carriedMomentum method
Returns the velocity carried on repeated flings.
The function is applied to the existing scroll velocity when another scroll drag is applied in the same direction.
By default, physics for platforms other than iOS doesn't carry momentum.
Implementation
double carriedMomentum(double existingVelocity) {
if (parent == null)
return 0.0;
return parent.carriedMomentum(existingVelocity);
}