CustomRoundRect.swift

import UIKit

private func angle(_ degree: Double) -> CGFloat {
    return CGFloat(Double.pi * degree / 180.0)
}

let bounds = CGRect(x: 0, y: 0, width: 150, height: 100)

// corner radius for bottom right, bottom left, top left, and top right
let r: (CGFloat, CGFloat, CGFloat, CGFloat) = (5, 10, 20, 40)

let path = UIBezierPath()
path.addArc(withCenter: CGPoint(x: bounds.maxX - r.0, y: bounds.maxY - r.0), radius: r.0, startAngle: angle(0),   endAngle: angle(90),  clockwise: true)
path.addArc(withCenter: CGPoint(x: bounds.minX + r.1, y: bounds.maxY - r.1), radius: r.1, startAngle: angle(90),  endAngle: angle(180), clockwise: true)
path.addArc(withCenter: CGPoint(x: bounds.minX + r.2, y: bounds.minY + r.2), radius: r.2, startAngle: angle(180), endAngle: angle(270), clockwise: true)
path.addArc(withCenter: CGPoint(x: bounds.maxX - r.3, y: bounds.minY + r.3), radius: r.3, startAngle: angle(270), endAngle: angle(0),   clockwise: true)
path.close()

let maskLayer = CAShapeLayer()
maskLayer.frame = bounds
maskLayer.path = path.cgPath

let view = UIView(frame: bounds)
view.layer.mask = maskLayer
view.backgroundColor = .red