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) {
  FractionalOffset alignment;
  if (axis == Axis.vertical)
    alignment = new FractionalOffset(0.0, axisAlignment);
  else
    alignment = new FractionalOffset(axisAlignment, 0.0);
  return new ClipRect(
    child: new Align(
      alignment: alignment,
      heightFactor: axis == Axis.vertical ? sizeFactor.value : null,
      widthFactor: axis == Axis.horizontal ? sizeFactor.value : null,
      child: child
    )
  );
}