slider.dart

class MySlider extends StatefulWidget {
  @override
  _MySliderState createState() => _MySliderState();
}

class _MySliderState extends State<MySlider> {
  double _value = 0.5;

  @override
  Widget build(BuildContext context) {
    return SliderTheme(
      data: SliderTheme.of(context).copyWith(
        thumbShape: RoundSliderThumbShape()
      ),
      child: Slider(
        value: _value,
        onChanged: (newValue){
          setState(() {
            _value = newValue;
          });
        },
      ),
    );
  }
}