如何在flutter的下拉菜单中创建单选按钮？

import 'package:flutter/material.dart';

class MyDropdownButton extends StatefulWidget {
  @override
  _MyDropdownButtonState createState() => _MyDropdownButtonState();
}

class _MyDropdownButtonState extends State<MyDropdownButton> {
  String _selectedOption;

  @override
  Widget build(BuildContext context) {
    return DropdownButtonFormField<String>(
      value: _selectedOption,
      onChanged: (String newValue) {
        setState(() {
          _selectedOption = newValue;
        });
      },
      items: <String>['Option 1', 'Option 2', 'Option 3', 'Option 4']
          .map<DropdownMenuItem<String>>((String value) {
        return DropdownMenuItem<String>(
          value: value,
          child: Text(value),
        );
      }).toList(),
      decoration: InputDecoration(
        labelText: 'Select an option',
        border: OutlineInputBorder(),
      ),
    );
  }
}