onLoad method
Implementation
Future<void> onLoad() async {
await _loader.load();
if (_backgroundSprite == null) {
_paintBackground = Paint()
..color = color.withOpacity(0.5)
..style = PaintingStyle.fill;
}
if (_paintKnob == null) {
_paintKnob = Paint()
..color = color.withOpacity(0.8)
..style = PaintingStyle.fill;
}
}