attach method
Register the given position with this controller.
After this function returns, the animateTo and jumpTo methods on this controller will manipulate the given position.
Implementation
@override
void attach(ScrollPosition position) {
super.attach(position);
final ExtendedPagePosition pagePosition = position as ExtendedPagePosition;
pagePosition.viewportFraction = viewportFraction;
pagePosition.pageSpacing = pageSpacing;
}