RelativePositionedTransition({Key key, @required Animation<Rect> rect, @required Size size, Widget child })

Create an animated version of Positioned.

Each frame, the Positioned widget will be configured to represent the current value of the rect argument assuming that the stack has the given size. Both rect and size must be non-null.

Source 

RelativePositionedTransition({
  Key key,
  @required Animation<Rect> rect,
  @required this.size,
  this.child
}) : super(key: key, animation: rect);