Swift Collision Boundary not working - ios

Question

Swift 2.0, xcode 7, Ios 9
Intention: I want the two squares to drop to the bottom and stay at the bottom of the screen.
What is happening now: There is no error preventing the code from running, the gravity works fine but the collision just seems to be ignored. Thus resulting in the two objects falling through the bottom of the screen.
Note: the subclass is UIView not UIViewController and this view is accessed from a segue.
Many Thanks!
Code :
import UIKit
class graphs: UIView {
//create two shapes
var greenSquare: UIView?
var redSquare: UIView?
var animator: UIDynamicAnimator?
override func drawRect(rect: CGRect) {
var dimen = CGRectMake(25, 25, 60, 60)
greenSquare = UIView(frame: dimen)
greenSquare?.backgroundColor = UIColor.greenColor()
dimen = CGRectMake(130, 25, 90, 90)
redSquare = UIView(frame: dimen)
redSquare?.backgroundColor = UIColor.redColor()
//add them to the screen
self.addSubview(greenSquare!)
self.addSubview(redSquare!)
}
#IBAction func startbutton(sender: AnyObject) {
//initialise the animator
animator = UIDynamicAnimator()
//colision
let boundries = UICollisionBehavior(items:[greenSquare!,redSquare!])
boundries.translatesReferenceBoundsIntoBoundary = true
//add gravity
let gravity = UIGravityBehavior(items: [greenSquare!, redSquare!])
let direction = CGVectorMake(0.0, 1.0)
gravity.gravityDirection = direction
animator?.addBehavior(boundries)
animator?.addBehavior(gravity)
}
}

Answer 1

I fixed it by changing the subclass to a UIViewController
why did the previous code not work?
this is the new code:
import UIKit
class DrawView: UIViewController {
//Create two shapes
var greenSquare: UIView?
var redSquare: UIView?
var animator: UIDynamicAnimator?
override func viewDidLoad() {
super.viewDidLoad()
}
#IBAction func startbutton(sender: UIButton) {
//Create the shapes
var dimen = CGRectMake(25, 25, 60, 60)
greenSquare = UIView(frame: dimen)
greenSquare?.backgroundColor = UIColor.greenColor()
dimen = CGRectMake(130, 25, 90, 90)
redSquare = UIView(frame: dimen)
redSquare?.backgroundColor = UIColor.redColor()
//Add them to the screen
self.view.addSubview(greenSquare!)
self.view.addSubview(redSquare!)
//Initialize the animator
animator = UIDynamicAnimator(referenceView: self.view)
//Gravity
let gravity = UIGravityBehavior(items: [greenSquare!, redSquare!] )
let direction = CGVectorMake(0.0, 1.0)
gravity.gravityDirection = direction
//Collision
let boundries = UICollisionBehavior(items: [greenSquare!, redSquare!] )
boundries.translatesReferenceBoundsIntoBoundary = true
//Elasticity
let bounce = UIDynamicItemBehavior(items: [greenSquare!, redSquare!] )
bounce.elasticity = 0.5
animator?.addBehavior(bounce)
animator?.addBehavior(boundries)
animator?.addBehavior(gravity)
}
}