smooth rounded corners in swift

Starting iOS 13.0, you can just use this property in addition to setting the cornerRadius:

sampleButton.layer.cornerCurve = .continuous

This is also easily animatable, which previously resulted in problems using a UIBezierPath.

See more at https://developer.apple.com/documentation/quartzcore/calayer/3152596-cornercurve

Prior to iOS 13

To get that effect you can use UIBezierPath

Edit: Usage in UIView

class MyView: UIView {

    let myButton UIButton = UIButton()

    override func layoutSubviews() {
        super.layoutSubviews()

        // your roundPath code here
    }

}

Edit 2:

Use this approach for rounding specific corners:

let roundPath = UIBezierPath(
    roundedRect: bounds,
    byRoundingCorners: [.topLeft, .topRight],
    cornerRadii: CGSize(width: 10, height: 10)
)

Source