更改DropDownButton選擇時出現顫動錯誤

Question

錯誤：

Failed assertion: line 609 pos 15: 'items == null ||
        I/flutter (24295): items.isEmpty || value == null || items.where((DropdownMenuItem<T> item) => item.value ==
        I/flutter (24295): value).length == 1': is not true.

我做了一些研究，但沒弄清楚導致問題的原因。 這是我的代碼：

class StatusList extends StatefulWidget {
  @override
  _StatusListState createState() => _StatusListState();
}

class _DispositionListState extends State<DispositionList> {

  var _currentSelectedValue = '';

  Future<RecordList> recordList;

  @override
  void initState() {
    recordList = getRecord();
    super.initState();
  }

  int i = 1;
  List<String> statusList = List<String>();

  @override
  Widget build(BuildContext context) {
    return FutureBuilder<RecordList>(
        future: recordList,
        builder: (context, snapshot) {
          if (snapshot.hasData) {
            String current = snapshot.data.record[0].status.trim();
            statusList.add(snapshot.data.record[0].status.trim());

            while (i < snapshot.data.record.length) {
              if (snapshot.data.record[i].status.trim() != current) {
                statusList.add(snapshot.data.record[i].status.trim());
                current = snapshot.data.record[i].status.trim();
              }
              i++;
            }
            _currentSelectedValue = statusList[0]; //not set this will straight getting error

            return DropdownButton(
              items: statusList.map((String dropDownStringItem) {
                return DropdownMenuItem<String>(
                    value: dropDownStringItem,
                    child: SizedBox(
                      width: 200.0,
                      child: Text(
                        dropDownStringItem,
                        overflow: TextOverflow.ellipsis,
                      ),
                    ));
              }).toList(),
              onChanged: (String valueSelected) {
                onDropDownSelected(valueSelected);
              },
              value: _currentSelectedValue,
            );
          } else if (snapshot.hasError) {
            return Text("${snapshot.error}");
          }
          return CircularProgressIndicator();
        });
  }

  void onDropDownSelected(String valueSelected) {
    setState(() {
      this._currentSelectedValue = valueSelected;
    });
  }
}

我嘗試比較recordList和onDropDownSelected中的valueSelected返回true。 甚至在沒有任何代碼的情況下調用setState時引起的問題。 誰知道基於這些代碼導致問題的原因？

補充代碼：這是我的getRecord（）

Future<RecordList> getRecord() async {
  String url = 'some url';
  final response = await http.get(url, headers: {"Accept": "application/json"});

  if (response.statusCode == 200) {
    return RecordList.fromJson(json.decode(response.body));
  } else {
    throw Exception('Failed to load post');
  }
}

class Record {
  final String status;
  final String disposition;
  final int total;

  Record({this.status, this.disposition, this.total});

  factory Record.fromJson(Map<String, dynamic> json) {
    return Record(
        status: json['status'],
        disposition: json['disposition'],
        total: json['total']);
  }
}

class RecordList {
  final List<Record> record;

  RecordList({this.record});

  factory RecordList.fromJson(List<dynamic> parsedJson) {
    List<Record> record = new List<Record>();
    record = parsedJson.map((i) => Record.fromJson(i)).toList();
    return new RecordList(
      record: record,
    );
  }
}

Answer 1

可能是DropdownButton的value屬性的值不是item的值之一。 value屬性應為null或項目值之一。

此外，也許你不需要這里的FutureBuilder 。 你可以這樣做：

class _DispositionListState extends State<DispositionList> {
  bool _isLoading = true;
  String _currentSelectedValue;
  List<String> statusList = List<String>();

  @override
  void initState() {
    super.initState();
    _loadStatusList();
  }

  _loadStatusList() async {
    final recordList = await getRecord();
    final list = recordList.record.map((r) { 
      return r.status.trim();
    }).toSet().toList();
    setState(() {
      statusList = list;
      _currentSelectedValue = list.first;
      _isLoading = false;
    });
  }

  @override
  Widget build(BuildContext context) {
    return isLoading ? CircularProgressIndicator() : _buildDropdown();
  }

  Widget _buildDropdown() {
    return DropdownButton(
      items: statusList.map((dropDownStringItem) {
        return DropdownMenuItem<String>(
          value: dropDownStringItem,
          child: SizedBox(
            width: 200.0,
            child: Text(
              dropDownStringItem,
              overflow: TextOverflow.ellipsis,
            ),
          ),
        );
      }).toList(),
      onChanged: (valueSelected) {
        onDropDownSelected(valueSelected);
      },
      value: _currentSelectedValue,
    );
  }

  void onDropDownSelected(String valueSelected) {
    setState(() {
      this._currentSelectedValue = valueSelected;
    });
  }
}

更新：添加.toSet()以從statusList過濾掉重復statusList

我認為問題是狀態列表有重復的項目。 我添加.toSet()之前.toList()過濾掉重復。 Set是一組對象，其中每個對象只能出現一次。