如何在 Flutter 中设置默认选中的单选按钮？

Question

默认情况下，Flutter 将所有单选按钮显示为空（未选中）。

如何设置默认选中的单选按钮？

我发布这个问题是为了记录我的解决方案，这可能会对某些人有所帮助，并且还开始讨论这个问题，因为我在这里没有找到任何相关信息。

在单选按钮代码下方：

class _ProductTypeScreen extends State<ProductType> {

  String _radioValue; //Initial definition of radio button value
  String choice;

  void radioButtonChanges(String value) {
    setState(() {
      _radioValue = value;
      switch (value) {
        case 'one':
          choice = value;
          break;
        case 'two':
          choice = value;
          break;
        case 'three':
          choice = value;
          break;
        default:
          choice = null;
      }
      debugPrint(choice); //Debug the choice in console
    });
  }

  // Now in the BuildContext... body widget:

  @override
  Widget build(BuildContext context) {
  //First of the three radio buttons

  Row(
    children: <Widget>[
      Radio(
        value: 'one',
        groupValue: _radioValue,
        onChanged: radioButtonChanges,
      ),
      Text(
        "One selected",
      ),
    ],
  ),

Answer 1

添加初始状态

class _ProductTypeScreen extends State<ProductType> {

  String _radioValue; //Initial definition of radio button value
  String choice;

  // ------ [add the next block] ------ 
  @override
  void initState() {
    setState(() {
      _radioValue = "one";
    });
    super.initState();
  }
  // ------ end: [add the next block] ------  

  void radioButtonChanges(String value) {
    setState(() {
      _radioValue = value;
      switch (value) {
        case 'one':
          choice = value;
          break;
        case 'two':
          choice = value;
          break;
        case 'three':
          choice = value;
          break;
        default:
          choice = null;
      }
      debugPrint(choice); //Debug the choice in console
    });
  }

  @override
  Widget build(BuildContext context) {

Answer 2

我举一个简单的例子来理解：

int _radioSelected = 1;**
String _radioVal;

     child: Row(
                mainAxisAlignment: MainAxisAlignment.center,
                children: [
                  Text('Male'),
                  Radio(
                    value: 1,
                    groupValue: _radioSelected,
                    activeColor: Colors.blue,
                    onChanged: (value) {
                      setState(() {
                        _radioSelected = value;
                        _radioVal = 'male';
                      });
                    },
                  ),
                  Text('Female'),
                  Radio(
                    value: 2,
                    groupValue: _radioSelected,
                    activeColor: Colors.pink,
                    onChanged: (value) {
                      setState(() {
                        _radioSelected = value;
                        _radioVal = 'female';
                      });
                    },
                  )
                ],
              ),