Bdtjtk

I have just started learning flutter.

I have two DropdownButtons I am trying to populate the second based on selection from the first filtering by cid in regions json.

{"countries":[{"cid":"2","name":"Country A"},{"cid":"3","name":"Country B"}]}

{"regions":[{"rid":"1","cid":"2","name":"Region 1 of Country A"},{"rid":"2","cid":"2","name":"Region 2 of Country A"},{"rid":"3","cid":"3","name":"Region 1 of Country B"},{"rid":"4","cid":"3","name":"Region 2 of Country B"}]}


void main () {



  runApp(new SearchAssembly());

}

class SearchAssembly extends StatefulWidget{

 SearchAssembly({Key key}):super(key:key);



 @override

 _SearchAssemblyState createState() => new _SearchAssemblyState ();



}

class _SearchAssemblyState extends State<SearchAssembly> {



 List _countryData = new List();

 List _regionData = new List();





 List<DropdownMenuItem<String>> _dropDownMenuItemsCountries;

 List<DropdownMenuItem<String>> _dropDownMenuItemsRegions;



 String _selectedCountry;

 String _selectedRegion;





 Future<List> getCountry() async{



http.Response response = await http.get(countryUrl);



var results= await json.decode(response.body)['countries'];

return results;

}



Future<List> getRegions() async{



http.Response response = await http.get(regionsUrl);



var results= await json.decode(response.body)['regions'];

return results;

}

void loadData () async{



   _countryData = await getCountry();

    _regionData = await getRegions();



    setState(() {

      _selectedCountry="2";

      _selectedRegion="1";

      _dropDownMenuItemsCountries = getDropDownMenuItemCountries();



_dropDownMenuItemsRegions=getDropDownMenuItemRegions(_selectedCountry);

});

}



@override

 void initState() {

  loadData ();

  super.initState();

}



List<DropdownMenuItem<String>> getDropDownMenuItemCountries( ) {



    List<DropdownMenuItem<String>> items = new List();



     if (_countryData.isNotEmpty) {



        for (int i = 0; i < _countryData.length; i++) {



        String cid= _countryData[i]['cid'].toString();

        String name=_countryData[i]['name'].toString();



        items.add(new DropdownMenuItem(value: cid,

         child: new Text(name)



       ));

      }

    }



    return items;

 }







  void changeDropDownItemCountry(String selectedCountry){



      setState(() {

      _selectedCountry=selectedCountry;



       _dropDownMenuItemsRegions = 

  getDropDownMenuItemRegions(_selectedCountry);





 });

}





 List<DropdownMenuItem<String>> getDropDownMenuItemRegions(String filterBy) 

{



List<DropdownMenuItem<String>> items = new List();





if(_regionData.isNotEmpty) {



  for (int i = 0; i < _regionData.length; i++) {



    if (_regionData[i]['cid'].toString() == filterBy) {

      String rid = _regionData[i]['rid'].toString();

      String name = _regionData[i]['name'].toString();



      items.add(new DropdownMenuItem(value: rid,

          child: new Text(name)



      ));

    }

  }





  }





 return items;

 }



void changeDropDownItemRegion(String selectedRegion){



    setState(() {

     _selectedRegion=selectedRegion;



     print ("The selected Region is: $_selectedRegion");

   });

 }





@override

Widget build(BuildContext context) {





// TODO: implement build

 return new MaterialApp(

   home: Scaffold(

    appBar: AppBar(

      title: Text(" Locator"),

      elevation: 0.1,

      backgroundColor: new Color(0xFF546E7A),

    ),



    backgroundColor: Colors.blueGrey.shade500,

    body:  ListView(

         padding: const EdgeInsets.all(10.0),

      children: <Widget>[







       new DropdownButtonHideUnderline(child:

       new DropdownButton(

         value: _selectedCountry,

         items: _dropDownMenuItemsCountries,

         onChanged: changeDropDownItemCountry,

         isExpanded: true,

         isDense: true,



       ),



       ),



       new SizedBox(

         child: null,height: 10.0,

       ),



       new DropdownButtonHideUnderline(child:

       new DropdownButton(

         value: _selectedRegion,

         items: _dropDownMenuItemsRegions,

         onChanged: changeDropDownItemRegion,

         isExpanded: true,

         isDense: true,



       ),



       )





      ],





     )

   ));



 }





}

Error:

I/flutter ( 6426): ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════

I/flutter ( 6426): The following assertion was thrown building SearchAssembly(dirty, state:

I/flutter ( 6426): _SearchAssemblyState#4c4ac):

I/flutter ( 6426): 'package:flutter/src/material/dropdown.dart': Failed assertion: line 514 pos 15: 'items == null ||

I/flutter ( 6426): value == null || items.where((DropdownMenuItem<T> item) => item.value == value).length == 1': is not

I/flutter ( 6426): true.

I/flutter ( 6426): 

I/flutter ( 6426): Either the assertion indicates an error in the framework itself, or we should provide substantially

I/flutter ( 6426): more information in this error message to help you determine and fix the underlying cause.

I/flutter ( 6426): In either case, please report this assertion by filing a bug on GitHub:

I/flutter ( 6426):   https://github.com/flutter/flutter/issues/new?template=BUG.md

I/flutter ( 6426): 

I/flutter ( 6426): When the exception was thrown, this was the stack:

I/flutter ( 6426): #2      new DropdownButton (package:flutter/src/material/dropdown.dart:514:15)

I/flutter ( 6426): #3      _SearchAssemblyState.build (package:flutter_dashboard/search_assembly_widget.dart:212:16)

I/flutter ( 6426): #4      StatefulElement.build (package:flutter/src/widgets/framework.dart:3809:27)

I/flutter ( 6426): #5      ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3721:15)

I/flutter ( 6426): #6      Element.rebuild (package:flutter/src/widgets/framework.dart:3547:5)

I/flutter ( 6426): #7      BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2286:33)

I/flutter ( 6426): #8      _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding&PaintingBinding&SemanticsBinding&RendererBinding&WidgetsBinding.drawFrame (package:flutter/src/widgets/binding.dart:676:20)

I/flutter ( 6426): #9      _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding&PaintingBinding&SemanticsBinding&RendererBinding._handlePersistentFrameCallback (package:flutter/src/rendering/binding.dart:219:5)

I/flutter ( 6426): #10     _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:990:15)

I/flutter ( 6426): #11     _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:930:9)

I/flutter ( 6426): #12     _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding._handleDrawFrame (package:flutter/src/scheduler/binding.dart:842:5)

I/flutter ( 6426): #13     _invoke (dart:ui/hooks.dart:154:13)

I/flutter ( 6426): #14     _drawFrame (dart:ui/hooks.dart:143:3)

I/flutter ( 6426): (elided 2 frames from class _AssertionError)

I/flutter ( 6426): ════════════════════════════════════════════════════════════════════════════════════════════════════

issues happened because the _selectedRegion does not update.

List<DropdownMenuItem<String>> getDropDownMenuItemRegions(String filterBy) {

    List<DropdownMenuItem<String>> items = new List();

    if (_regionData.isNotEmpty) {

      for (int i = 0; i < _regionData.length; i++) {

        if (_regionData[i]['cid'] == filterBy) {

          print(_regionData[i]['cid']);

          String rid = _regionData[i]['rid'];

          String name = _regionData[i]['name'];

          setState(() {

            _selectedRegion = _regionData[i]['rid'];

          });

          items.add(new DropdownMenuItem(value: rid, child: new Text(name)));

        }

      }

    }



    return items;

  }

this select last item in DropdownMenuItem