更改两级 DropdownButtonFormField ：应该只有一项具有 [DropdownButton] 的值

尽管这里有多个条目似乎有类似的问题，但我无法让它真正发挥作用。

我有两个依赖的 DropdownButtonFormFields 的设置，其中第二个在第一个列表更改后更改为另一个列表。

1. 我能够将问题分解为第二个选择的选定值的持续剩余。我预计它会随着我在代码中提供的值信息而改变。

提供以下错误

════════ Exception caught by widgets library ═══════════════════════════════════

There should be exactly one item with [DropdownButton]'s value: GreenBananas. 

Either zero or 2 or more [DropdownMenuItem]s were detected with the same value
'package:flutter/src/material/dropdown.dart':
Failed assertion: line 827 pos 15: 'items == null || items.isEmpty || value == null ||
              items.where((DropdownMenuItem<T> item) {
                return item.value == value;
              }).length == 1'
The relevant error-causing widget was
    DropdownButtonFormField<String> 
lib/…/testing/test.dart:242
══════════════════════════════════════════════════════════════════

我简化了示例并重建了错误，以便更好地分析问题并从你们那里获得更有价值的输入:)

class InputRowTest extends StatefulWidget {
  @override
  _InputRowTestState createState() => _InputRowTestState();
}

class _InputRowTestState extends State<InputRowTest> {
  List<String> list1 = ['Apples', 'Bananas', 'Peaches'];

  List<String> list1_1 = ['GreenApples', 'RedApples', 'YellowApples'];

  List<String> list1_2 = [
    'YellowBananas',
    'BrownBananas',
    'GreenBananas',
    'GreenApples'
  ];

  List<String> list1_3 = [
    'RedPeaches',
    'YellowPeaches',
    'GreenPeaches',
    'GreenApples'
  ];

  List<String> _fromparent;
  int _fromparentint;
  //String selected;

  @override
  void initState() {
    _fromparent = list1_1;
    _fromparentint = 0;
    //selected = list1[0];
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    List<List<String>> subLists = [list1_1, list1_2, list1_3];
    _fromparent = subLists[_fromparentint];

    DropdownButtonFormField ddff = DropdownButtonFormField(
      //value: selected, //list1[0],
      //items: list1.map((category) {
      value: _fromparent[0], //Seems this value wont change.
      items: _fromparent.map((category) {
        return DropdownMenuItem(
          value: category,
          child: Container(
            child: Text(category),
          ),
        );
      }).toList(),
      onChanged: (val) => print(val),
    );

    return Center(
      child: Row(
        children: <Widget>[
          Expanded(
            child: DropdownButtonFormField(
              value: list1[0],
              items: list1.map((category) {
                return DropdownMenuItem(
                  value: category,
                  child: Container(
                    child: Text(category),
                  ),
                );
              }).toList(),
              onChanged: (val) {
                setState(() {
                  //selected = val;
                  _fromparentint = list1.indexOf(val);
                });
              },
            ),
          ),
          Expanded(
            child: ddff,
          ),
        ],
      ),
    );
  }
}

我找到了解决该问题的方法。

看起来 Flutter 并没有重建 DropDownFormField，而是认为保留它是完全可以的。在这种情况下，它也相当顽固。

由于我找不到重建场的方法，所以我创建了一个非常令人讨厌但有效的方法。还需要一些打磨。

我基本上让 flutter 相信我每次都提供不同的小部件。

class InputRowTest extends StatefulWidget {
  @override
  _InputRowTestState createState() => _InputRowTestState();
}

class _InputRowTestState extends State<InputRowTest> {
  List<String> list1 = ['Apples', 'Bananas', 'Peaches'];

  List<String> list1_1 = ['GreenApples', 'RedApples', 'YellowApples'];

  List<String> list1_2 = [
    'YellowBananas',
    'BrownBananas',
    'GreenBananas',
    'GreenApples'
  ];

  List<String> list1_3 = [
    'RedPeaches',
    'YellowPeaches',
    'GreenPeaches',
    'GreenApples'
  ];

  List<String> _fromparent;
  int _fromparentint;
  Widget ddbff;
  var selected;
  bool chance;

  Widget ddff(List<String> list, bool chance) {
    return (chance)
        ? DropdownButtonFormField(
            value: list[0], //Seems this value wont change.
            items: list.map((category) {
              return DropdownMenuItem(
                value: category,
                child: Container(
                  child: Text(category),
                ),
              );
            }).toList(),
            onChanged: (val) {
              print(val);
            },
          )
        : Container(
            child: DropdownButtonFormField(
              value: list[0], //Seems this value wont change.
              items: list.map((category) {
                return DropdownMenuItem(
                  value: category,
                  child: Container(
                    child: Text(category),
                  ),
                );
              }).toList(),
              onChanged: (val) {
                print(val);
              },
            ),
          );
  }

  @override
  void initState() {
    _fromparent = list1_1;
    _fromparentint = 0;
    selected = list1_1[0];
    chance = true;
    ddbff = ddff(_fromparent, chance);
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    List<List<String>> subLists = [list1_1, list1_2, list1_3];
    _fromparent = subLists[_fromparentint];

    chance = !chance;
    ddbff = ddff(_fromparent, chance);

    return Center(
      child: Container(
        child: Row(
          children: <Widget>[
            Expanded(
              child: DropdownButtonFormField(
                value: list1[0],
                items: list1.map((category) {
                  return DropdownMenuItem(
                    value: category,
                    child: Container(
                      child: Text(category),
                    ),
                  );
                }).toList(),
                onChanged: (val) {
                  setState(() {
                    _fromparentint = list1.indexOf(val);
                  });
                },
              ),
            ),
            Expanded(
              child: ddbff,
            ),
          ],
        ),
      ),
    );
  }
}