;

  Data({this.year});

  Data.fromJson(Map<String, dynamic> json) {
    year = json['year'];
  }

  Map<String, dynamic> toJson() {
    final Map<String, dynamic> data = new Map<String, dynamic>();
    data['year'] = this.year;
    return data;
  }
}

Answer 1

最好与ChangeNotifierProvider一起使用

在使用MultiProvider创建的主文件中

  @override
  Widget build(BuildContext context) {
    return MultiProvider(
      providers: [
        ChangeNotifierProvider(create: (context) => Service()),
      ],
      child: MaterialApp(..)
    )
  }

在我添加的extends ChangeNotifiere服务类中 notifyListeners()通话时，所有使用者均会更新

class Service extends ChangeNotifier {

  // Variable set in Service
  CarYear _carYear = new CarYear();
  CarYear get carYear => _carYear;

  // init MultiProvider create() =>
  Service() {
    fetchCarYear();
  }

  Future<CarYear> fetchCarYear() async {
    _carYear = CarYear();

    final response = await Future.delayed(
      Duration(milliseconds: 500),
          () => jsonEncode({
        "data": [
          {"year": "2020"},
          {"year": "2019"},
          {"year": "2018"}
        ]
      }),
    );

    // update _carYear Variable
    _carYear = CarYear.fromJson(jsonDecode(response));

    // all Consumer get a refresh()
    notifyListeners();
  }

}

在CardVehicle中，我放置了一个Consumer，但是CarYear carYear = Provider.of<Service>(context).carYear也可以工作。但是使用Provider.of，您每notifyListeners()都会更新整个小部件

class CardVehicle extends StatelessWidget {
  @override
  Widget build(BuildContext context) {

    return Container(
      padding: EdgeInsets.all(20),
      decoration: BoxDecoration(
        borderRadius: BorderRadius.circular(10),
        color: Colors.white,
      ),
      child: Column(
        children: <Widget>[

          // its better you do it here with a Consumer, because not the whole widget is updating
          new Consumer<Service>(
              builder: (context, service, child) => new DropdownButton(
                  isExpanded: true,
                  icon: Icon(Icons.keyboard_arrow_down),
                  items: service.carYear.data
                      .map((item) => DropdownMenuItem(child: Text(item.year), value: item.year,))
                      .toList(),
                  onChanged: (value) {
                    print(value);
                  },
                ),
            ),
        ],
      ),
    );
  }

}

Answer 2

Comming from the other question you posted.

您需要更改catchError中的futureProvider，以返回 CarYear 而不是void。现在futureProvider的类型为Dynamic，而provider.of的搜索对象不是 dynamic 的提供者，而是 CarYear 的提供者。

为避免这种情况，请使catchError返回一个空的CarYear或使其引发异常。

我希望这会有所帮助！

Answer 3

仅适用于FutureProvider的示例

MultiProvider(
  providers: [
    FutureProvider(create: (_) => Service().fetchCarYear()),
  ],
  child: MaterialApp(..)

返回CarYear的服务类

class Service {

  // Variable set in Service
  CarYear _carYear;

  Future<CarYear> fetchCarYear() async {
    _carYear = CarYear(data: new List());

    final response = await Future.delayed(
      Duration(milliseconds: 500),
          () => jsonEncode({
        "data": [
          {"year": "2020"},
          {"year": "2019"},
          {"year": "2018"}
        ]
      }),
    );

    // update _carYear Variable
    _carYear = CarYear.fromJson(jsonDecode(response));

    // all Consumer get a refresh()
    //notifyListeners();
    return _carYear;
  }

}

和带有CarYear carYear = Provider.of<CarYear>(context);的CardVehicle

class CardVehicle extends StatelessWidget {
  @override
  Widget build(BuildContext context) {

    CarYear carYear = Provider.of<CarYear>(context);

    return Container(
      padding: EdgeInsets.all(20),
      decoration: BoxDecoration(
        borderRadius: BorderRadius.circular(10),
        color: Colors.white,
      ),
      child: Column(
        children: <Widget>[
          new DropdownButton(
            isExpanded: true,
            icon: Icon(Icons.keyboard_arrow_down),
            items: carYear != null ? carYear.data
                .map((item) => DropdownMenuItem(child: Text(item.year), value: item.year,))
                .toList() : null,
            onChanged: (value) {
              print(value);
            },
          ),
        ],
      ),
    );
  }

}

在此小部件上方找不到错误的正确提供者

3 个答案: