createAnimation method
Called to create the animation that exposes the current progress of the transition controlled by the animation controller created by createAnimationController().
Implementation
@override
Animation<double> createAnimation() {
assert(_animation == null);
_animation = CurvedAnimation(
parent: super.createAnimation(),
curve: Curves.linearToEaseOut,
reverseCurve: Curves.linearToEaseOut.flipped,
);
_offsetTween = Tween<Offset>(
begin: const Offset(0, 1),
end: Offset.zero,
);
return _animation!;
}