Radio

material design 的单选按钮。

 const Radio({
    Key key,
    @required this.value, //值
    @required this.groupValue,//组值
    @required this.onChanged,//返回value值
    this.activeColor, //激活时颜色
    this.focusColor,
    this.hoverColor,
    this.materialTapTargetSize,
    this.visualDensity,
    this.focusNode,
    this.autofocus = false,
  })