How to make a UIView have an effect of transparent gradient?

In Swift 4: for top to bottom transparent gradient.

let gradient = CAGradientLayer()
gradient.startPoint = CGPoint(x: 0.5, y: 0.0)
gradient.endPoint = CGPoint(x: 0.5, y: 0.6)
let whiteColor = UIColor.white
gradient.colors = [whiteColor.withAlphaComponent(0.0).cgColor, whiteColor.withAlphaComponent(1.0).cgColor, whiteColor.withAlphaComponent(1.0).cgColor]
gradient.locations = [NSNumber(value: 0.0),NSNumber(value: 0.2),NSNumber(value: 1.0)]
gradient.frame = gradientView.bounds
gradientView.layer.mask = gradient

Screenshot

Swift code

let mask = CAGradientLayer()
mask.startPoint = CGPointMake(0.0, 0.5)
mask.endPoint = CGPointMake(1.0, 0.5)
let whiteColor = UIColor.whiteColor()
mask.colors = [whiteColor.colorWithAlphaComponent(0.0).CGColor,whiteColor.colorWithAlphaComponent(1.0),whiteColor.colorWithAlphaComponent(1.0).CGColor]
mask.locations = [NSNumber(double: 0.0),NSNumber(double: 0.2),NSNumber(double: 1.0)]
mask.frame = label.bounds
label.layer.mask = mask