main.dart

class _MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Column(
      children: [
        ExpensiveWidget(focusNode: _node),
        StateBuilder<double>(
          create: (BuildContext context) => 1.0,
          builder: (BuildContext context, double opacity, SetStateCallback<double> setOpacity) {
            return GestureDetector(
              onTap: () => setOpacity(opacity == 1.0 ? 0.5 : 1.0),
              child: Opacity(opacity: opacity, child: widget.child),
            );
          },
        ),
      ],
    );
  }
}