当然，我可以为你提供一个关于如何使用Flutter中的country_state_city插件来选择国家、州/省、城市的代码示例。这个插件能够帮助你轻松地在Flutter应用中实现地理位置的选择功能。

首先，你需要在pubspec.yaml文件中添加这个插件的依赖：

dependencies:
  flutter:
    sdk: flutter
  country_state_city: ^latest_version  # 请替换为最新版本号

然后运行flutter pub get来安装依赖。

接下来，是一个简单的示例代码，展示如何使用country_state_city插件来实现国家、州/省、城市的选择：

import 'package:flutter/material.dart';
import 'package:country_state_city/country_state_city.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Country, State, City Picker',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  Country _selectedCountry;
  StateModel _selectedState;
  CityModel _selectedCity;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Country, State, City Picker'),
      ),
      body: Padding(
        padding: const EdgeInsets.all(16.0),
        child: Column(
          children: <Widget>[
            DropdownButtonFormField<Country>(
              value: _selectedCountry,
              hint: Text('Select Country'),
              onChanged: (newValue) {
                setState(() {
                  _selectedCountry = newValue;
                  _selectedState = null;
                  _selectedCity = null;
                });
                // Fetch states for the selected country
                fetchStatesForCountry(newValue.countryCode);
              },
              items: CountryStateCityPicker.getCountries()
                  .map<DropdownMenuItem<Country>>((Country value) {
                return DropdownMenuItem<Country>(
                  value: value,
                  child: Text(value.name),
                );
              }).toList(),
            ),
            SizedBox(height: 16),
            _selectedCountry != null
                ? DropdownButtonFormField<StateModel>(
                    value: _selectedState,
                    hint: Text('Select State'),
                    onChanged: (newValue) {
                      setState(() {
                        _selectedState = newValue;
                        _selectedCity = null;
                      });
                      // Fetch cities for the selected state
                      fetchCitiesForState(_selectedCountry.countryCode, newValue.code);
                    },
                    items: _selectedCountry.states
                        .map<DropdownMenuItem<StateModel>>((StateModel value) {
                      return DropdownMenuItem<StateModel>(
                        value: value,
                        child: Text(value.name),
                      );
                    }).toList(),
                  )
                : Container(),
            SizedBox(height: 16),
            _selectedState != null
                ? DropdownButtonFormField<CityModel>(
                    value: _selectedCity,
                    hint: Text('Select City'),
                    onChanged: (newValue) {
                      setState(() {
                        _selectedCity = newValue;
                      });
                    },
                    items: _selectedState.cities
                        .map<DropdownMenuItem<CityModel>>((CityModel value) {
                      return DropdownMenuItem<CityModel>(
                        value: value,
                        child: Text(value.name),
                      );
                    }).toList(),
                  )
                : Container(),
          ],
        ),
      ),
    );
  }

  // Dummy function to simulate fetching states and cities
  void fetchStatesForCountry(String countryCode) async {
    // In a real scenario, you might fetch this data from an API or a local database
    List<StateModel> states = CountryStateCityPicker.getStatesByCountryCode(countryCode);
    setState(() {
      _selectedCountry?.states = states;
    });
  }

  void fetchCitiesForState(String countryCode, String stateCode) async {
    // In a real scenario, you might fetch this data from an API or a local database
    List<CityModel> cities = CountryStateCityPicker.getCitiesByCountryCodeAndStateCode(countryCode, stateCode);
    setState(() {
      _selectedState?.cities = cities;
    });
  }
}

注意：

1. 在实际应用中，你可能需要从API或本地数据库中获取国家和城市数据。这里为了简化，我们直接使用了插件提供的方法。
2. fetchStatesForCountry和fetchCitiesForState函数是模拟的，它们应该在实际应用中异步获取数据。
3. Country, StateModel, CityModel等类型是从country_state_city插件中导入的。

这个示例展示了如何使用country_state_city插件来创建一个简单的国家、州/省、城市选择器。你可以根据自己的需求进一步定制和扩展这个示例。