NoSuchMethodError: Invalid member on null:'length'

Question

我尝试按日期过滤交易。用户选择日期后，数据将自动过滤，但我收到一个错误，即 NoSuchMethodError: Invalid member on null:'length'。我知道列表有问题，但由于我传递值的方式或比较日期的方式，我不确定它。该列表通过 GET 请求从数据库中获取数据。我测试了 GET 请求并在无状态小部件上显示数据，它运行良好。

class BudgetDateRangePicker extends StatefulWidget {
  final isSelected = <bool>[true, false];
  final List<Transaction> transactions;
  BudgetDateRangePicker({Key key, this.transactions}) : super(key: key);
  @override
  State<StatefulWidget> createState() => _BudgetDateRangePickerState();
}

class _BudgetDateRangePickerState extends State<BudgetDateRangePicker> {
  DateTime _toDate = DateTime.now();
  DateTime _fromDate = DateTime.utc(2020, 1, 1);
  List<Transaction> trans;
  Future<void> _selectDate(BuildContext context) async {
    final List<DateTime> picked = await DateRangePicker.showDatePicker(
        context: context,
        initialLastDate: new DateTime.now(),
        initialFirstDate: (new DateTime.utc(2020, 1, 1)),
        firstDate: new DateTime(2015),
        lastDate: new DateTime(DateTime.now().year + 2));
    if (picked != null && picked.length == 2) {
      setState(() {
        _fromDate = picked[0];
        _toDate = picked[1];
        for (var item in widget.transactions) {
          if (DateTime.parse(item.date).isBefore(_toDate) &&
              DateTime.parse(item.date).isAfter(_fromDate)) {
            trans.add(item);
          }
        }
      });
    }
  }

  @override
  Widget build(BuildContext context) {
    return Row(mainAxisAlignment: MainAxisAlignment.end, children: <Widget>[
      Container(
          margin: const EdgeInsets.only(right: 10),
          child: Text("From: ${DateFormat.yMMMd().format(_fromDate)}")),
      Container(
        margin: const EdgeInsets.only(right: 10),
        child: Text("To: ${DateFormat.yMMMd().format(_toDate)}"),
      ),
      Container(
        margin: const EdgeInsets.only(right: 10),
        child: RaisedButton(
          color: Colors.green[200],
          shape: RoundedRectangleBorder(
            borderRadius: new BorderRadius.circular(6.0),
            side: BorderSide(color: Colors.white),
          ),
          onPressed: () => _selectDate(context),
          child: Text('Select date'),
        ),
      ),
      ListView.separated(
        separatorBuilder: (context, index) {
          return Divider(color: Colors.grey[400]);
        },
        itemCount: trans.length, // this one has error
        itemBuilder: (context, index) {
          final transaction = trans[index];

          return ListTile(
            contentPadding: EdgeInsets.all(10),
            title: Column(
                crossAxisAlignment: CrossAxisAlignment.start,
                children: <Widget>[
                  Text("${transaction.category}",
                      style:
                          TextStyle(fontSize: 24, fontWeight: FontWeight.w500)),
                  Text("${transaction.date}",
                      style: TextStyle(color: Colors.grey[500], fontSize: 14)),
                ]),
            trailing: Column(children: <Widget>[
              Text("${transaction.amount}",
                  style: TextStyle(fontSize: 24, fontWeight: FontWeight.w500)),
            ]),
          );
        },
      )
    ]);
  }
}

Future<List<Transaction>> fetchBudgetTrans(http.Client client) async {
  final response =
      await client.get(Uri.parse('http://127.0.0.1:8000/budget/trans'));
  return compute(parseDataBudget, response.body);
}

Answer 1

尝试初始化您的列表，这样如果没有应用过滤器，它就不会导致空引用错误

 List<Transaction> trans = [];

NoSuchMethodError: Invalid member on null:'length'

NoSuchMethodError: Invalid member on null:'length'

future

list

flutter