Lets draw a square in the middle of our view and make it fall to the bottom and stop at the bottom edge collising with the screen bottom boundary.
@IBOutlet var animationView: UIView!
var squareView:UIView!
var collision: UICollisionBehavior!
var animator: UIDynamicAnimator!
var gravity: UIGravityBehavior!
override func viewDidLoad() {
super.viewDidLoad()
let squareSize = CGSize(width: 30.0, height: 30.0)
let centerPoint = CGPoint(x: self.animationView.bounds.midX - (squareSize.width/2), y: self.animationView.bounds.midY - (squareSize.height/2))
let frame = CGRect(origin: centerPoint, size: squareSize)
squareView = UIView(frame: frame)
squareView.backgroundColor = UIColor.orangeColor()
animationView.addSubview(squareView)
animator = UIDynamicAnimator(referenceView: view)
gravity = UIGravityBehavior(items: [squareView])
animator.addBehavior(gravity)
collision = UICollisionBehavior(items: [square])
collision.translatesReferenceBoundsIntoBoundary = true
animator.addBehavior(collision)
}