下拉列表不适用于字符串列表

Question

我在设置下拉菜单功能时遇到了一些问题。我不明白这是为什么，因为小部件本身接受 String: DropdownMenuItem 我收到错误：

type string is not a subtype of type DropdownMenuItem<dynamic>

我尝试将我的值转换为字符串或动态值，如下所示：值：map["breed"].cast() 或值：地图[“品种”].cast()

但似乎没有任何效果。

class DogForm extends StatefulWidget {
  @override
  _DogFormState createState() => _DogFormState();
}

class _DogFormState extends State<DogForm> {
  final todoController = TextEditingController();
  final List<DropdownMenuItem> breeds = [];
  String? _mySelection = '';
  final List<Map> _mybreedJson = [
    {
      "breed": "Cavalier King Charles Spaniel",
      "img":
          "https://upload.wikimedia.org/wikipedia/commons/thumb/5/5f/CarterBIS.Tiki.13.6.09.jpg/220px-CarterBIS.Tiki.13.6.09.jpg"
    },
    {
      "breed": "Curly-Coated Retriever",
      "img":
          "https://upload.wikimedia.org/wikipedia/commons/thumb/5/58/Curly_Coated_Retriever.jpg/220px-Curly_Coated_Retriever.jpg"
    },
  ];

  void addToDo() async {
    if (todoController.text.trim().isEmpty) {
      ScaffoldMessenger.of(context).showSnackBar(SnackBar(
        content: Text("Empty title"),
        duration: Duration(seconds: 2),
      ));
      return;
    }
    await saveTodo(todoController.text);
    setState(() {
      todoController.clear();
    });
  }

  @override
  Widget build(BuildContext context) {

    return Scaffold(
        appBar: AppBar(
          title: Text("Parse Todo List"),
          backgroundColor: Colors.blueAccent,
          centerTitle: true,
        ),
        body: Column(
          children: <Widget>[
            Container(
              padding: EdgeInsets.fromLTRB(17.0, 1.0, 7.0, 1.0),
              child: Row(
                  mainAxisAlignment: MainAxisAlignment.center,
                  children: <Widget>[
                    Expanded(
                      child: DropdownButtonHideUnderline(
                        child: ButtonTheme(
                          alignedDropdown: true,
                          child: DropdownButton<String>(
                            isDense: true,
                            hint: Text('Select Breed'),
                            value: _mySelection,
                            onChanged: (String? newValue) {
                              setState(() {
                                _mySelection = newValue;
                              });
                            },
                            items: _mybreedJson.map((Map map) {
                              return DropdownMenuItem<String>(
                                value: map["breed"],
                                // value: _mySelection,
                                child: Row(
                                  children: <Widget>[
                                    Image.asset(
                                      map["img"],
                                      width: 25,
                                    ),
                                    Container(
                                        margin: EdgeInsets.only(left: 10),
                                        child: Text(map["breed"])),
                                  ],
                                ),
                              );
                            }).toList(),
                          ),
                        ),
                      ),
                    ),
                  ]),
            )
          ],
        ));
  }
}

Future<void> saveTodo(String title) async {
  await Future.delayed(Duration(seconds: 1), () {});
  final todo = ParseObject('Todo')
    ..set('title', title)
    ..set('done', false);
  await todo.save();
}

拜托！有人可以告诉我如何使用这样的列表吗？

_mybreedJson = [
    {
      "breed": "Cavalier King Charles Spaniel",
      "img":
          "https://upload.wikimedia.org/wikipedia/commons/thumb/5/5f/CarterBIS.Tiki.13.6.09.jpg/220px-CarterBIS.Tiki.13.6.09.jpg"
    },
    {
      "breed": "Curly-Coated Retriever",
      "img":
          "https://upload.wikimedia.org/wikipedia/commons/thumb/5/58/Curly_Coated_Retriever.jpg/220px-Curly_Coated_Retriever.jpg"
    },
  ];

Answer 1

如果你制作 Map 地图 --> Map 地图，也许 flutter 会告诉你错误在哪里。

Answer 2

这就是你想要的？请运行代码。我更改了 .asset => .network 并且，我将实际值设置为默认值

import 'package:flutter/material.dart';
import 'package:parse_server_sdk_flutter/parse_server_sdk.dart';

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

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

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Material App',
      theme: ThemeData.dark(),
      home: DogForm(),
    );
  }
}

class DogForm extends StatefulWidget {
  @override
  _DogFormState createState() => _DogFormState();
}

class _DogFormState extends State<DogForm> {
  final todoController = TextEditingController();
  final List<DropdownMenuItem> breeds = [];
  String? _mySelection = 'Cavalier King Charles Spaniel';
  final List<Map> _mybreedJson = [
    {
      "breed": "Cavalier King Charles Spaniel",
      "img":
          "https://images.pexels.com/photos/45201/kitty-cat-kitten-pet-45201.jpeg?auto=compress&cs=tinysrgb&dpr=1&w=500"
    },
    {
      "breed": "Curly-Coated Retriever",
      "img":
          "https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcSjiLOiEP-qSR6OgMrPELypnHToVToGPEc_qTkuLq5mMKwCCMoQ4x6Fsn19uvBoDO0qZaQ&usqp=CAU"
    },
  ];

  void addToDo() async {
    if (todoController.text.trim().isEmpty) {
      ScaffoldMessenger.of(context).showSnackBar(const SnackBar(
        content: Text("Empty title"),
        duration: Duration(seconds: 2),
      ));
      return;
    }
    await saveTodo(todoController.text);
    setState(() {
      todoController.clear();
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: const Text("Parse Todo List"),
          backgroundColor: Colors.blueAccent,
          centerTitle: true,
        ),
        body: Column(
          children: <Widget>[
            Container(
              padding: const EdgeInsets.fromLTRB(17.0, 1.0, 7.0, 1.0),
              child: Row(mainAxisAlignment: MainAxisAlignment.center, children: <Widget>[
                Expanded(
                  child: DropdownButtonHideUnderline(
                    child: ButtonTheme(
                      alignedDropdown: true,
                      child: DropdownButton<String>(
                        isDense: true,
                        hint: const Text('Select Breed'),
                        value: _mySelection,
                        onChanged: (String? newValue) {
                          setState(() {
                            _mySelection = newValue;
                          });
                        },
                        items: _mybreedJson.map((Map map) {
                          return DropdownMenuItem<String>(
                            value: map["breed"],
                            // value: _mySelection,
                            child: Row(
                              children: <Widget>[
                                Image.network(
                                  map["img"],
                                  width: 25,
                                ),
                                Container(margin: const EdgeInsets.only(left: 10), child: Text(map["breed"])),
                              ],
                            ),
                          );
                        }).toList(),
                      ),
                    ),
                  ),
                ),
              ]),
            )
          ],
        ));
  }
}

Future<void> saveTodo(String title) async {
  await Future.delayed(const Duration(seconds: 1), () {});
  final todo = ParseObject('Todo')
    ..set('title', title)
    ..set('done', false);
  await todo.save();
}

下拉列表不适用于字符串列表

Dropdown not working with List of strings flutter

flutter