Here is my current code:

override func viewDidLoad() {
    super.viewDidLoad()

    let rect = Draw(frame: CGRect(
        origin: CGPoint(x: 50, y: 50),
        size: CGSize(width: 100, height: 100)))
    self.view.addSubview(rect)
}

Use this custom class, basically you need create a bezier path, using lines and quad curves, handling every corner with values in @IBInspectable properties.

This is the code

//
//  CornerView.swift
//  UIViewCornerRounded
//
//  Created by Reinier Melian on 21/07/2017.
//  Copyright © 2017 Pruebas. All rights reserved.
//

import UIKit


@IBDesignable
class CornerView: UIView {
    
    @IBInspectable var leftTopRadius : CGFloat = 0{
        didSet{
            self.applyMask()
        }
    }
    @IBInspectable var rightTopRadius : CGFloat = 0{
        didSet{
            self.applyMask()
        }
    }
    @IBInspectable var rightBottomRadius : CGFloat = 0{
        didSet{
            self.applyMask()
        }
    }
    
    @IBInspectable var leftBottomRadius : CGFloat = 0{
        didSet{
            self.applyMask()
        }
    }

    
    override func layoutSubviews() {
        super.layoutSubviews()
        self.applyMask()
    }
    
    // Only override draw() if you perform custom drawing.
    // An empty implementation adversely affects performance during animation.
    /*override func draw(_ rect: CGRect) {
        super.draw(rect)

    }*/
    
    func applyMask()
    {
        let shapeLayer = CAShapeLayer(layer: self.layer)
        shapeLayer.path = self.pathForCornersRounded(rect:self.bounds).cgPath
        shapeLayer.frame = self.bounds
        shapeLayer.masksToBounds = true
        self.layer.mask = shapeLayer
    }
    
    func pathForCornersRounded(rect:CGRect) ->UIBezierPath
    {
        let path = UIBezierPath()
        path.move(to: CGPoint(x: 0 + leftTopRadius , y: 0))
        path.addLine(to: CGPoint(x: rect.size.width - rightTopRadius , y: 0))
        path.addQuadCurve(to: CGPoint(x: rect.size.width , y: rightTopRadius), controlPoint: CGPoint(x: rect.size.width, y: 0))
        path.addLine(to: CGPoint(x: rect.size.width , y: rect.size.height - rightBottomRadius))
        path.addQuadCurve(to: CGPoint(x: rect.size.width - rightBottomRadius , y: rect.size.height), controlPoint: CGPoint(x: rect.size.width, y: rect.size.height))
        path.addLine(to: CGPoint(x: leftBottomRadius , y: rect.size.height))
        path.addQuadCurve(to: CGPoint(x: 0 , y: rect.size.height - leftBottomRadius), controlPoint: CGPoint(x: 0, y: rect.size.height))
        path.addLine(to: CGPoint(x: 0 , y: leftTopRadius))
        path.addQuadCurve(to: CGPoint(x: 0 + leftTopRadius , y: 0), controlPoint: CGPoint(x: 0, y: 0))
        path.close()
        
        return path
    }
    
}

Here is the results

Using this values

Hope this helps