带JSON的颤振迭代图

我试图用API JSON数据显示一个图表。我做了一些迭代并将结果保存在一个全局变量上,然后将这个变量放在图表中,但变量返回空值,没有任何数据。如果我在foreach中打印列表有数据,但如果我在widget中打印,则为空。我读 this 但代码看起来很不一样,所以我无法理解。

变量

  List<charts.Series<TimeSeriesSales, DateTime>> seriesList;
  List<TimeSeriesSales> data = [];

获取数据

final String url = 'URL';

class TimeSeriesSales {
  final DateTime fecha;
  final double valor;

  TimeSeriesSales(this.fecha, this.valor);
}


 Future<Null> getUserDetails() async {
    final response = await http.get(
      url,
      headers: {
        HttpHeaders.authorizationHeader:
            "HEADER "
      },
    );
    final responseJson = json.decode(response.body);   
      setState(() {
      for (Map user in responseJson) {
          var fecha= new DateTime.fromMillisecondsSinceEpoch(user['fecha']).toLocal();
          data.add(TimeSeriesSales(new DateTime(fecha.year, fecha.month,fecha.day), user['valor']));
          print (data.toString());
      }
    });
  }

初始状态

void initState() {
    super.initState();
    getUserDetails();
    seriesList = _createSampleData();
    animate = false;
     }

填充时间

 List<charts.Series<TimeSeriesSales, DateTime>> _createSampleData() {
    return [
      new charts.Series<TimeSeriesSales, DateTime>(
        id: 'Sales',
        colorFn: (_, __) => charts.MaterialPalette.blue.shadeDefault,
        domainFn: (TimeSeriesSales valor, _) => valor.fecha,
        measureFn: (TimeSeriesSales valor, _) => valor.valor,
        data: data,
      )
    ];
  }
}

小装置

 Widget build(BuildContext context) {
    return new Scaffold(
   new Expanded(
          child: new charts.TimeSeriesChart(
            seriesList,
            animate: animate,
            defaultRenderer: new charts.BarRendererConfig<DateTime>(),
            domainAxis: new charts.DateTimeAxisSpec(usingBarRenderer: true),
            defaultInteractions: false,
            behaviors: [
              new charts.SelectNearest(),
              new charts.DomainHighlighter()
            ],
          ),
        ),

)

有人知道为什么列表在widget中没有数据吗?有人知道在init上迭代图表的正确方法是什么?