Ios 在Swift中为图像淡入淡出动画

我有一组图像，我想有淡入我的登录屏幕背景。我在斯威夫特找不到任何能做到这一点的东西。有没有一种方法我能做到

这是我目前的代码：

override func viewDidLoad() {
    super.viewDidLoad()
    startAnimating()
    // Do any additional setup after loading the view, typically from a nib.
}

func startAnimating(){
    backgroundImage.animationImages = [
        UIImage(named: "background1.jpg"),
        UIImage(named: "background2.jpg")
    ]

    backgroundImage.animationDuration = 3
    backgroundImage.startAnimating()

}

---

下面的代码将帮助您设置具有淡出效果的图像动画，并更改背景

import UIKit

class ViewController: UIViewController {

    var imgView:UIImageView?  //class variable to display you bg image

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.


        imgView = UIImageView(frame: self.view.frame) //imgView to display background
        self.view.insertSubview(imgView, atIndex: 0)  //imgview add above the background of main view

        animateImage(1) /*call animageImage with parameter number as image number as i user image name as avatar1.png, avatar2.png, avatar3.png and avatar4.png*/

    }


    func animateImage(no:Int)
    {
        var imgNumber:Int = no
        let t:NSTimeInterval = 1;
        let t1:NSTimeInterval = 0;
        var name:String = "avatar\(imgNumber).png"
        imgView!.alpha = 0.4
        imgView!.image = UIImage(named:name);

        //code to animate bg with delay 2 and after completion it recursively calling animateImage method 
        UIView.animateWithDuration(2.0, delay: 0, options:UIViewAnimationOptions.CurveEaseOut, animations: {() in 
                                                      self.imgView!.alpha = 1.0;
                                                   }, 
                                         completion: {(Bool) in
                                                      imgNumber++;
                                                      if imgNumber>4  //only for 4 image
                                                      {
                                                          imgNumber = 1
                                                      }
                                                      self.animateImage(imgNumber); 
                                                   })
    }
}

---

这段代码运行良好。你想用另一种方法来实现这个动画吗？是的，在图像之间有一种缓慢的淡入淡出