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) {
  if ((isScrollable && velocity.abs() > 0) || position < minScrollOffset || position > maxScrollOffset) {
    // If the triggering gesture starts at or beyond the contentExtent's limits
    // then the simulation only serves to settle the scrollOffset back to its
    // minimum or maximum value.
    if (position < minScrollOffset || position > maxScrollOffset)
      velocity = 0.0;
    return super.createScrollSimulation(position, velocity);
  }
  return null;
}