custom_page_route.dart

Navigator.of(context).push(
      PageRouteBuilder<Null>(
          pageBuilder: (BuildContext context, Animation<double> animation,
              Animation<double> secondaryAnimation) {
            return AnimatedBuilder(
                animation: animation,
                builder: (BuildContext context, Widget child) {
                  return Opacity(
                    opacity: animation.value,
                    child: page,
                  );
                });
          },
          transitionDuration: Duration(milliseconds: 600)),
    );