1. override
Widget build(BuildContext context)

Override this method to build widgets that depend on the current value of the animation.

Source 

@override
Widget build(BuildContext context) {
  final RelativeRect offsets = new RelativeRect.fromSize(rect.value, size);
  return new Positioned(
    top: offsets.top,
    right: offsets.right,
    bottom: offsets.bottom,
    left: offsets.left,
    child: child
  );
}