您可以很轻松地使用CoreAnimation实现此目的。 我添加了一个简单的演示，您可以使用它here进行播放（只需构建项目并点击任意位置即可观看动画）。

逻辑如下：

1. 创建UIView的自定义子类。
2. 设置了某些文本后，创建两个相似的CATextLayers，每个文本和框架都相同。
3. 为这些图层设置不同的foregroundColor和mask。左侧层的mask将是视图的左侧，右侧层的mask将是视图的右侧。
4. 为这些图层同时动画foregroundColor。

自定义视图的代码：

class CustomTextLabel: UIView {
    override init(frame: CGRect) {
        super.init(frame: frame)
        backgroundColor = .green
    }

    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    private var textLayer1: CATextLayer?
    private var textLayer2: CATextLayer?

    func setText(_ text: String,fontSize: CGFloat) {
        // create 2 layers with the same text and size,we'll set the colors for them later
        textLayer1 = createTextLayer(text,fontSize: fontSize)
        textLayer2 = createTextLayer(text,fontSize: fontSize)

        // estimate the frame size needed for the text layer with such text and font size
        let textSize = textLayer1!.preferredFrameSize()
        let w = frame.width,h = frame.height

        // calculate the frame such that both layers will be in center of view
        let centeredTextFrame = CGRect(x: (w-textSize.width)/2,y: (h-textSize.height)/2,width: textSize.width,height: textSize.height)
        textLayer1!.frame = centeredTextFrame
        textLayer2!.frame = centeredTextFrame

        // set up default color for the text
        textLayer1!.foregroundColor = UIColor.yellow.cgColor
        textLayer2!.foregroundColor = UIColor.yellow.cgColor

        // set background transparent,that's very important
        textLayer1!.backgroundColor = UIColor.clear.cgColor
        textLayer2!.backgroundColor = UIColor.clear.cgColor

        // set up masks,such that each layer's text is visible only in its part
        textLayer1!.mask = createMaskLayer(CGRect(x: 0,y: 0,width: textSize.width/2,height: textSize.height))
        textLayer2!.mask = createMaskLayer(CGRect(x: textSize.width/2,height: textSize.height))

        layer.addSublayer(textLayer1!)
        layer.addSublayer(textLayer2!)
    }

    private var finishColor1: UIColor = .black,finishColor2: UIColor = .black
    func animateText(leftPartColor1: UIColor,leftPartColor2: UIColor,rightPartColor1: UIColor,rightPartColor2: UIColor) {
        finishColor1 = leftPartColor2
        finishColor2 = rightPartColor2

        if let layer1 = textLayer1,let layer2 = textLayer2 {
            CATransaction.begin()
            let animation1 = CABasicAnimation(keyPath: "foregroundColor")
            animation1.fromValue = leftPartColor1.cgColor
            animation1.toValue = leftPartColor2.cgColor
            animation1.duration = 3.0
            layer1.add(animation1,forKey: "animation1")


            let animation2 = CABasicAnimation(keyPath: "foregroundColor")
            animation2.fromValue = rightPartColor1.cgColor
            animation2.toValue = rightPartColor2.cgColor
            animation2.duration = 3.0
            layer2.add(animation2,forKey: "animation2")

            CATransaction.setCompletionBlock {
                self.textLayer1?.foregroundColor = self.finishColor1.cgColor
                self.textLayer2?.foregroundColor = self.finishColor2.cgColor
            }

            CATransaction.commit()
        }
    }

    private func createTextLayer(_ text: String,fontSize: CGFloat) -> CATextLayer {
        let textLayer = CATextLayer()
        textLayer.string = text
        textLayer.fontSize = fontSize // TODO: also set font name
        textLayer.contentsScale = UIScreen.main.scale

        return textLayer
    }

    private func createMaskLayer(_ holeRect: CGRect) -> CAShapeLayer {
        let layer = CAShapeLayer()

        let path = CGMutablePath()

        path.addRect(holeRect)
        path.addRect(bounds)

        layer.path = path
        layer.fillRule = CAShapeLayerFillRule.evenOdd
        layer.opacity = 1

        return layer
    }
}

自定义视图的调用：

class ViewController: UIViewController {

    var customLabel: CustomTextLabel!
    override func viewDidLoad() {
        super.viewDidLoad()

        let viewW = view.frame.width,viewH = view.frame.height
        let labelW: CGFloat = 200,labelH: CGFloat = 50

        customLabel = CustomTextLabel(frame: CGRect(x: (viewW-labelW)/2,y: (viewH-labelH)/2,width: labelW,height: labelH))
        customLabel.setText("Optimizing...",fontSize: 20)
        view.addSubview(customLabel)

        let tapRecogniner = UITapGestureRecognizer(target: self,action: #selector(onTap))
        view.addGestureRecognizer(tapRecogniner)
    }

    @objc func onTap() {
        customLabel.animateText(leftPartColor1: UIColor.blue,leftPartColor2: UIColor.red,rightPartColor1: UIColor.white,rightPartColor2: UIColor.black)
    }

}

由于Olha（@OlhaPavliuk）的回答，我为文本层使用了两个CATextLayer形状和两个CAShapeLayer蒙版。在绘制方法中，我只是将蒙版帧更改为计算出的大小（bounds.width *进度值），然后将第二个蒙版原点更改为新的起点（bounds.width-bounds.width *进度值）。

此外，在创建遮罩时设置layer.fillRule = CAShapeLayerFillRule.evenOdd非常重要，这样两层都可见。

事实证明，我实际上不需要任何动画代码，因为更改帧看起来还可以。

运动中：https://giphy.com/gifs/LMbmlMoxY9oaWhXfO1

完整代码：https://gist.github.com/joliejuly/a792c2ab8d97d304d731a4a5202f741a