let centerXLC : NSLayoutConstraint = NSLayoutConstraint(
  item: self.asvScrollView, attribute: NSLayoutAttribute.CenterX, relatedBy: NSLayoutRelation.Equal, 
  toItem: self.view, attribute: NSLayoutAttribute.CenterX, multiplier: 1.0, constant: 0)
  
let centerYLC : NSLayoutConstraint = NSLayoutConstraint(
  item: self.asvScrollView, attribute: NSLayoutAttribute.CenterY, relatedBy: NSLayoutRelation.Equal, 
  toItem: self.view, attribute: NSLayoutAttribute.CenterY, multiplier: 1.0, constant: 0)
  self.view.addConstraints([centerXLC,centerYLC])