Pageable({Key key, double initialScrollOffset, Axis scrollDirection: Axis.vertical, ViewportAnchor scrollAnchor: ViewportAnchor.start, ScrollListener onScrollStart, ScrollListener onScroll, ScrollListener onScrollEnd, SnapOffsetCallback snapOffsetCallback, bool itemsWrap: false, PageableListFlingBehavior itemsSnapAlignment: PageableListFlingBehavior.stopAtNextPage, ValueChanged<int> onPageChanged, Duration duration: const Duration(milliseconds: 200), Curve curve: Curves.ease })

Initializes fields for subclasses.

The scrollDirection, scrollAnchor, and itemsSnapAlignment arguments must not be null.

Source 

Pageable({
  Key key,
  double initialScrollOffset,
  Axis scrollDirection: Axis.vertical,
  ViewportAnchor scrollAnchor: ViewportAnchor.start,
  ScrollListener onScrollStart,
  ScrollListener onScroll,
  ScrollListener onScrollEnd,
  SnapOffsetCallback snapOffsetCallback,
  this.itemsWrap: false,
  this.itemsSnapAlignment: PageableListFlingBehavior.stopAtNextPage,
  this.onPageChanged,
  this.duration: const Duration(milliseconds: 200),
  this.curve: Curves.ease
}) : super(
  key: key,
  initialScrollOffset: initialScrollOffset,
  scrollDirection: scrollDirection,
  scrollAnchor: scrollAnchor,
  onScrollStart: onScrollStart,
  onScroll: onScroll,
  onScrollEnd: onScrollEnd,
  snapOffsetCallback: snapOffsetCallback
) {
  assert(itemsSnapAlignment != null);
}