应该只有一项具有 [DropdownButton] 的值：

Posted 2023-03-07

【中文标题】应该只有一项具有 [DropdownButton] 的值：

【英文标题】：There should be exactly one item with [DropdownButton]'s value:

【发布时间】：2020-11-10 00:27:37

【问题描述】：

String selectedItem = 'Instalação Fibra';

DropdownButton<String>(
                value: selectedItem,
                onChanged: (value) 
                  setState(() 
                    selectedItem = value;
                  );
                ,
                items: <String>['Instalação de TV', 'Instalação Rádio']
                    .map<DropdownMenuItem<String>>((String e) 
                  return DropdownMenuItem<String>(
                    value: e,
                    child: Text(e),
                  );
                ).toList(),
              )

这怎么可能是错的？ 这不是编译。我尝试了几种方法，但都出现了同样的错误。

【参考方案1】：

您可以在下面复制粘贴运行完整代码selectedItem 'Instalação Fibra' 在<String>['Instalação de TV', 'Instalação Rádio'] 中不存在 您可以使用List<String> items 并设置selectedItem = items[0] 代码sn-p

List<String> items = ['Instalação Fibra','Instalação de TV', 'Instalação Rádio'];
String selectedItem;

@override
void initState() 
    selectedItem = items[0];
    super.initState();

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();


class _MyHomePageState extends State<MyHomePage> 
  List<String> items = ['Instalação Fibra','Instalação de TV', 'Instalação Rádio'];
  String selectedItem;

  @override
  void initState() 
    selectedItem = items[0];
    super.initState();
  

  @override
  Widget build(BuildContext context) 
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: DropdownButton<String>(
        value: selectedItem,
        onChanged: (value) 
          setState(() 
            selectedItem = value;
          );
        ,
        items: items.map<DropdownMenuItem<String>>((String e) 
          return DropdownMenuItem<String>(
            value: e,
            child: Text(e),
          );
        ).toList(),
      ),
    );