gistfile1.swift

@IBAction func viewDragged(sender: UIPanGestureRecognizer) {
  let yTranslation = sender.translationInView(view).y
  if (hasExceededVerticalLimit(topViewConstraint.constant)){
    totalTranslation += yTranslation
    topViewConstraint.constant = logConstraintValueForYPosition(totalTranslation)
    if(sender.state == UIGestureRecognizerState.Ended ){
       animateViewBackToLimit()
    }
  } else {
    topViewConstraint.constant += yTranslation
  }
  sender.setTranslation(CGPointZero, inView: view)
}

func logConstraintValueForYPosition(yPosition : CGFloat) -> CGFloat {
  return verticalLimit * (1 + log10(yPosition/verticalLimit))
}