UIImageView is moving back after new UIView is created - ios

Question

Hi I am trying to make a game using swift but am currently very stuck. My game has two buttons that move a UIImageView left and right which works perfectly although when my NSTimer calls to my animateBalls function every 2.5 seconds the UIImageView that was moved goes back to its original position. How would I be able to still be able to create a New UIView every 2.5 seconds but keep the UIIMageViews current position? Thanks for any help that you can give me, Here is my code:
import UIKit
class ViewController: UIViewController {
let speed = 10.0
#IBOutlet weak var mainBar: UIImageView!
override func viewDidLoad() {
super.viewDidLoad()
_ = NSTimer.scheduledTimerWithTimeInterval(2.5, target: self, selector: Selector("animateBalls"), userInfo: nil, repeats: true)
}
#IBAction func moveRightIsTapped(sender: AnyObject) {
if(mainBar.frame.origin.x < -158.5)
{
let xPosition = mainBar.frame.origin.x + 38.5
let yPosition = mainBar.frame.origin.y
let height = mainBar.frame.height
let width = mainBar.frame.width
mainBar.frame = CGRectMake(xPosition, yPosition, width, height)
}
}
#IBAction func moveLeftIsTapped(sender: AnyObject) {
if(mainBar.frame.origin.x > -389.5)
{
let xPosition = mainBar.frame.origin.x - 38.5
let yPosition = mainBar.frame.origin.y
let height = mainBar.frame.height
let width = mainBar.frame.width
mainBar.frame = CGRectMake(xPosition, yPosition, width, height)
}
}
#IBAction func playButtonTapped(sender: AnyObject) {
animateBalls()
}
func animateBalls() {
randomNum = Int(arc4random_uniform(1))
if(randomNum == 0)
{
let ball1 = UIView()
ball1.frame = CGRect(x: 121, y: -20, width: 20, height: 20)
ball1.layer.cornerRadius = 10
ball1.clipsToBounds = true
ball1.backgroundColor = UIColor.purpleColor()
self.view.addSubview(ball1)
UIView.animateWithDuration(speed, animations:{ ball1.frame = CGRect(x: 121, y: 705, width: ball1.frame.width, height: ball1.frame.height) })
}
if(randomNum == 1)
{
let ball2 = UIView()
ball2.frame = CGRect(x: 159, y: -20, width: 20, height: 20)
ball2.layer.cornerRadius = 10
ball2.clipsToBounds = true
ball2.backgroundColor = UIColor.greenColor()
self.view.addSubview(ball2)
UIView.animateWithDuration(speed, animations:{ ball2.frame = CGRect(x: 159, y: 705, width: ball2.frame.width, height: ball2.frame.height) })
}