1. override
Widget buildTransitions(BuildContext context, Animation<double> animation, Animation<double> forwardAnimation, Widget child)

Override this method to wrap the route in a number of transition widgets.

For example, to create a fade entrance transition, wrap the given child widget in a FadeTransition using the given animation as the opacity.

By default, the child is not wrapped in any transition widgets.

  • context The context in which the route is being built.
  • animation The animation for this route's transition. When entering, the animation runs forward from 0.0 to 1.0. When exiting, this animation runs backwards from 1.0 to 0.0.
  • forwardAnimation The animation for the route being pushed on top of this route. This animation lets this route coordinate with the entrance and exit transition of routes pushed on top of this route.

Source 

@override
Widget buildTransitions(BuildContext context, Animation<double> animation, Animation<double> forwardAnimation, Widget child) {
  if (Theme.of(context).platform == TargetPlatform.iOS &&
      Navigator.of(context).userGestureInProgress) {
    return new _CupertinoPageTransition(
      animation: new AnimationMean(left: animation, right: forwardAnimation),
      child: child
    );
  } else {
    return new _MountainViewPageTransition(
      animation: animation,
      child: child
    );
  }
}