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

Creates a new physics simulation with the given initial conditions.

Source 

@override
Simulation release(double position, double velocity) {
  double target = velocity < 0.0 ? this.left - tolerance.distance
                                 : this.right + tolerance.distance;
  return new SpringSimulation(spring, position, target, velocity)
    ..tolerance = tolerance;
}