应该只有一项具有 [DropdownButton] 的值：答案

【问题标题】：There should be exactly one item with [DropdownButton]'s value:应该只有一项具有 [DropdownButton] 的值：
【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。

【问题讨论】：

标签： flutter

【解决方案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(),
      ),
    );
  }
}

【讨论】：