[英]Cannot create Dropdown button in Flutter
I am trying to use a Dropdown from a json in flutter, but have an error, I have not been able to see it:我正在尝试使用 flutter 中的 json 的下拉菜单,但出现错误,我无法看到它:
Widget _listaCiudades(){
//citiesProvider.getCities().then((value) => print(value));
return FutureBuilder(
future: citiesProvider.getCities(),
builder: (context, AsyncSnapshot <List<dynamic>> snapshot) {
//print('project snapshot data is: ${snapshot.data}');
return DropdownButton(
style: TextStyle(
fontFamily: 'Monserrat',
fontStyle: FontStyle.normal,
fontSize: 20,
),
disabledHint: Text("You can't select anything."),
items: snapshot.data.map((valorCity) {
print(valorCity['name']);
return DropdownMenuItem<String>(
value: valorCity['pk'].toString(),
child: Text(valorCity['name'].toString()),
);
}).toList(),//allCities(snapshot.data),
onChanged: (String newValue) {
setState(() {
value = newValue;
});
}
);
},
);
}
}
This is the output:这是 output:
Performing hot restart...
Syncing files to device Android SDK built for x86...
Restarted application in 5.373ms.
════════ Exception caught by widgets library ═══════════════════════════════════════════════════════
The following NoSuchMethodError was thrown building FutureBuilder<List<dynamic>>(dirty, state: _FutureBuilderState<List<dynamic>>#819ba):
The method 'map' was called on null.
Receiver: null
Tried calling: map<DropdownMenuItem<String>>(Closure: (dynamic) => DropdownMenuItem<String>)
The relevant error-causing widget was:
FutureBuilder<List<dynamic>> file:///C:/Users/FALABELLA/Desktop/Flutter/flutter-app-login-ui/lib/screens/signup_screen.dart:186:12
When the exception was thrown, this was the stack:
#0 Object.noSuchMethod (dart:core-patch/object_patch.dart:53:5)
#1 _SignupScreenState._listaCiudades.<anonymous closure> (package:app_login_ui/screens/signup_screen.dart:199:34)
#2 _FutureBuilderState.build (package:flutter/src/widgets/async.dart:732:55)
#3 StatefulElement.build (package:flutter/src/widgets/framework.dart:4623:28)
#4 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4506:15)
...
════════════════════════════════════════════════════════════════════════════════════════════════════
════════ Exception caught by widgets library ═══════════════════════════════════════════════════════
The following NoSuchMethodError was thrown building FutureBuilder<List<dynamic>>(dirty, state: _FutureBuilderState<List<dynamic>>#819ba):
The method 'map' was called on null.
Receiver: null
Tried calling: map<DropdownMenuItem<String>>(Closure: (dynamic) => DropdownMenuItem<String>)
The relevant error-causing widget was:
FutureBuilder<List<dynamic>> file:///C:/Users/FALABELLA/Desktop/Flutter/flutter-app-login-ui/lib/screens/signup_screen.dart:186:12
When the exception was thrown, this was the stack:
#0 Object.noSuchMethod (dart:core-patch/object_patch.dart:53:5)
#1 _SignupScreenState._listaCiudades.<anonymous closure> (package:app_login_ui/screens/signup_screen.dart:199:34)
#2 _FutureBuilderState.build (package:flutter/src/widgets/async.dart:732:55)
#3 StatefulElement.build (package:flutter/src/widgets/framework.dart:4623:28)
#4 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4506:15)
...
════════════════════════════════════════════════════════════════════════════════════════════════════
I/flutter ( 7314): Aartselaar
I/flutter ( 7314): Abancay
I/flutter ( 7314): Abbotsford
I/flutter ( 7314): Abingdon
I/flutter ( 7314): Absecon
I/flutter ( 7314): Abu Dhabi
I/flutter ( 7314): Acacias
I/flutter ( 7314): Acarigua
I/flutter ( 7314): Adelaide
I/flutter ( 7314): Aeropuerto
I/flutter ( 7314): Agua
I/flutter ( 7314): Agua Amarilla
I/flutter ( 7314): Agua Fria
I/flutter ( 7314): Agua Salada
You should check snapshot has data or not before return drop down widget, using below condition您应该在返回下拉小部件之前检查快照是否有数据,使用以下条件
if (snapshot.hasData) {
return DropdownButton(
...
...
);
}
And You can also try,你也可以试试,
items: snapshot.data?.map((valorCity) {
print(valorCity['name']);
return new DropdownMenuItem<String>(
value: valorCity['pk'].toString(),
child: Text(valorCity['name'].toString()),
);
})?.toList() ?? [],
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