slider-2.dart

  @override
  Widget build(BuildContext context) {
    return SliderTheme(
      data: SliderTheme.of(context).copyWith(
        thumbShape: RoundSliderThumbShape(enabledThumbRadius: 20)
      ),
      child: Slider(
        value: _value,
        onChanged: (newValue){
          setState(() {
            _value = newValue;
          });
        },
      ),
    );
  }