1. override
Simulation createSnapScrollSimulation(double startOffset, double endOffset, double startVelocity, double endVelocity)

Returns an animation that ends at the snap offset.

This function is called when a drag gesture ends and a SnapOffsetCallback is specified for the scrollable.

Returns null if the behavior is to do nothing.

Source 

@override
Simulation createSnapScrollSimulation(double startOffset, double endOffset, double startVelocity, double endVelocity) {
  return _createSnapScrollSimulation(startOffset, endOffset, startVelocity, endVelocity);
}