如何对依赖于其他下拉列表的下拉列表进行编码-使用颤振

Question

我有两个颤音下降。第一种方法是从数据库中加载一份大学列表，并运行得非常完美。第二个从数据库中加载所选大学的校园。

目前，我使用的是大学下拉列表的setState()函数，该函数可以工作80%的时间。另外的20%似乎是由一个缓慢的网络造成的，在屏幕刷新之前，下拉列表没有时间填充，所以它是空的。如果大学辍学被点击一次，两次或三次，校园的辍学通常会填补。以下是加载代码(其中loadDatabaseSubTable()填充校园下拉列表)：

 Container(
      decoration: new BoxDecoration(
        border: Border.all(width: 1),
        borderRadius: new BorderRadius.all(Radius.circular(5.0)),
        shape: BoxShape.rectangle,
      ),
      padding: EdgeInsets.symmetric(
          horizontal: kContainerPaddingHorizontal,
          vertical: kContainerPaddingVertical),
      child: DropdownButton<String>(
       hint: Text('Select the Institution'),
        value: selectedInstitute,
        underline: Container(
          height: 0,
        ),
        onChanged: (String value) {
          // Use setState() to load next dropdown
          if (institutionBefore != value) {
              setState(() {
              // Get matching campuses
              dropCampus = [];
              campusBuild.institutionId = shInstitutionId;
              campusBuild.campusId = '';
              loadDatabaseSubTable(
                  tableName: 'sa_campus_by_institute',
                  listVariable: dropCampus,
                  object: campusBuild);
              //
              selectedInstitute = value;
            });
          }
        },
        items: dropInstitution.map((String description) {
          return DropdownMenuItem<String>(
            value: description,
            child: Text(
              description,
              style: TextStyle(color: Colors.black),
            ),
          );
        }).toList(),
      ),
    ),

以上方法是正确的做法吗？如果是这样的话，我如何确保刷新不会在填充下拉列表之前发生？

还是使用FutureBuilder来处理它更好呢？

我曾想过，首先要把所有大学的校园都载入内存--不过是安卓应用程序。这似乎对带宽的使用造成了过度的影响。

我有另一个屏幕，它有6个下拉列表，每个都依赖于前面的一个--对每个屏幕使用一个FutureBuilder似乎是处理下拉列表的一种非常混乱的方法。提前谢谢。

Answer 1

问题是，loadDatabaseSubTable是异步的，您没有等待它完成，所以当它获取您的setState将执行的数据时。

您需要将onChanged更改为异步：

onChanged: (String value) async {

然后等待loadDatabaseSubTable：

await loadDatabaseSubTable(
  tableName: 'sa_campus_by_institute',
  listVariable: dropCampus,
  object: campusBuild);

另请参阅

• https://dart.dev/codelabs/async-await

Answer 2

您可以复制粘贴，运行下面的完整代码

在示例代码中，选择第一个下拉state后，模拟第二个下拉Local government area的检索项有5秒的延迟。

步骤1:您可以在onChanged中设置第二个下拉列表，并使用async await加载数据库表

DropdownButton<String>(
            ...
            onChanged: (value) => _onSelectedState(value),

步骤2:您可以将bool _isLoading设置为true，并在数据尚未准备就绪时显示CircularProgressIndicator()

_isLoading
              ? CircularProgressIndicator()
              : DropdownButton<String>(

步骤3:数据准备就绪后，将bool _isLoading设置为false

setState(() {
      _isLoading = false;
    });

代码段

void _onSelectedState(String value) async {
    setState(() {
      _selectedLGA = "Choose ..";
      _selectedState = value;
      _lgas = ["Choose .."];
      _isLoading = true;
    });

    _lgas = List.from(_lgas)..addAll(await repo.getLocalByState(value));

    setState(() {
      _isLoading = false;
    });
  }

工作演示

​

​

全码

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,
      ),
      home: Home(),
    );
  }
}

class Home extends StatefulWidget {
  @override
  _HomeState createState() => _HomeState();
}

class _HomeState extends State<Home> {
  Repository repo = Repository();

  List<String> _states = ["Choose a state"];
  List<String> _lgas = ["Choose .."];
  String _selectedState = "Choose a state";
  String _selectedLGA = "Choose ..";
  bool _isLoading = false;

  @override
  void initState() {
    _states = List.from(_states)..addAll(repo.getStates());
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("STATES MULTI DROPDOWN"),
        elevation: 0.1,
      ),
      body: SafeArea(
        child: Container(
          padding: EdgeInsets.symmetric(horizontal: 50, vertical: 30),
          child: Column(
            children: <Widget>[
              DropdownButton<String>(
                isExpanded: true,
                items: _states.map((String dropDownStringItem) {
                  return DropdownMenuItem<String>(
                    value: dropDownStringItem,
                    child: Text(dropDownStringItem),
                  );
                }).toList(),
                onChanged: (value) => _onSelectedState(value),
                value: _selectedState,
              ),
              _isLoading
                  ? CircularProgressIndicator()
                  : DropdownButton<String>(
                      isExpanded: true,
                      items: _lgas.map((String dropDownStringItem) {
                        return DropdownMenuItem<String>(
                          value: dropDownStringItem,
                          child: Text(dropDownStringItem),
                        );
                      }).toList(),
                      // onChanged: (value) => print(value),
                      onChanged: (value) => _onSelectedLGA(value),
                      value: _selectedLGA,
                    ),
            ],
          ),
        ),
      ),
    );
  }

  void _onSelectedState(String value) async {
    setState(() {
      _selectedLGA = "Choose ..";
      _selectedState = value;
      _lgas = ["Choose .."];
      _isLoading = true;
    });

    _lgas = List.from(_lgas)..addAll(await repo.getLocalByState(value));

    setState(() {
      _isLoading = false;
    });
  }

  void _onSelectedLGA(String value) {
    setState(() => _selectedLGA = value);
  }
}

class Repository {
  // http://locationsng-api.herokuapp.com/api/v1/lgas
  // test() => _nigeria.map((map) => StateModel.fromJson(map));
  List<Map> getAll() => _nigeria;

  /*getLocalByState(String state) => _nigeria
      .map((map) => StateModel.fromJson(map))
      .where((item) => item.state == state)
      .map((item) => item.lgas)
      .expand((i) => i)
      .toList();*/

  Future<List<String>> getLocalByState(String state) async {
    await Future.delayed(Duration(seconds: 5), () {
      print("Future.delayed");
    });

    return Future.value(_nigeria
        .map((map) => StateModel.fromJson(map))
        .where((item) => item.state == state)
        .map((item) => item.lgas)
        .expand((i) => i)
        .toList());
  }
  // _nigeria.where((list) => list['state'] == state);
  // .map((item) => item['lgas'])
  // .expand((i) => i)
  // .toList();

  List<String> getStates() => _nigeria
      .map((map) => StateModel.fromJson(map))
      .map((item) => item.state)
      .toList();
  // _nigeria.map((item) => item['state'].toString()).toList();

  List _nigeria = [
    {
      "state": "Adamawa",
      "alias": "adamawa",
      "lgas": [
        "Demsa",
        "Fufure",
        "Ganye",
        "Gayuk",
        "Gombi",
        "Grie",
        "Hong",
        "Jada",
        "Larmurde",
        "Madagali",
        "Maiha",
        "Mayo Belwa",
        "Michika",
        "Mubi North",
        "Mubi South",
        "Numan",
        "Shelleng",
        "Song",
        "Toungo",
        "Yola North",
        "Yola South"
      ]
    },
    {
      "state": "Akwa Ibom",
      "alias": "akwa_ibom",
      "lgas": [
        "Abak",
        "Eastern Obolo",
        "Eket",
        "Esit Eket",
        "Essien Udim",
        "Etim Ekpo",
        "Etinan",
        "Ibeno",
        "Ibesikpo Asutan",
        "Ibiono-Ibom",
        "Ikot Abasi",
        "Ika",
        "Ikono",
        "Ikot Ekpene",
        "Ini",
        "Mkpat-Enin",
        "Itu",
        "Mbo",
        "Nsit-Atai",
        "Nsit-Ibom",
        "Nsit-Ubium",
        "Obot Akara",
        "Okobo",
        "Onna",
        "Oron",
        "Udung-Uko",
        "Ukanafun",
        "Oruk Anam",
        "Uruan",
        "Urue-Offong/Oruko",
        "Uyo"
      ]
    },
    {
      "state": "Anambra",
      "alias": "anambra",
      "lgas": [
        "Aguata",
        "Anambra East",
        "Anaocha",
        "Awka North",
        "Anambra West",
        "Awka South",
        "Ayamelum",
        "Dunukofia",
        "Ekwusigo",
        "Idemili North",
        "Idemili South",
        "Ihiala",
        "Njikoka",
        "Nnewi North",
        "Nnewi South",
        "Ogbaru",
        "Onitsha North",
        "Onitsha South",
        "Orumba North",
        "Orumba South",
        "Oyi"
      ]
    },
    {
      "state": "Ogun",
      "alias": "ogun",
      "lgas": [
        "Abeokuta North",
        "Abeokuta South",
        "Ado-Odo/Ota",
        "Egbado North",
        "Ewekoro",
        "Egbado South",
        "Ijebu North",
        "Ijebu East",
        "Ifo",
        "Ijebu Ode",
        "Ijebu North East",
        "Imeko Afon",
        "Ikenne",
        "Ipokia",
        "Odeda",
        "Obafemi Owode",
        "Odogbolu",
        "Remo North",
        "Ogun Waterside",
        "Shagamu"
      ]
    },
    {
      "state": "Ondo",
      "alias": "ondo",
      "lgas": [
        "Akoko North-East",
        "Akoko North-West",
        "Akoko South-West",
        "Akoko South-East",
        "Akure North",
        "Akure South",
        "Ese Odo",
        "Idanre",
        "Ifedore",
        "Ilaje",
        "Irele",
        "Ile Oluji/Okeigbo",
        "Odigbo",
        "Okitipupa",
        "Ondo West",
        "Ose",
        "Ondo East",
        "Owo"
      ]
    },
    {
      "state": "Rivers",
      "alias": "rivers",
      "lgas": [
        "Abua/Odual",
        "Ahoada East",
        "Ahoada West",
        "Andoni",
        "Akuku-Toru",
        "Asari-Toru",
        "Bonny",
        "Degema",
        "Emuoha",
        "Eleme",
        "Ikwerre",
        "Etche",
        "Gokana",
        "Khana",
        "Obio/Akpor",
        "Ogba/Egbema/Ndoni",
        "Ogu/Bolo",
        "Okrika",
        "Omuma",
        "Opobo/Nkoro",
        "Oyigbo",
        "Port Harcourt",
        "Tai"
      ]
    },
    {
      "state": "Bauchi",
      "alias": "bauchi",
      "lgas": [
        "Alkaleri",
        "Bauchi",
        "Bogoro",
        "Damban",
        "Darazo",
        "Dass",
        "Gamawa",
        "Ganjuwa",
        "Giade",
        "Itas/Gadau",
        "Jama'are",
        "Katagum",
        "Kirfi",
        "Misau",
        "Ningi",
        "Shira",
        "Tafawa Balewa",
        "Toro",
        "Warji",
        "Zaki"
      ]
    },
    {
      "state": "Benue",
      "alias": "benue",
      "lgas": [
        "Agatu",
        "Apa",
        "Ado",
        "Buruku",
        "Gboko",
        "Guma",
        "Gwer East",
        "Gwer West",
        "Katsina-Ala",
        "Konshisha",
        "Kwande",
        "Logo",
        "Makurdi",
        "Obi",
        "Ogbadibo",
        "Ohimini",
        "Oju",
        "Okpokwu",
        "Oturkpo",
        "Tarka",
        "Ukum",
        "Ushongo",
        "Vandeikya"
      ]
    },
    {
      "state": "Borno",
      "alias": "borno",
      "lgas": [
        "Abadam",
        "Askira/Uba",
        "Bama",
        "Bayo",
        "Biu",
        "Chibok",
        "Damboa",
        "Dikwa",
        "Guzamala",
        "Gubio",
        "Hawul",
        "Gwoza",
        "Jere",
        "Kaga",
        "Kala/Balge",
        "Konduga",
        "Kukawa",
        "Kwaya Kusar",
        "Mafa",
        "Magumeri",
        "Maiduguri",
        "Mobbar",
        "Marte",
        "Monguno",
        "Ngala",
        "Nganzai",
        "Shani"
      ]
    },
    {
      "state": "Bayelsa",
      "alias": "bayelsa",
      "lgas": [
        "Brass",
        "Ekeremor",
        "Kolokuma/Opokuma",
        "Nembe",
        "Ogbia",
        "Sagbama",
        "Southern Ijaw",
        "Yenagoa"
      ]
    },
    {
      "state": "Cross River",
      "alias": "cross_river",
      "lgas": [
        "Abi",
        "Akamkpa",
        "Akpabuyo",
        "Bakassi",
        "Bekwarra",
        "Biase",
        "Boki",
        "Calabar Municipal",
        "Calabar South",
        "Etung",
        "Ikom",
        "Obanliku",
        "Obubra",
        "Obudu",
        "Odukpani",
        "Ogoja",
        "Yakuur",
        "Yala"
      ]
    },
    {
      "state": "Delta",
      "alias": "delta",
      "lgas": [
        "Aniocha North",
        "Aniocha South",
        "Bomadi",
        "Burutu",
        "Ethiope West",
        "Ethiope East",
        "Ika North East",
        "Ika South",
        "Isoko North",
        "Isoko South",
        "Ndokwa East",
        "Ndokwa West",
        "Okpe",
        "Oshimili North",
        "Oshimili South",
        "Patani",
        "Sapele",
        "Udu",
        "Ughelli North",
        "Ukwuani",
        "Ughelli South",
        "Uvwie",
        "Warri North",
        "Warri South",
        "Warri South West"
      ]
    },
    {
      "state": "Ebonyi",
      "alias": "ebonyi",
      "lgas": [
        "Abakaliki",
        "Afikpo North",
        "Ebonyi",
        "Afikpo South",
        "Ezza North",
        "Ikwo",
        "Ezza South",
        "Ivo",
        "Ishielu",
        "Izzi",
        "Ohaozara",
        "Ohaukwu",
        "Onicha"
      ]
    },
    {
      "state": "Edo",
      "alias": "edo",
      "lgas": [
        "Akoko-Edo",
        "Egor",
        "Esan Central",
        "Esan North-East",
        "Esan South-East",
        "Esan West",
        "Etsako Central",
        "Etsako East",
        "Etsako West",
        "Igueben",
        "Ikpoba Okha",
        "Orhionmwon",
        "Oredo",
        "Ovia North-East",
        "Ovia South-West",
        "Owan East",
        "Owan West",
        "Uhunmwonde"
      ]
    },
    {
      "state": "Ekiti",
      "alias": "ekiti",
      "lgas": [
        "Ado Ekiti",
        "Efon",
        "Ekiti East",
        "Ekiti South-West",
        "Ekiti West",
        "Emure",
        "Gbonyin",
        "Ido Osi",
        "Ijero",
        "Ikere",
        "Ilejemeje",
        "Irepodun/Ifelodun",
        "Ikole",
        "Ise/Orun",
        "Moba",
        "Oye"
      ]
    },
    {
      "state": "Enugu",
      "alias": "enugu",
      "lgas": [
        "Awgu",
        "Aninri",
        "Enugu East",
        "Enugu North",
        "Ezeagu",
        "Enugu South",
        "Igbo Etiti",
        "Igbo Eze North",
        "Igbo Eze South",
        "Isi Uzo",
        "Nkanu East",
        "Nkanu West",
        "Nsukka",
        "Udenu",
        "Oji River",
        "Uzo Uwani",
        "Udi"
      ]
    },
    {
      "state": "Federal Capital Territory",
      "alias": "abuja",
      "lgas": [
        "Abaji",
        "Bwari",
        "Gwagwalada",
        "Kuje",
        "Kwali",
        "Municipal Area Council"
      ]
    },
    {
      "state": "Gombe",
      "alias": "gombe",
      "lgas": [
        "Akko",
        "Balanga",
        "Billiri",
        "Dukku",
        "Funakaye",
        "Gombe",
        "Kaltungo",
        "Kwami",
        "Nafada",
        "Shongom",
        "Yamaltu/Deba"
      ]
    },
    {
      "state": "Jigawa",
      "alias": "jigawa",
      "lgas": [
        "Auyo",
        "Babura",
        "Buji",
        "Biriniwa",
        "Birnin Kudu",
        "Dutse",
        "Gagarawa",
        "Garki",
        "Gumel",
        "Guri",
        "Gwaram",
        "Gwiwa",
        "Hadejia",
        "Jahun",
        "Kafin Hausa",
        "Kazaure",
        "Kiri Kasama",
        "Kiyawa",
        "Kaugama",
        "Maigatari",
        "Malam Madori",
        "Miga",
        "Sule Tankarkar",
        "Roni",
        "Ringim",
        "Yankwashi",
        "Taura"
      ]
    },
    {
      "state": "Oyo",
      "alias": "oyo",
      "lgas": [
        "Afijio",
        "Akinyele",
        "Atiba",
        "Atisbo",
        "Egbeda",
        "Ibadan North",
        "Ibadan North-East",
        "Ibadan North-West",
        "Ibadan South-East",
        "Ibarapa Central",
        "Ibadan South-West",
        "Ibarapa East",
        "Ido",
        "Ibarapa North",
        "Irepo",
        "Iseyin",
        "Itesiwaju",
        "Iwajowa",
        "Kajola",
        "Lagelu",
        "Ogbomosho North",
        "Ogbomosho South",
        "Ogo Oluwa",
        "Olorunsogo",
        "Oluyole",
        "Ona Ara",
        "Orelope",
        "Ori Ire",
        "Oyo",
        "Oyo East",
        "Saki East",
        "Saki West",
        "Surulere Oyo State"
      ]
    },
    {
      "state": "Imo",
      "alias": "imo",
      "lgas": [
        "Aboh Mbaise",
        "Ahiazu Mbaise",
        "Ehime Mbano",
        "Ezinihitte",
        "Ideato North",
        "Ideato South",
        "Ihitte/Uboma",
        "Ikeduru",
        "Isiala Mbano",
        "Mbaitoli",
        "Isu",
        "Ngor Okpala",
        "Njaba",
        "Nkwerre",
        "Nwangele",
        "Obowo",
        "Oguta",
        "Ohaji/Egbema",
        "Okigwe",
        "Orlu",
        "Orsu",
        "Oru East",
        "Oru West",
        "Owerri Municipal",
        "Owerri North",
        "Unuimo",
        "Owerri West"
      ]
    },
    {
      "state": "Kaduna",
      "alias": "kaduna",
      "lgas": [
        "Birnin Gwari",
        "Chikun",
        "Giwa",
        "Ikara",
        "Igabi",
        "Jaba",
        "Jema'a",
        "Kachia",
        "Kaduna North",
        "Kaduna South",
        "Kagarko",
        "Kajuru",
        "Kaura",
        "Kauru",
        "Kubau",
        "Kudan",
        "Lere",
        "Makarfi",
        "Sabon Gari",
        "Sanga",
        "Soba",
        "Zangon Kataf",
        "Zaria"
      ]
    },
    {
      "state": "Kebbi",
      "alias": "kebbi",
      "lgas": [
        "Aleiro",
        "Argungu",
        "Arewa Dandi",
        "Augie",
        "Bagudo",
        "Birnin Kebbi",
        "Bunza",
        "Dandi",
        "Fakai",
        "Gwandu",
        "Jega",
        "Kalgo",
        "Koko/Besse",
        "Maiyama",
        "Ngaski",
        "Shanga",
        "Suru",
        "Sakaba",
        "Wasagu/Danko",
        "Yauri",
        "Zuru"
      ]
    },
    {
      "state": "Kano",
      "alias": "kano",
      "lgas": [
        "Ajingi",
        "Albasu",
        "Bagwai",
        "Bebeji",
        "Bichi",
        "Bunkure",
        "Dala",
        "Dambatta",
        "Dawakin Kudu",
        "Dawakin Tofa",
        "Doguwa",
        "Fagge",
        "Gabasawa",
        "Garko",
        "Garun Mallam",
        "Gezawa",
        "Gaya",
        "Gwale",
        "Gwarzo",
        "Kabo",
        "Kano Municipal",
        "Karaye",
        "Kibiya",
        "Kiru",
        "Kumbotso",
        "Kunchi",
        "Kura",
        "Madobi",
        "Makoda",
        "Minjibir",
        "Nasarawa",
        "Rano",
        "Rimin Gado",
        "Rogo",
        "Shanono",
        "Takai",
        "Sumaila",
        "Tarauni",
        "Tofa",
        "Tsanyawa",
        "Tudun Wada",
        "Ungogo",
        "Warawa",
        "Wudil"
      ]
    },
    {
      "state": "Kogi",
      "alias": "kogi",
      "lgas": [
        "Ajaokuta",
        "Adavi",
        "Ankpa",
        "Bassa",
        "Dekina",
        "Ibaji",
        "Idah",
        "Igalamela Odolu",
        "Ijumu",
        "Kogi",
        "Kabba/Bunu",
        "Lokoja",
        "Ofu",
        "Mopa Muro",
        "Ogori/Magongo",
        "Okehi",
        "Okene",
        "Olamaboro",
        "Omala",
        "Yagba East",
        "Yagba West"
      ]
    },
    {
      "state": "Osun",
      "alias": "osun",
      "lgas": [
        "Aiyedire",
        "Atakunmosa West",
        "Atakunmosa East",
        "Aiyedaade",
        "Boluwaduro",
        "Boripe",
        "Ife East",
        "Ede South",
        "Ife North",
        "Ede North",
        "Ife South",
        "Ejigbo",
        "Ife Central",
        "Ifedayo",
        "Egbedore",
        "Ila",
        "Ifelodun",
        "Ilesa East",
        "Ilesa West",
        "Irepodun",
        "Irewole",
        "Isokan",
        "Iwo",
        "Obokun",
        "Odo Otin",
        "Ola Oluwa",
        "Olorunda",
        "Oriade",
        "Orolu",
        "Osogbo"
      ]
    },
    {
      "state": "Sokoto",
      "alias": "sokoto",
      "lgas": [
        "Gudu",
        "Gwadabawa",
        "Illela",
        "Isa",
        "Kebbe",
        "Kware",
        "Rabah",
        "Sabon Birni",
        "Shagari",
        "Silame",
        "Sokoto North",
        "Sokoto South",
        "Tambuwal",
        "Tangaza",
        "Tureta",
        "Wamako",
        "Wurno",
        "Yabo",
        "Binji",
        "Bodinga",
        "Dange Shuni",
        "Goronyo",
        "Gada"
      ]
    },
    {
      "state": "Plateau",
      "alias": "plateau",
      "lgas": [
        "Bokkos",
        "Barkin Ladi",
        "Bassa",
        "Jos East",
        "Jos North",
        "Jos South",
        "Kanam",
        "Kanke",
        "Langtang South",
        "Langtang North",
        "Mangu",
        "Mikang",
        "Pankshin",
        "Qua'an Pan",
        "Riyom",
        "Shendam",
        "Wase"
      ]
    },
    {
      "state": "Taraba",
      "alias": "taraba",
      "lgas": [
        "Ardo Kola",
        "Bali",
        "Donga",
        "Gashaka",
        "Gassol",
        "Ibi",
        "Jalingo",
        "Karim Lamido",
        "Kumi",
        "Lau",
        "Sardauna",
        "Takum",
        "Ussa",
        "Wukari",
        "Yorro",
        "Zing"
      ]
    },
    {
      "state": "Yobe",
      "alias": "yobe",
      "lgas": [
        "Bade",
        "Bursari",
        "Damaturu",
        "Fika",
        "Fune",
        "Geidam",
        "Gujba",
        "Gulani",
        "Jakusko",
        "Karasuwa",
        "Machina",
        "Nangere",
        "Nguru",
        "Potiskum",
        "Tarmuwa",
        "Yunusari"
      ]
    },
    {
      "state": "Zamfara",
      "alias": "zamfara",
      "lgas": [
        "Anka",
        "Birnin Magaji/Kiyaw",
        "Bakura",
        "Bukkuyum",
        "Bungudu",
        "Gummi",
        "Gusau",
        "Kaura Namoda",
        "Maradun",
        "Shinkafi",
        "Maru",
        "Talata Mafara",
        "Tsafe",
        "Zurmi"
      ]
    },
    {
      "state": "Lagos",
      "alias": "lagos",
      "lgas": [
        "Agege",
        "Ajeromi-Ifelodun",
        "Alimosho",
        "Amuwo-Odofin",
        "Badagry",
        "Apapa",
        "Epe",
        "Eti Osa",
        "Ibeju-Lekki",
        "Ifako-Ijaiye",
        "Ikeja",
        "Ikorodu",
        "Kosofe",
        "Lagos Island",
        "Mushin",
        "Lagos Mainland",
        "Ojo",
        "Oshodi-Isolo",
        "Shomolu",
        "Surulere Lagos State"
      ]
    },
    {
      "state": "Katsina",
      "alias": "katsina",
      "lgas": [
        "Bakori",
        "Batagarawa",
        "Batsari",
        "Baure",
        "Bindawa",
        "Charanchi",
        "Danja",
        "Dandume",
        "Dan Musa",
        "Daura",
        "Dutsi",
        "Dutsin Ma",
        "Faskari",
        "Funtua",
        "Ingawa",
        "Jibia",
        "Kafur",
        "Kaita",
        "Kankara",
        "Kankia",
        "Katsina",
        "Kurfi",
        "Kusada",
        "Mai'Adua",
        "Malumfashi",
        "Mani",
        "Mashi",
        "Matazu",
        "Musawa",
        "Rimi",
        "Sabuwa",
        "Safana",
        "Sandamu",
        "Zango"
      ]
    },
    {
      "state": "Kwara",
      "alias": "kwara",
      "lgas": [
        "Asa",
        "Baruten",
        "Edu",
        "Ilorin East",
        "Ifelodun",
        "Ilorin South",
        "Ekiti Kwara State",
        "Ilorin West",
        "Irepodun",
        "Isin",
        "Kaiama",
        "Moro",
        "Offa",
        "Oke Ero",
        "Oyun",
        "Pategi"
      ]
    },
    {
      "state": "Nasarawa",
      "alias": "nasarawa",
      "lgas": [
        "Akwanga",
        "Awe",
        "Doma",
        "Karu",
        "Keana",
        "Keffi",
        "Lafia",
        "Kokona",
        "Nasarawa Egon",
        "Nasarawa",
        "Obi",
        "Toto",
        "Wamba"
      ]
    },
    {
      "state": "Niger",
      "alias": "niger",
      "lgas": [
        "Agaie",
        "Agwara",
        "Bida",
        "Borgu",
        "Bosso",
        "Chanchaga",
        "Edati",
        "Gbako",
        "Gurara",
        "Katcha",
        "Kontagora",
        "Lapai",
        "Lavun",
        "Mariga",
        "Magama",
        "Mokwa",
        "Mashegu",
        "Moya",
        "Paikoro",
        "Rafi",
        "Rijau",
        "Shiroro",
        "Suleja",
        "Tafa",
        "Wushishi"
      ]
    },
    {
      "state": "Abia",
      "alias": "abia",
      "lgas": [
        "Aba North",
        "Arochukwu",
        "Aba South",
        "Bende",
        "Isiala Ngwa North",
        "Ikwuano",
        "Isiala Ngwa South",
        "Isuikwuato",
        "Obi Ngwa",
        "Ohafia",
        "Osisioma",
        "Ugwunagbo",
        "Ukwa East",
        "Ukwa West",
        "Umuahia North",
        "Umuahia South",
        "Umu Nneochi"
      ]
    }
  ];
}

class StateModel {
  String state;
  String alias;
  List<String> lgas;

  StateModel({this.state, this.alias, this.lgas});

  StateModel.fromJson(Map<String, dynamic> json) {
    state = json['state'];
    alias = json['alias'];
    lgas = json['lgas'].cast<String>();
  }

  Map<String, dynamic> toJson() {
    final Map<String, dynamic> data = new Map<String, dynamic>();
    data['state'] = this.state;
    data['alias'] = this.alias;
    data['lgas'] = this.lgas;
    return data;
  }
}

Answer 3

“这对我很管用，”春亨。但是，如果我想对汽车制造商列表中的两个依赖下拉按钮(第一个下拉按钮)和汽车模型(第二个下拉按钮)进行操作，如何使用相同的方法？但是当我点击模型(第二个)时，我希望能够得到模型的类别，这样我就可以打印并显示它是否是汽车、卡车、公共汽车等等。在实施上遇到了困难。

List _cars = [
    {
      "make": "Acura",
      "brand": [
        {"name": "CL", "category": "Coupe"},
        {"name": "ILX", "category": "Sedan"},
        {"name": "Integra", "category": "Sedan"},
        {"name": "Legend", "category": "Sedan"},
        {"name": "MDX", "category": "SUV"},
        {"name": "NSX", "category": "Coupe"},
        {"name": "RDX", "category": "SUV"},
        {"name": "RL", "category": "Sedan"},
        {"name": "RLX", "category": "Sedan"},
        {"name": "RSX", "category": "Coupe"},
        {"name": "SLX", "category": "SUV"},
        {"name": "TL", "category": "Sedan"},
        {"name": "TLX", "category": "Sedan"},
        {"name": "TSX", "category": "Sedan"},
        {"name": "Vigor", "category": "Sedan"},
        {"name": "ZDX", "category": "SUV"},
      ]
    },
    {
      "make": "Alfa Romeo",
      "brand": [
        {"name": "147", "category": "Sedan"},
        {"name": "164", "category": "Sedan"},
        {"name": "4C", "category": "Coupe"},
        {"name": "4C Spider", "category": "Convertible"},
        {"name": "Brera", "category": "Sedan"},
        {"name": "Giulia", "category": "Sedan"},
        {"name": "Giulietta", "category": "Sedan"},
        {"name": "GT", "category": "Sedan"},
        {"name": "Mito", "category": "Sedan"},
        {"name": "Stelvio", "category": "SUV"},
      ]
    }
  ];

上面的json文件的一个例子。

class CarsModel {
  late String make;
  late List<Brand> brand;

  CarsModel({required this.make, required this.brand});

  CarsModel.fromJSON(Map<String, dynamic> json) {
    make = json['make'];
    brand = json['brand'].map<Brand>((json) => Brand.fromJSON(json)).toList();
  }

  Map<String, dynamic> toJson() {
    final Map<String, dynamic> data = Map<String, dynamic>();
    data['make'] = this.make;
    data['brand'] = this.brand;
    return data;
  }
}

class Brand {
  late String name;
  late String category;

  Brand({required this.name, required this.category});

  Brand.fromJSON(Map<String, dynamic> json) {
    name = json['name'];
    category = json['category'];
  }

  Map<String, dynamic> toJson() {
    final Map<String, dynamic> data = Map<String, dynamic>();
    data['name'] = this.name;
    data['category'] = this.category;

    return data;
  }
}

JSON文件的模型