DropdownButtonFormField 断言失败，而 DropdownButtonHideUnderline 没有

Question

这适用于 DropdownButtonHideUnderline，但不适用于 DropdownButtonFormField。我想要使​​用 DropdownButtonFormField 获得的 inputDecoration，但是当我更改项目时，此代码在运行时失败。

我要么需要修复它以使用 DropdownButtonFormField 运行，要么我应该找到一种方法将 inputDecoration 添加到 DropdownButtonHideUnderline；

在运行时出现的错误是：

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'

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

Map data = {
  'Project 1': ['Entrance', 'Main Hallway', 'Kitchen'],
  'Project 2': ['Patio', 'Dining Room'],
};

class _MyHomePageState extends State<MyHomePage> {
  String _project;
  String _room;
  List<String> _roomList = [];

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: Text(widget.title),
        ),
        body: Center(
          child: Column(
            children: <Widget>[
              DropdownButtonFormField(
                decoration: InputDecoration(labelText: 'Project'),
                value: _project,
                onChanged: (value) {
                  setState(() {
                    _project = value;
                    _room = null;
                    _roomList = data[_project];
                  });
                },
                items: data.keys.map((item) {
                      return DropdownMenuItem(
                        child: Text(item),
                        value: item,
                      );
                    })?.toList() ??
                    [],
              ),
              DropdownButtonFormField(
                decoration: InputDecoration(labelText: 'Room'),
                value: _room,
                onChanged: (value) {
                  setState(() {
                    _room = value;
                    print(_project);
                    print(_room);
                  });
                },
                items: _roomList.map((item) {
                      return DropdownMenuItem(
                        child: Text(item),
                        value: item,
                      );
                    })?.toList() ??
                    [],
              ),
            ],
          ),
        ));
  }
}

Answer 1

您可以在下面复制粘贴运行完整代码
当下拉列表数据完全不同时会触发此错误
对于RoomDropdownButtonFormField，您可以使用key: UniqueKey()，小部件将重新创建

代码sn-p

DropdownButtonFormField(
        key: UniqueKey(),
        decoration: InputDecoration(labelText: 'Room'),

工作演示

完整代码

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

Map data = {
  'Project 1': ['Entrance', 'Main Hallway', 'Kitchen'],
  'Project 2': ['Patio', 'Dining Room'],
};

class _MyHomePageState extends State<MyHomePage> {
  String _project;
  String _room;
  List<String> _roomList = [];

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: Text(widget.title),
        ),
        body: Center(
          child: Column(
            children: <Widget>[
              DropdownButtonFormField(
                decoration: InputDecoration(labelText: 'Project'),
                value: _project,
                onChanged: (value) {
                  setState(() {
                    _project = value;
                    _room = null;
                    _roomList = data[_project];
                  });
                },
                items: data.keys.map((item) {
                      return DropdownMenuItem(
                        child: Text(item),
                        value: item,
                      );
                    })?.toList() ??
                    [],
              ),
              DropdownButtonFormField(
                key: UniqueKey(),
                decoration: InputDecoration(labelText: 'Room'),
                value: _room,
                onChanged: (value) {
                  setState(() {
                    _room = value;
                    print(_project);
                    print(_room);
                  });
                },
                items: _roomList.map((item) {
                      return DropdownMenuItem(
                        child: Text(item),
                        value: item,
                      );
                    })?.toList() ??
                    [],
              ),
            ],
          ),
        ));
  }
}