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