Flutter Dio Interceptor for refresh token

dio_helper.dart

class DioHelper {
  final Dio dio;
  DioHelper({@required this.dio});

  final CustomSharedPreferences _customSharedPreferences =
      new CustomSharedPreferences();
  static String _baseUrl = BASE_URL;
  String token = "";
  
  void initializeToken(String savedToken) {
    token = savedToken;
    _initApiClient();
  }

  Future<void> initApiClient() {
    dio.interceptors
        .add(InterceptorsWrapper(onRequest: (RequestOptions options) {
      options.headers["Authorization"] = "Bearer " + token;
      return options;
    }, onResponse: (Response response) {
      return response;
    }, onError: (DioError error) async {
      RequestOptions origin = error.response.request;
      if (error.response.statusCode == 401) {
        try {
          Response<dynamic> data = await dio.get("your_refresh_url");
          token = data.data['newToken'];
          _customSharedPreferences.setToken(data.data['newToken']);
          origin.headers["Authorization"] = "Bearer " + data.data['newToken'];
          return dio.request(origin.path, options: origin);
        } catch (err) {
          return err;
        }
      }
      return error;
    }));
    dio.options.baseUrl = _baseUrl;
  }

  Future<dynamic> get(String url) async {
    try {
      final response = await dio.get(url);
      var apiResponse = ApiResponse.fromJson(response.data);
      if (apiResponse.status != 200) {
        throw Exception(apiResponse.message);
      }
      return apiResponse.data;
    } on DioError catch (e) {
      // debugging purpose
      print('[Dio Helper - GET] Connection Exception => ' + e.message);
      throw e;
    }
  }

  Future<dynamic> post(String url,
      {Map headers, @required data, encoding}) async {
    try {
      final response =
          await dio.post(url, data: data, options: Options(headers: headers));
      ApiResponse apiResponse = ApiResponse.fromJson(response.data);
      if (apiResponse.status != 200) {
        throw Exception(apiResponse.message);
      }
      return apiResponse.data;
    } on DioError catch (e) {
      // debugging purpose
      print('[Dio Helper - GET] Connection Exception => ' + e.message);
      throw e;
    }
  }
}

there is an example here to navigate to login page. https://medium.com/flutter-community/dio-interceptors-in-flutter-17be4214f363