Adding UICollisionBehavior

FallingSquareViewController.swift

    lazy var collider:UICollisionBehavior = {
        let lazyCollider = UICollisionBehavior()
        // This line, makes the boundries of our reference view a boundary
        // for the added items to collide with.
        lazyCollider.translatesReferenceBoundsIntoBoundary = true
        return lazyCollider
        }()
    
    // This is how our animate function will look like    
    func animateSquare(){
        animator.addBehavior(gravity)
        // Add the collider also to the animator
        animator.addBehavior(collider)

        // Add the squareView to both behaviors
        collider.addItem(squareView)
        gravity.addItem(squareView)
    }