关于 Flutter DropdownButton 值的问题

如何解决关于 Flutter DropdownButton 值的问题

您好，我是 Flutter 新手，我正在尝试根据 Firebase 后端中的不同文档创建多个 DropdownButton。由于可能有多个 DropdownButton（不同的文档有不同数量的下拉列表要创建），我想用 List<String> formInput 维护这些值（formInput[0] 维护第一个 DropdownButton 值等）。

但我一直遇到这个错误，并得出结论是formInput[i] 错误。所以我的问题是为什么我不能使用 formInput[i] 作为下拉按钮的值？ formInput[i] 不是一件物品吗？是否有更好的解决方案来维护未知数量的值？

非常感谢

代码：

class _SubmissionFormState extends State<SubmissionForm> {
  List<String> formInput;
  var test;
  @override
  Widget build(BuildContext context) {
    Size size = MediaQuery.of(context).size;
    return FutureBuilder(
      future: getStringFromPref('site'),builder: (context,snapshot) {
        if (snapshot.hasData) {
          String site = snapshot.data;
          return StreamBuilder(
              stream: FirebaseFirestore.instance
                  .collection('form')
                  .where('name',isEqualTo: site)
                  .snapshots(),snapshot) {
                if (snapshot.hasData) {
                  int inputSize = snapshot.data.documents.length;
                  List<Widget> listofWidget = [];
                  formInput = new List<String>.filled(inputSize,'-');
                  print(formInput.length);

                  for (int i = 0; i < inputSize; i++) {
                    DocumentSnapshot snap = snapshot.data.documents[i];
                    List<DropdownMenuItem> options = [];
                    // adding drop down menu items
                    for (int j = 0; j < snap['formVal'].length; j++) {
                      String formVal = snap['formVal'][j];
                      options.add(DropdownMenuItem(
                        child: Text(
                          formVal,),value: formVal,));
                      print('[inner loop] for loop $i ' + snap['formVal'][j]);
                    }

                    // the list of DropdownButtons
                    listofWidget.add(DropdownButton(
                      value: formInput[i],items: options,hint: Text(
                        "-",onChanged: (value) {
                        setState(() {
                          formInput[i] = value;
                        });
                      },));

                    // listofWidget.add(Text(snap['formTxt']));
                  }
                  return Column(
                    children: listofWidget,);
                } else {
                  return CircularProgressIndicator();
                }
              });
        } else {
          return CircularProgressIndicator();
        }
      },);
  }
}

错误：

I/Flutter (30860): ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
I/Flutter (30860): The following assertion was thrown building StreamBuilder<QuerySnapshot>(dirty,state:
I/Flutter (30860): _StreamBuilderBaseState<QuerySnapshot,AsyncSnapshot<QuerySnapshot>>#60d57):
I/Flutter (30860): There should be exactly one item with [DropdownButton]'s value: -.
I/Flutter (30860): Either zero or 2 or more [DropdownMenuItem]s were detected with the same value
I/Flutter (30860): 'package:Flutter/src/material/dropdown.dart':
I/Flutter (30860): Failed assertion: line 839 pos 15: 'items == null || items.isEmpty || value == null ||
I/Flutter (30860):               items.where((DropdownMenuItem<T> item) {
I/Flutter (30860):                 return item.value == value;
I/Flutter (30860):               }).length == 1'
I/Flutter (30860):