LogoutFab02

logout_fab_navigator_02.dart

class LogoutFab extends StatelessWidget {
  
  final VoidCallback onLogout;
  final Color color;

  const LogoutFab({Key key, @required this.onLogout, this.color}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    final authViewModel = context.watch<AuthViewModel>();
    return authViewModel.logingOut ? _inProgressFab() :  _extendedFab(authViewModel);
  }

  FloatingActionButton _inProgressFab() {
    return FloatingActionButton(
      onPressed: null,
      child: CircularProgressIndicator(backgroundColor: Colors.white),
      foregroundColor: color == null ? Colors.white : color.onTextColor(),
      backgroundColor: color,
    );
  }

  FloatingActionButton _extendedFab(AuthViewModel authViewModel) {
    return FloatingActionButton.extended(
      icon: Icon(Icons.exit_to_app),
      onPressed: () async {
        await authViewModel.onLogoutTap();
        onLogout();
      },
      foregroundColor: color == null ? Colors.white : color.onTextColor(),
      backgroundColor: color,
      label: Padding(
        padding: const EdgeInsets.all(16.0),
        child: Text('Logout'),
      ),
    );
  }
}

According to this video creating own FloatingActionButton and FloatingActionButton Widgets is preferrable over helper methods