Swift 设置动画时UIImageView位置错误

我只是想用UIImageView做一个小转盘，当我添加第二个图像时，奇怪的事情发生了。从只有一个图像开始，我只想把它发送到左边，所以我只做了下面的代码，效果非常好，我的意思是，图像按预期向左移动

        let frame = self.mainImage.frame
        var newFrame = frame

        UIView.animateWithDuration(0.9, animations: { () -> Void in
            self.mainImage.frame.origin.x = 0 - frame.size.width

            }, completion: { (done) -> Void in
                if done {
                     self.mainImage.frame = frame
                }
            }
        )

但是，当我添加第二个图像时，两个图像都来自右侧。也许这很简单，但我看不出发生了什么

        let frame = self.mainImage.frame
        var newFrame = frame
        newFrame.origin.x = self.view.frame.size.width + 50

        let newImageView = UIImageView(frame: newFrame)
        newImageView.image = UIImage(named: "C2")
        newImageView.contentMode = .ScaleAspectFit
        self.view.addSubview(newImageView)
        UIView.animateWithDuration(0.9, animations: { () -> Void in
            self.mainImage.frame.origin.x = 0 - frame.size.width
            newImageView.frame = CGRectMake(frame.origin.x, frame.origin.y, frame.size.width, frame.size.height)
            }, completion: { (done) -> Void in
                if done {
                    self.mainImage.frame = frame
                    newImageView.removeFromSuperview()
                }
            }
        )