class _MyHomePageState extends StatelessWidget {

  @override
  Widget build(BuildContext context) {
    return Stack(
      children: [
        OverlappingPanels(
          // Using the Builder widget is not required. You can pass your widget directly. But to use `OverlappingPanelsState.of(context)` you need to wrap your widget in a Builder
          left: Builder(builder: (context) {
            return const LeftPage();
          }),
          right: Builder(
            builder: (context) => const RightPage(),
          ),
          main: Builder(
            builder: (context) {
              return const MainPage();
            },
          ),
          onSideChange: (side) {
            setState(() {
              if (side == RevealSide.main) {
                // hide something
              } else if (side == RevealSide.left) {
                // show something
              }
            });
          },
        ),
      ],
    );
  }
}

IconButton(
  icon: const Icon(Icons.menu),
  onPressed: () {
    // This is the main point
    OverlappingPanelsState.of(context)?.reveal(RevealSide.left);
  },
)