gravity.magnitude = 12 let collision = UICollisionBehavior(items: [destination]) collision.addBoundaryWithIdentifier("bottom", fromPoint: CGPoint(x: 0, y: source.bounds.height), toPoint: CGPoint(x: source.bounds.width, y: source.bounds.height)) let item = UIDynamicItemBehavior(items: [destination]) item.elasticity = 0.5 item.resistance = 0.8 animator.delegate = self animator.addBehavior(gravity) animator.addBehavior(collision) animator.addBehavior(item)