[DropdownButton] 应该只有一项

Question

我想我想要的东西很简单，flutter 告诉我只有一个项目或者没有项目。但我确定项目不止一个，IDK，我应该如何针对这个问题提出问题。我只是想拿一个列表并用 CustomClass 的 items.I 做一个下拉菜单从 https://api.flutter.dev/flutter/material/DropdownButton-class.html 复制粘贴这行得通，但是当我用我的更改数据时......

import 'package:flutter/material.dart';

class BreedJson {
  final String breed;
  final String img;

  BreedJson(this.breed, this.img);
}

void main() => runApp(const MyApp());

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  static const String _title = 'Flutter Code Sample';

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: _title,
      home: Scaffold(
        appBar: AppBar(title: const Text(_title)),
        body: const Center(
          child: MyStatefulWidget(),
        ),
      ),
    );
  }
}

class MyStatefulWidget extends StatefulWidget {
  const MyStatefulWidget({Key? key}) : super(key: key);

  @override
  State<MyStatefulWidget> createState() => _MyStatefulWidgetState();
}

class _MyStatefulWidgetState extends State<MyStatefulWidget> {
  String dropdownValue = 'default ';

  final List<BreedJson> myBreedJson = [
    BreedJson("Cavalier King Charles Spaniel",
        'https://images.pexels.com/photos/104827/cat-pet-animal-domestic-104827.jpeg?auto=compress&cs=tinysrgb&dpr=1&w=500'),
    BreedJson('Curly-Coated Retriever',
        'https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcSLqpKzfZ6L0TSvUKBhXwJQOnqfWzoaQWoIjCZu1s0evNhfSFWeNVMYWJYt0MqInbznRgE&usqp=CAU')
  ];

  @override
  Widget build(BuildContext context) {
    return DropdownButton<String>(
        value: dropdownValue,
        icon: const Icon(Icons.arrow_downward),
        elevation: 16,
        style: const TextStyle(color: Colors.deepPurple),
        underline: Container(
          height: 2,
          color: Colors.deepPurpleAccent,
        ),
        onChanged: (String? newValue) {
          setState(() {
            dropdownValue = newValue!;
          });
        },
        items: myBreedJson
            .map<DropdownMenuItem<String>>((BreedJson value) => DropdownMenuItem<String>(
                  value: value.breed,
                  child: Text(value.img),
                ))
            .toList());
  }
}

Answer 1

myBreedJson 列表不包含 default 这就是它抛出此错误的原因。

您可以在值上初始传递 null，为此使 dropdownValue 可为空

 String? dropdownValue;

Answer 2

试试这个：

String dropdownValue = "Cavalier King Charles Spaniel";

[DropdownButton] 应该只有一项

There should be exactly one item with [DropdownButton]'s

dart

flutter