在下拉菜单中显示自定义值以及api数据波动

Question

这是一个显示api数据的下拉列表，共有6个主要类别，我需要与api数据一起添加ALL，或者我想显示ALL或如果用户未选择任何内容，则选择ALL，我正尝试遵循此操作 https://stackoverflow.com/a/54948635/13418165，但我对此颇为困惑，我可以向其显示api数据没有问题，显示提示是我需要暂时在下拉菜单中连同api数据一起使用

变量初始化

List<CategoryModel> job_category_model = [];
String _selectedjobcategory; 

查看

Flexible( flex: 0,
                    child: Padding(
                      padding: EdgeInsets.only(left: 10, right: 10, top: 15),
                      child: Container(
                        height: 45,
                        width: 320,
                        decoration: BoxDecoration(
                            color: Colors.white,
                            borderRadius: BorderRadius.circular(5),
                            border: Border.all(
                                color: Color.fromRGBO(92, 52, 76, 1))),
                        child: DropdownButton<String>(
                            isExpanded: true,
                            itemHeight: 50,
                            underline: SizedBox(),
                            value: _selectedjobcategory,
                            items: job_category_model.map((maincateogry) {
                              return DropdownMenuItem<String>(
                                value: maincateogry.id,
                                child: new Text(
                                  language != "ml" ? maincateogry.english: maincateogry.malayalam,
                                  style: TextStyle(
                                      fontSize: 15,
                                      color: Color.fromRGBO(
                                          92, 52, 76, 1)),
                                ),
                              );
                            }).toList(),
                            hint: Padding(
                              padding: const EdgeInsets.all(8.0),
                              child: Text("ALL"),
                            ),
                            onChanged: (newValues) {
                              setState(() {
                                _selectedjobcategory = newValues;
                                GetSubCategories(_selectedjobcategory);
                                subcategoryvisible=true;
                                
                              });
                            }),
                      ),
                    ),
                  ),

API

 Future<String> GetWorkCategories() async {
    Future auth_token = SharedPrefrence().getToken();
    auth_token.then((data) async {
      token = data;
      var response = await http.post(
        Urls.MENU,
        headers: {"Content-Type": "application/json", "Authorization": token},
      );
      print("response:" + response.body.toString());

      Map<String, dynamic> value = json.decode(response.body);
      var message = value['status'];
      try {
        if (response.statusCode == 200) {
          try {
            if (message == true) {
              var menu_list = value['doc'];
              for (int i = 0; i < menu_list.length; i++) {
                var data = menu_list[i];
                job_category_model.add(CategoryModel.fromJson(data));
              }
              setState(() {
                print("UI Updated");
              });
            } else {
              final snackBar = SnackBar(content: Text(message));
              _scaffoldKey.currentState.showSnackBar(snackBar);
            }
          } catch (e) {
            e.toString();
          }
        } else {
          final snackBar = SnackBar(content: Text(message));
          _scaffoldKey.currentState.showSnackBar(snackBar);
        }
      } catch (e) {
        e.toString();
      }
    });
  }

模型类

class CategoryModel{

  String id = "";
  String menu_id = "";
  String malayalam = "";
  String english = "";
  String image = "";


  CategoryModel(
      {this.id, this.menu_id, this.malayalam, this.english, this.image});

  CategoryModel.fromJson(json)
      : id = json['_id'].toString(),
        menu_id = json['menuId'].toString(),
        malayalam = json['translations']['malayalam'].toString(),
        english = json['translations']['english'].toString(),
        image = json['image'].toString();
}