1. override
Simulation createScrollSimulation(double position, double velocity)

Returns a simulation that propels the scrollOffset.

This function is called when a drag gesture ends.

Returns null if the behavior is to do nothing.

Source 

@override
Simulation createScrollSimulation(double position, double velocity) {
  return new ScrollSimulation(
    position: position,
    velocity: velocity,
    leadingExtent: minScrollOffset,
    trailingExtent: maxScrollOffset,
    platform: platform,
  );
}