如何获得RadioButtonGroup选择的位置

Question

我已经选择了一个RadioButtonGroup并为其设置了一些值。但是我希望用户选择单选按钮时我想要那个位置。

RadioButtonGroup(
        labels: <String>[
          GetAssessorLoginModel.response.eventData.students[0].theoryQuestions[mainIndex].optiona,
          GetAssessorLoginModel.response.eventData.students[0].theoryQuestions[mainIndex].optionb,
          GetAssessorLoginModel.response.eventData.students[0].theoryQuestions[mainIndex].optionc,
          GetAssessorLoginModel.response.eventData.students[0].theoryQuestions[mainIndex].optiond,
        ],
        onSelected: (String selected)=>  setState((){
          print("selected answer="+selected);              question_json[index].qstn_id=GetAssessorLoginModel.response.eventData.students[0].theoryQuestions[mainIndex].id;
          question_json[index].qstn_answer=selected;
         question_status[index].isAnswer=true;
         if(!question_status[index].isReviewed){
           question_status[index].isReviewed=false;
         }
          question_status[index].isVisited=true;
           print("value of answer"+question_status.length.toString());
        }),
       picked:question_json[index].qstn_answer,
    ),

RadioButtonGroup(
        labels: <String>[
          GetAssessorLoginModel.response.eventData.students[0].theoryQuestions[mainIndex].optiona,
          GetAssessorLoginModel.response.eventData.students[0].theoryQuestions[mainIndex].optionb,
          GetAssessorLoginModel.response.eventData.students[0].theoryQuestions[mainIndex].optionc,
          GetAssessorLoginModel.response.eventData.students[0].theoryQuestions[mainIndex].optiond,
        ],
        onSelected: (String selected)=>  setState((){
          /*radio_select = true;
          nothing_select=false;
          both_markreview_radioselect=false;
          if(mark_review){
            both_markreview_radioselect=true;
          }else{
            mark_review=false;
            both_markreview_radioselect=false;
          }*/
          print("selected answer="+selected);
          question_json[index].qstn_id=GetAssessorLoginModel.response.eventData.students[0].theoryQuestions[mainIndex].id;
          question_json[index].qstn_answer=selected;
         question_status[index].isAnswer=true;
         if(!question_status[index].isReviewed){
           question_status[index].isReviewed=false;
         }
          question_status[index].isVisited=true;
           print("value of answer"+question_status.length.toString());
        }),
       picked:question_json[index].qstn_answer,
    ),

在Onselected内部的代码中，我正在获取该单选按钮的值，但我想要该位置。