1. override
Future<Null> fling(double scrollVelocity)

If scrollVelocity is greater than PixelScrollTolerance.velocity then fling the scroll offset with the given velocity in logical pixels/second. Otherwise, if this scrollable is overscrolled or a snapOffsetCallback was given, animate the scroll offset to its final value with settleScrollOffset.

Calling this function starts a physics-based animation of the scroll offset with the given value as the initial velocity. The physics simulation is determined by the scroll behavior.

The returned Future completes when the scrolling animation is complete.

Source 

@override
Future<Null> fling(double scrollVelocity) {
  switch(config.itemsSnapAlignment) {
    case PageableListFlingBehavior.canFlingAcrossMultiplePages:
      return super.fling(scrollVelocity).then(_notifyPageChanged);
    case PageableListFlingBehavior.stopAtNextPage:
      return _flingToAdjacentItem(scrollVelocity);
  }
  assert(config.itemsSnapAlignment != null);
  return null;
}