如何编写依赖于其他下拉菜单的下拉菜单 - 使用 Flutter
How to code dropdowns that depend on other dropdowns - using Flutter
我有两个 Flutter 下拉菜单。第一个从数据库加载大学列表并且完美运行。第二个从数据库中加载所选大学的校园。
目前，我正在使用大学下拉菜单的 onChanged() 函数中的 setState()，该函数在 80% 的时间内都有效。另外 20% 似乎是由于网络速度慢造成的，因此下拉列表在屏幕刷新之前没有时间填充，因此它是空的。如果大学下拉菜单被点击一次，校园下拉菜单通常会被点击两次或三次。这是加载代码（其中 loadDatabaseSubTable() 填写校园下拉列表：
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(),
),
),
以上方法是否正确？如果是这样，我如何确保在下拉列表填满之前不进行刷新？
还是用 FutureBuilder 来处理更好？
我曾想过首先将所有大学的所有校园加载到内存中 - 但对于 Android 应用程序。这似乎对带宽使用有点矫枉过正。
我有另一个屏幕，它有 6 个下拉菜单，每个都依赖于前一个 - 对每个下拉菜单使用 FutureBuilder 似乎是一种非常不整洁的处理下拉菜单的方式。提前致谢。
问题是 loadDatabaseSubTable 是异步的，你没有等待它完成，所以当它获取数据时你的 setState 已经被执行了。
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);
另见
您可以复制粘贴运行下面的完整代码
在示例代码中，在选择第一个下拉菜单 state 后， 模拟 检索第二个下拉菜单 Local government area
的项目有 5 秒的延迟
第 1 步：您可以在 onChanged 中设置第二个下拉菜单并使用 async await 加载数据库 table
DropdownButton<String>(
...
onChanged: (value) => _onSelectedState(value),
第2步：您可以将bool _isLoading设置为true并在数据未准备好时显示CircularProgressIndicator()
_isLoading
? CircularProgressIndicator()
: DropdownButton<String>(
第 3 步：数据准备就绪后，将 bool _isLoading 设置为 false
setState(() {
_isLoading = false;
});
代码片段
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
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,
),
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;
}
}
它对我有用@chunhunghan。但是，如果我想做汽车制造商列表（第一个下拉列表）和汽车型号（第二个下拉列表）的两个相关下拉按钮，我该如何使用相同的方法？但是当我单击模型（第二个）时，我希望能够获得模型的类别，这样我就可以打印并显示它是汽车、卡车、公共汽车等。实施时遇到问题。
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"},
]
}
];
上述 json 文件的示例。
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
我有两个 Flutter 下拉菜单。第一个从数据库加载大学列表并且完美运行。第二个从数据库中加载所选大学的校园。
目前，我正在使用大学下拉菜单的 onChanged() 函数中的 setState()，该函数在 80% 的时间内都有效。另外 20% 似乎是由于网络速度慢造成的，因此下拉列表在屏幕刷新之前没有时间填充，因此它是空的。如果大学下拉菜单被点击一次，校园下拉菜单通常会被点击两次或三次。这是加载代码（其中 loadDatabaseSubTable() 填写校园下拉列表：
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(),
),
),
以上方法是否正确？如果是这样，我如何确保在下拉列表填满之前不进行刷新？
还是用 FutureBuilder 来处理更好？
我曾想过首先将所有大学的所有校园加载到内存中 - 但对于 Android 应用程序。这似乎对带宽使用有点矫枉过正。
我有另一个屏幕，它有 6 个下拉菜单，每个都依赖于前一个 - 对每个下拉菜单使用 FutureBuilder 似乎是一种非常不整洁的处理下拉菜单的方式。提前致谢。
问题是 loadDatabaseSubTable 是异步的，你没有等待它完成，所以当它获取数据时你的 setState 已经被执行了。
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);
另见
您可以复制粘贴运行下面的完整代码
在示例代码中，在选择第一个下拉菜单 state 后， 模拟 检索第二个下拉菜单 Local government area
的项目有 5 秒的延迟
第 1 步：您可以在 onChanged 中设置第二个下拉菜单并使用 async await 加载数据库 table
DropdownButton<String>(
...
onChanged: (value) => _onSelectedState(value),
第2步：您可以将bool _isLoading设置为true并在数据未准备好时显示CircularProgressIndicator()
_isLoading
? CircularProgressIndicator()
: DropdownButton<String>(
第 3 步：数据准备就绪后，将 bool _isLoading 设置为 false
setState(() {
_isLoading = false;
});
代码片段
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
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,
),
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;
}
}
它对我有用@chunhunghan。但是，如果我想做汽车制造商列表（第一个下拉列表）和汽车型号（第二个下拉列表）的两个相关下拉按钮，我该如何使用相同的方法？但是当我单击模型（第二个）时，我希望能够获得模型的类别，这样我就可以打印并显示它是汽车、卡车、公共汽车等。实施时遇到问题。
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"},
]
}
];
上述 json 文件的示例。
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