如何根据第一个下拉列表中选择的选项创建一个下拉选项

Question

I have two dropdowns. the first permit to choose the country and the second the city that depends on the country. Both are widgets build in different files but used in the another same file. my difficulties: How to retrieve the choice of countrie and how to transmit it when i am calling the city widget to use it to retrieve the cities of the chosen country because my variable _maselectionpays that contains the choice of the country is not declared in this widget

                          //debut
                 Listepayswidget(),
                 Listevillewidget(pays:_maselectionpays),

EDIT:

my code of widget country (pays)

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


class Listepayswidget extends StatefulWidget {
 Listepayswidget({Key key}) : super(key: key);

  @override
  State<StatefulWidget> createState() => new _ListepayswidgetState();

}
class _ListepayswidgetState extends State<Listepayswidget> {

   String _maselectionpays;
   List data=List();
  //List _allville= new List();
   Future<String> getpays() async {
  // paradisimo.000webhostapp.com
  final response = await http.post(ressource);

  var resbody= json.decode(response.body);
  setState((){
  data=resbody;
   });
  print (resbody);
  return "success";
 //datab=data.toString();
   }

    @override
   void initState(){
    super.initState();
    this.getpays();

}
  @override
   Widget build(BuildContext context) {
   return new Container(

              child: 

              new DropdownButton(


                items: data.map((item){
                  return new DropdownMenuItem(
                    child: new Text(item['libelle_pays']),
                    value: item['code_pays'].toString(),

                  );
                }).toList(),
                onChanged: (newVal){
                  setState(() {
                   _maselectionpays=newVal; 
                  });
                },
                value: _maselectionpays,
              ),
            );




  }
}

The code of widget city(ville) is similar than the code of widget country(pays).

This is my code of statefulwidget when I call widget pays

import 'package:flutter/material.dart';
import 'package:paradisimmo/utilitaire/combo_offre.dart';
import 'package:paradisimmo/utilitaire/combo_pays.dart';




class EnregistreroffrePage extends StatefulWidget {
   EnregistreroffrePage({this.username});
final String username;
  @override
  State<StatefulWidget> createState() => new _EnregistreroffrePage();
}

class _EnregistreroffreData {
  String email = '';
  String password = '';

}

class _EnregistreroffrePage extends State<EnregistreroffrePage> {
  bool monVal = false;
  bool tuVal = false;
  bool wedVal = false;
  int _radioValue1 = -1;

 void _handleRadioValueChange1(int value) {
    setState(() {
      _radioValue1 = value;

      switch (_radioValue1) {
        case 0:

          //correctScore++;
          break;
        case 1:

          break;
        case 2:

          break;
      }
    });
  }

  final GlobalKey<FormState> _formKey = new GlobalKey<FormState>();
  _EnregistreroffreData _data = new  _EnregistreroffreData();

  void submit() {
    // First validate form.
    if (this._formKey.currentState.validate()) {
      _formKey.currentState.save(); // Save our form now.

    }
  }

  @override
  Widget build(BuildContext context) {
    //final Size screenSize = MediaQuery.of(context).size;

    return new Scaffold(
      appBar: new AppBar(
        title: new Text('Enregistrer une offre'),
      ),
        body: Center(
            child: ListView(
              shrinkWrap: true,
              padding: EdgeInsets.all(15.0),
              children: <Widget>[
                Center(
                  child: Card(
                    elevation: 8.0,
                    child: Container(
                      padding: EdgeInsets.all(10.0),
                      child: Column(
                        children: <Widget>[
                           //debut
                  Listepayswidget(),
                  Listevillewidget(pays:_maselectionpays),

Answer 1

You need to make use of StatefulWidget for your input form. When you select a country , then trigger setState() and that will rebuild the city drop-down widget. You can find more examples on StatefulWidget here . Here is sample code from google.

Read your newly added code and here is what i think:

_maselectionpays .aka country need to be accessed from the statefullwidget city(Listevillewidget). It is needed there so you can rebuild the city upon country selection . There is a couple of ways of doing this, but the easiest is to make maselectionpays a variable inside EnregistreroffrePage , then pass it to Listepayswidget(maselectionpays) and to Listevillewidget(maselectionpays) . When it is changed you call setState inside Listepayswidget and you shall use it inside Listevillewidget in the DropdownButton . Would have used it as an key in a HashMap<key, value> data , the value would of course be a list of citys and the key is countries. So if the key changes, the country then the city-list changes. Therefor Flutter would re-build the element.

For a better management of this could you should look into the BLoC-pattern , it's less code. Or look into RxDart , that uses observables and Streams for handling changes, also this uses less code.