Adding Gravity behavior to multiple objects in Swift

Question

I am building a game in which there are lots of square blocks floating around the screen, bouncing off each other, walls, etc until they get destroyed. Currently, I have a master UIGravityBehavior and I add each square (a UIView class called SquareView) as it's created, then remove it when it's destroyed. But I think I should be doing this inside the SquareView object. Here's the relevant code:

class ViewController: UIViewController, UICollisionBehaviorDelegate  {
    var gravity: UIGravityBehavior!

...

// next sequence adds a square
let newSquare = SquareView()
newSquare.createSquare(touch.locationInView(view).x, y: touch.locationInView(view).y)
view.addSubview(newSquare)
gravity.addItem(newSquare)

...

// this removes a square
gravity.removeItem(currentSquare)

So I believe I should be moving this code outside of the ViewController into my definition of my SquareView class:

func createSquare(x: CGFloat, y: CGFloat){
    self.backgroundColor = color[touchCount]
    self.frame = CGRect(x: x, y: y, width: size[touchCount], height: size[touchCount])
}

Incidentally, I do the same thing with UICollisionBehavior and assume that's not optimal as well.

Any help would be great!