效果图：
highlight.gif
实现思路：
主要通过CAGradientLayer，实现渐变，然后将这个渐变图层添加到View上，通过添加基本动画，不行的改变渐变layer的位置，最终形成文字高亮的显示效果。
代码实现
创建一个UIView的分类，在分类中提供方法以方便在外部调用：
extension UIView {
func startPreviewLoading() {
layer.masksToBounds = true
let bgColor = UIColor.lightGray
let duration: TimeInterval = 1.5
backgroundColor = bgColor
let gradienLayer = CAGradientLayer()
gradienLayer.frame = CGRect(x: 0, y: 0, width: kShadowWidth, height: self.frame.size.height)
gradienLayer.colors = [ bgColor.withAlphaComponent(0.1).cgColor,
UIColor.white.withAlphaComponent(0.25).cgColor,
UIColor.white.withAlphaComponent(0.4).cgColor,
UIColor.white.withAlphaComponent(0.25).cgColor,
bgColor.withAlphaComponent(0.1).cgColor
]
gradienLayer.startPoint = CGPoint(x: 0, y: 0.5)
gradienLayer.endPoint = CGPoint(x: 1, y: 0.5)
layer.addSublayer(gradienLayer)
func animate() {
let animation = CABasicAnimation(keyPath: "position")
animation.fromValue = NSValue(cgPoint: CGPoint(x: -kShadowWidth / 2.0, y: self.frame.size.height / 2.0))
animation.toValue = NSValue(cgPoint: CGPoint(x: self.frame.size.width + kShadowWidth / 2.0, y: self.frame.size.height / 2.0))
animation.duration = duration
animation.repeatCount = MAXFLOAT
gradienLayer.add(animation, forKey: "position")
}
animate()
}
}
