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) {
  return new Positioned(
    top: rect.value.top,
    right: rect.value.right,
    bottom: rect.value.bottom,
    left: rect.value.left,
    child: child
  );
}