Create your own rounded rect path with rounding corners.

RoundedCornerLayer

//
//  RoundedCornerLayer.swift
//  OmnyStudio
//
//  Created by Jin Wang on 20/03/2015.
//  Copyright (c) 2015 121cast. All rights reserved.
//

import UIKit

struct RoundedCornerType : RawOptionSetType {
    typealias RawValue = UInt
    private var value: UInt = 0
    init(_ value: UInt) { self.value = value }
    init(rawValue value: UInt) { self.value = value }
    init(nilLiteral: ()) { self.value = 0 }
    static var allZeros: RoundedCornerType { return self(0) }
    static func fromMask(raw: UInt) -> RoundedCornerType { return self(raw) }
    var rawValue: UInt { return self.value }
    
    static var None: RoundedCornerType { return self(0) }
    static var UpperLeft: RoundedCornerType { return RoundedCornerType(1 << 0) }
    static var UpperRight: RoundedCornerType { return RoundedCornerType(1 << 1) }
    static var BottomLeft: RoundedCornerType { return RoundedCornerType(1 << 2) }
    static var BottomRight: RoundedCornerType { return RoundedCornerType(1 << 3) }
}

class RoundedCornerLayer {
    static func layerWithCorner(size: CGSize, radius: CGFloat, type: RoundedCornerType) -> CALayer {
        //// Rectangle Drawing
        var rectanglePath = UIBezierPath()
        
        if (type & .UpperLeft) != nil {
            rectanglePath.moveToPoint(CGPointMake(radius, 0))
        } else {
            rectanglePath.moveToPoint(CGPointMake(0, 0))
        }
        if (type & .UpperRight) != nil {
            rectanglePath.addLineToPoint(CGPointMake(size.width - radius, 0))
            
            let center = CGPoint(x: size.width - radius, y: radius)
            rectanglePath.addArcWithCenter(center, radius: radius, startAngle: 3 * CGFloat(M_PI_2), endAngle: 0, clockwise: true)
        } else {
            rectanglePath.addLineToPoint(CGPointMake(size.width, 0))
        }
        if (type & .BottomRight) != nil {
            rectanglePath.addLineToPoint(CGPointMake(size.width, size.height - radius))
            
            let center = CGPoint(x: size.width - radius, y: size.height - radius)
            rectanglePath.addArcWithCenter(center, radius: radius, startAngle: 0, endAngle: CGFloat(M_PI_2), clockwise: true)
        } else {
            rectanglePath.addLineToPoint(CGPointMake(size.width, size.height))
        }
        if (type & .BottomLeft) != nil {
            rectanglePath.addLineToPoint(CGPointMake(radius, size.height))
            
            let center = CGPoint(x: radius, y: size.height - radius)
            rectanglePath.addArcWithCenter(center, radius: radius, startAngle: CGFloat(M_PI_2), endAngle: CGFloat(M_PI), clockwise: true)
        } else {
            rectanglePath.addLineToPoint(CGPointMake(0, size.height))
        }
        if (type & .UpperLeft) != nil {
            rectanglePath.addLineToPoint(CGPointMake(0, radius))
            
            let center = CGPoint(x: radius, y: radius)
            rectanglePath.addArcWithCenter(center, radius: radius, startAngle: CGFloat(M_PI), endAngle: 3 * CGFloat(M_PI_2), clockwise: true)
        } else {
            rectanglePath.addLineToPoint(CGPointMake(0, 0))
        }
        rectanglePath.closePath()
        
        let layer = CAShapeLayer()
        layer.path = rectanglePath.CGPath
        
        return layer
    }
}

let myView = UIView(frame: CGRect(x: 0, y: 0, width: 200, height: 200))
let centerView = UIView(frame: CGRect(x: 0, y: 0, width: 50, height: 50))
centerView.center = myView.center
myView.addSubview(centerView)

let myLayer = RoundedCornerLayer.layerWithCorner(centerView.bounds.size, radius: 10, type: RoundedCornerType.UpperRight | RoundedCornerType.BottomRight)
centerView.backgroundColor = UIColor.redColor()
centerView.layer.mask = myLayer

// Output
myView