关于flutter:Flutter中关于使用异步时获得FutureT中数据的问题

应用异步取得 Future<T>

在 flutter 我的项目中应用 async await 异步获取数据时，会返回一个 Future<T> 类型的数据，然而将其应用在 widget 中，会有一个问题：在 statefulWidget 中应用变量接管值，会呈现无奈赋值的状况

///index.dart
  class Index extends StatefulWidget {Index({Key key}) : super(key: key);

  @override

  _IndexState createState() => _IndexState();

}

 class _IndexState extends State<Index> {

  List<Widget> _temps;

  String _studentId = "abcd";

  String _token =

      "lkjncasfmalknfadwSadwawf1dawd";

  @override

  void initState() {super.initState();

    _temps = _getClassItems(getOneDaylessons(_dates, _studentId, _token));

  }

  var _dates = initDate();

  void _getDates(value) {_dates = DateFormat('EEE', "en_US").format(value).toString();

    setState(() {_temps = _getClassItems(getOneDaylessons(_dates, _studentId, _token));

    });

  }

  List<Widget> _getClassItems(List<Widget> classItems) {if (classItems.isEmpty) {

      return [

        Container(

          height: 60,

          child: Center(child: Text("今日暂无课程"),

          ),

        )

      ];

    } else {return classItems;}

  }
  
   @override

  Widget build(BuildContext context) {
    return Scaffold(

      body: Container(

        child:
                  Column(children: _temps,// 报错处),
                 )
            )
   }
  
///getOneDaylessons.dart
  List<Widget> getOneDaylessons(String date, String studentId, String token) {

  ClassResponseData _data;
  getTimeTableFromStudentID(studentId, token)

      .then((v) => {_data=v});

  List<ClassItemViewModel> datas =

      getADayClasses(data);

  List temp = datas

      .map((e) {if (e.date == date) {return ClassItem(data: e);

        }

      })

      .toList()

      .where((element) => element != null)

      .toList();

  return temp;

}

// 运行我的项目报错_temps 是 null

解决方案

将 getOneDaylessons 同样应用 async 包装，返回 Future<T> 到 index.dart. 在 index.dart 中，应用 FutureBuilder，取值

///getOneDaylessons.dart
Future<List<Widget>> getOneDaylessons(String date, String studentId, String token) async {
  ClassResponseData response =
      await getTimeTableFromStudentID(studentId, token);
  List<ClassItemViewModel> datas = getADayClasses(response);

  List temp = datas
      .map((e) {if (e.date == date) {return ClassItem(data: e);
        }
      })
      .toList()
      .where((element) => element != null)
      .toList();
  return temp;
}
///index.dart
...
_temps = _getClassItems();
...
Future<List<Widget>> _getClassItems() async {
    List<Widget> classItems =
        await getOneDaylessons(this._dates, this._studentId, this._token);
    if (classItems.isEmpty) {
      return [
        Container(
          height: 60,
          child: Center(child: Text("今日暂无课程"),
          ),
        )
      ];
    } else {return classItems;}
  }
 @override

  Widget build(BuildContext context) {
    return Scaffold(

      body: Container(

        child:
                  Container(
                    child: FutureBuilder(
                        future: _temps,
                        builder:
                            // ignore: missing_return
                            (BuildContext context, AsyncSnapshot snapshot) {
                          if (snapshot.connectionState ==
                              ConnectionState.done) {if (snapshot.hasError) {return Text(snapshot.error.toString());
                            }
                            return Column(children: snapshot.data);
                          }
                          return CircularProgressIndicator();}),
                  ),
                 )
   }