Playground for masking corners of a UIView

maskCorners.swift

import XCPlayground
import Foundation
import UIKit

func maskCorners(corners: [UIViewContentMode], ofView view: UIView, toDepth depth: CGFloat) {
    let s = 1 + 2 * depth
    let path = UIBezierPath()

    if corners.contains(.TopLeft) {
        path.moveToPoint(CGPoint(x: 0, y: 0 + depth))
        path.addLineToPoint(CGPoint(x: 0 + depth, y: 0))
    } else {
        path.moveToPoint(CGPoint(x: 0, y: 0))
    }

    if corners.contains(.TopRight) {
        path.addLineToPoint(CGPoint(x: s - depth, y: 0))
        path.addLineToPoint(CGPoint(x: s, y: 0 + depth))
    } else {
        path.addLineToPoint(CGPoint(x: s, y: 0))
    }

    if corners.contains(.BottomRight) {
        path.addLineToPoint(CGPoint(x: s, y: s - depth))
        path.addLineToPoint(CGPoint(x: s - depth, y: s))
    } else {
        path.addLineToPoint(CGPoint(x: s, y: s))
    }

    if corners.contains(.BottomLeft) {
        path.addLineToPoint(CGPoint(x: 0 + depth, y: s))
        path.addLineToPoint(CGPoint(x: 0, y: s - depth))
    } else {
        path.addLineToPoint(CGPoint(x: 0, y: s))
    }

    path.closePath()

    let colorSpace = CGColorSpaceCreateDeviceGray()
    let scale = UIScreen.mainScreen().scale
    let gc = CGBitmapContextCreate(nil, Int(s * scale), Int(s * scale), 8, 0, colorSpace, CGImageAlphaInfo.Only.rawValue)!
    CGContextScaleCTM(gc, scale, -scale)
    CGContextTranslateCTM(gc, 0, -s)
    CGContextSetFillColorWithColor(gc, UIColor.whiteColor().CGColor)
    CGContextAddPath(gc, path.CGPath)
    CGContextFillPath(gc)

    let image = UIImage(CGImage: CGBitmapContextCreateImage(gc)!, scale: scale, orientation: .Up)
    let maskImage = image.resizableImageWithCapInsets(UIEdgeInsets(top: depth, left: depth, bottom: depth, right: depth))

    let maskView = view.maskView as? UIImageView ?? UIImageView()
    maskView.image = maskImage

    if view.maskView != maskView {
        view.maskView = maskView
        maskView.frame = view.bounds
    }
}

let view = UIImageView(image: UIImage(named: "Kaz-256.jpg"))
view.autoresizingMask = [ .FlexibleWidth, .FlexibleHeight ]
let backgroundView = UIView(frame: view.frame)
backgroundView.backgroundColor = UIColor.purpleColor()
backgroundView.addSubview(view)
XCPlaygroundPage.currentPage.liveView = backgroundView

maskCorners([.TopLeft, .BottomRight], ofView: view, toDepth: 50)