打开和关闭Facebook的登录视图后，动画背景会停止

Question

在我的登录视图中，有一个连续的背景动画和一个用于登录的Facebook按钮。

问题是，当我打开Facebook登录页面（Safari）并关闭它时，动画停止，因为它可以在下面看到：

关注我的动画代码：

    //MARK: - View animations
    func animateLogin(){

        self.animateBackground(imageName: "backgroundLogin-layer1", scrolling: .Left, timing: 30.0)
        self.animateBackground(imageName: "backgroundLogin-layer2", scrolling: .Right, timing: 35.0)
        self.animateBackground(imageName: "backgroundLogin-layer3", scrolling: .Right, timing: 50.0)
        self.animateBackground(imageName: "backgroundLogin-layer4", scrolling: .Right, timing: 60.0)

    }

    func animateBackground(imageName img: String, scrolling side: Side, timing duration: NSTimeInterval){

        let backgroundImage = UIImage(named:img)!
        let amountToKeepImageSquare = backgroundImage.size.height - self.view.frame.size.height

        let backgroundImageView1 = UIImageView(image: backgroundImage)
        backgroundImageView1.frame = CGRect(x: self.view.frame.origin.x, y: self.view.frame.origin.y, width: backgroundImage.size.width - amountToKeepImageSquare, height: self.view.frame.size.height)
        self.view.addSubview(backgroundImageView1)

        var positionXSecondImage: CGFloat
        var multiplier: CGFloat

        if side == .Right{
            //Scrolling right
            positionXSecondImage = (backgroundImageView1.frame.size.width * -1.0)
            multiplier = 1
        }else{
            //Scrolling left
            positionXSecondImage = backgroundImageView1.frame.size.width
            multiplier = -1
        }

        let backgroundImageView2 = UIImageView(image: backgroundImage)
        backgroundImageView2.frame = CGRect(x: positionXSecondImage, y: self.view.frame.origin.y, width: backgroundImage.size.width - amountToKeepImageSquare, height: self.view.frame.height)
        self.view.addSubview(backgroundImageView2)

        self.view.sendSubviewToBack(backgroundImageView1)
        self.view.sendSubviewToBack(backgroundImageView2)

        // Animate background
        UIView.animateWithDuration(duration, delay: 0.0, options: [.Repeat, .CurveLinear], animations: {
            backgroundImageView1.frame = CGRectOffset(backgroundImageView1.frame, multiplier * backgroundImageView1.frame.size.width, 0.0)
            backgroundImageView2.frame = CGRectOffset(backgroundImageView2.frame, multiplier * backgroundImageView2.frame.size.width, 0.0)
            }, completion: nil)

    }

我在 ViewDidLoad 方法中调用 animateLogin 函数，我已经尝试在 ViewDidAppear

中调用它

    //MARK: - App's lifecycle
    override func viewDidLoad() {
        super.viewDidLoad()

        ...
        ...
        //Calling function taht makes background animted
        self.animateLogin()
        ...
        ...


    }

我需要在关闭Facebook登录视图后重新启动动画。