abstract class AnimatedWidget extends StatefulWidget {
  const AnimatedWidget({
    Key? key,
    required this.listenable,
  }) : assert(listenable != null),
       super(key: key);
  final Listenable listenable;
  @protected
  Widget build(BuildContext context);
  @override
  State<AnimatedWidget> createState() => _AnimatedState();
  @override
  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
    super.debugFillProperties(properties);
    properties.add(DiagnosticsProperty<Listenable>('animation', listenable));
  }
}
class _AnimatedState extends State<AnimatedWidget> {
  @override
  void initState() {
    super.initState();
    widget.listenable.addListener(_handleChange);
  }
  @override
  void didUpdateWidget(AnimatedWidget oldWidget) {
    super.didUpdateWidget(oldWidget);
    if (widget.listenable != oldWidget.listenable) {
      oldWidget.listenable.removeListener(_handleChange);
      widget.listenable.addListener(_handleChange);
    }
  }
  @override
  void dispose() {
    widget.listenable.removeListener(_handleChange);
    super.dispose();
  }
  void _handleChange() {
    setState(() {
      // The listenable's state is our build state, and it changed already.
    });
  }
  @override
  Widget build(BuildContext context) => widget.build(context);
}