ios - Updating constraints in animation with Swift -

i want animation move uiimageview new position defined new set of constraints, reason cannot work. i'm trying do:

@iboutlet weak var oldbottom: nslayoutconstraint!  ...  func animatemovement() {      // here i'm defining new constraint      var newbottom = nslayoutconstraint(         item: self.logo,         attribute: nslayoutattribute.bottom,         relatedby: nslayoutrelation.equal,         toitem: self.usernamefield,         attribute: nslayoutattribute.bottom,         multiplier: 1,         constant: 15)       self.view.layoutifneeded()      uiview.animatewithduration(1.0, animations: {         // here i'm setting existing constraint (defined in storyboard) new constraint          self.oldbottom = newbottom         self.view.layoutifneeded()          }, completion: {}) } 

any thoughts appreciated!

remove self.view.layoutifneeded() outside block. have 1 inside animation block , should sufficient.