Initial Colour slider build

ColourSlider1.dart

class ColourSlider extends StatefulWidget {
  static final track = "track";
  static final thumb = "thumb";

  @override
  _ColourSliderState createState() => _ColourSliderState();
}

class _ColourSliderState extends State<ColourSlider> {
  @override
  Widget build(BuildContext context) {
    return Container(
      width: 300,
      height: 50,
      child: CustomMultiChildLayout(
        delegate: _ColourSliderDelegate(),
        children: <Widget>[
          LayoutId(
            id: ColourSlider.track,
            child: Container(
              width: 300,
              height: 20,
              color: Colors.white,
            ),
          ),
          LayoutId(
            id: ColourSlider.thumb,
            child: Container(
              width: 50,
              height: 50,
              color: Colors.red,
            ),
          ),
        ],
      ),
    );
  }
}