flutter - 当您返回更改第一个下拉菜单时，取决于 Dropdown 失败断言错误

Question

我正在尝试此视频中的代码：https ://www.youtube.com/watch?v= 4AUuhhSakro（或 github：https ://github.com/khaliqdadmohmand/flutter_dynamic_dropdownLists/blob/master/lib/main.dart )

问题是当我们（视频的观看者）尝试“返回”以更改初始状态（不是应用程序状态，如县或省）时，应用程序崩溃并出现以下错误：

There should be exactly one item with [DropdownButton]'s value: 14. 
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'

我相信问题是当第一次构建下拉菜单时，它可以有一个空字符串作为value下拉菜单的参数，但是第二次它在断言上崩溃（如果你将它设置为 null 你会崩溃，value==null如果你不要重置您正在使用的变量，value然后在新的下拉列表中这个值不在项目中。（计数必须是== 1）

这在我自己的项目中也一直在绞尽脑汁，我以为我已经成功了，但显然它仍然很糟糕。

Flutter：使用拖曳下拉按钮从另一个加载一个时出现错误

这是一个类似的问题，并且该解决方案中包含异步（但这对我来说根本不起作用）。

标签： flutterdart

Answer 1

您可以在下面复制粘贴运行完整代码
您可以onChanged添加_myCity = null;
代码片段

onChanged: (String Value) {
      setState(() {
        _myState = Value;
        _myCity = null;
        _getCitiesList();
        print(_myState);
      });
    },

工作演示

完整代码

import 'dart:convert';

import 'package:flutter/material.dart';
import 'package:http/http.dart' as http;

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

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  @override
  void initState() {
    _getStateList();
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Dynamic DropDownList REST API'),
      ),
      body: Column(
        mainAxisAlignment: MainAxisAlignment.start,
        children: <Widget>[
          Container(
            alignment: Alignment.topCenter,
            margin: EdgeInsets.only(bottom: 100, top: 100),
            child: Text(
              'KDTechs',
              style: TextStyle(fontWeight: FontWeight.w800, fontSize: 20),
            ),
          ),
          //======================================================== State

          Container(
            padding: EdgeInsets.only(left: 15, right: 15, top: 5),
            color: Colors.white,
            child: Row(
              mainAxisAlignment: MainAxisAlignment.spaceBetween,
              children: <Widget>[
                Expanded(
                  child: DropdownButtonHideUnderline(
                    child: ButtonTheme(
                      alignedDropdown: true,
                      child: DropdownButton<String>(
                        value: _myState,
                        iconSize: 30,
                        icon: (null),
                        style: TextStyle(
                          color: Colors.black54,
                          fontSize: 16,
                        ),
                        hint: Text('Select State'),
                        onChanged: (String Value) {
                          setState(() {
                            _myState = Value;
                            _myCity = null;
                            _getCitiesList();
                            print(_myState);
                          });
                        },
                        items: statesList?.map((item) {
                              return DropdownMenuItem(
                                child: Text(item['name']),
                                value: item['id'].toString(),
                              );
                            })?.toList() ??
                            [],
                      ),
                    ),
                  ),
                ),
              ],
            ),
          ),
          SizedBox(
            height: 30,
          ),

          //======================================================== City

          Container(
            padding: EdgeInsets.only(left: 15, right: 15, top: 5),
            color: Colors.white,
            child: Row(
              mainAxisAlignment: MainAxisAlignment.spaceBetween,
              children: <Widget>[
                Expanded(
                  child: DropdownButtonHideUnderline(
                    child: ButtonTheme(
                      alignedDropdown: true,
                      child: DropdownButton<String>(
                        value: _myCity,
                        iconSize: 30,
                        icon: (null),
                        style: TextStyle(
                          color: Colors.black54,
                          fontSize: 16,
                        ),
                        hint: Text('Select City'),
                        onChanged: (String Value) {
                          setState(() {
                            _myCity = Value;
                            print(_myCity);
                          });
                        },
                        items: citiesList?.map((item) {
                              return DropdownMenuItem(
                                child: Text(item['name']),
                                value: item['id'].toString(),
                              );
                            })?.toList() ??
                            [],
                      ),
                    ),
                  ),
                ),
              ],
            ),
          ),
        ],
      ),
    );
  }

  //=============================================================================== Api Calling here

//CALLING STATE API HERE
// Get State information by API
  List statesList;
  String _myState;

  String stateInfoUrl = 'http://cleanions.bestweb.my/api/location/get_state';
  Future<String> _getStateList() async {
    await http.post(stateInfoUrl, headers: {
      'Content-Type': 'application/x-www-form-urlencoded'
    }, body: {
      "api_key": '25d55ad283aa400af464c76d713c07ad',
    }).then((response) {
      var data = json.decode(response.body);

//      print(data);
      setState(() {
        statesList = data['state'];
      });
    });
  }

  // Get State information by API
  List citiesList;
  String _myCity;

  String cityInfoUrl =
      'http://cleanions.bestweb.my/api/location/get_city_by_state_id';
  Future<String> _getCitiesList() async {
    await http.post(cityInfoUrl, headers: {
      'Content-Type': 'application/x-www-form-urlencoded'
    }, body: {
      "api_key": '25d55ad283aa400af464c76d713c07ad',
      "state_id": _myState,
    }).then((response) {
      var data = json.decode(response.body);

      setState(() {
        citiesList = data['cities'];
      });
    });
  }
}