NoSuchMethodError：null 上的无效成员：'length'

Posted 2023-02-23

【中文标题】NoSuchMethodError：null 上的无效成员：\'length\'

【英文标题】：NoSuchMethodError: Invalid member on null:'length'NoSuchMethodError：null 上的无效成员：'length'

【发布时间】：2021-07-18 18:28:30

【问题描述】：

我尝试按日期过滤交易。用户选择日期后，将自动过滤数据，但出现 NoSuchMethodError: Invalid member on null:'length' 错误。 我知道列表有问题，但我不确定，因为我传递值的方式或比较日期的方式。 该列表通过 GET 请求从 DB 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);

【参考方案1】：

尝试初始化您的列表，因此如果没有应用过滤器，则不会导致空引用错误

 List<Transaction> trans = [];

【讨论】：

如果解决了您的问题，请接受答案（将问题标记为已回答）。

抛出了另一个异常：无效参数：null，我从 ```new FutureBuilder>(future: fetchBudgetTrans(new http.Client()),```得到了这个错误