Combine overlapping UIBezierPaths to create a single mask - ios

Question

I'm looking to create a mask in my view that is shaped like several circles. Here blurView is the view I want to have cut outs in, and frames contains CGRect frame values for these circles/ovals.
let holes = UIBezierPath()
for f in frames {
let p = UIBezierPath(ovalInRect: f)
holes.appendPath(p)
}
let path = UIBezierPath(rect: blurView.bounds)
path.appendPath(holes)
path.usesEvenOddFillRule = true
let mask = CAShapeLayer()
mask.path = path.CGPath
mask.fillRule = kCAFillRuleEvenOdd
blurView.layer.mask = mask
This works quite well, except when circles overlap, the contents of the blurView show through instead of the view below it. How can I fix that?
Edit: Using Rob's answer, I got somewhere, but not quite where I want to be. The edges of the mask are ragged, they lack anti-aliasing:
Any way to fix that?

Answer 1

The basic idea is to fill a bitmap context with an opaque colour and then clear anything we want to clip. Grab an image of that bitmap context and use as a mask.
You can copy this into a playground if you like:
import UIKit
// A couple of overlapping circles to play with
let frames = [
CGRect(x: 100, y: 100, width: 200, height: 200),
CGRect(x: 50, y: 50, width: 200, height: 200)
]
let blurView = UIImageView(image: ...) // <== You can drag an image here. Literals are cool.
let size = blurView.bounds.size
let scale = UIScreen.mainScreen().scale
// First, create a bitmap context to work in.
let bitmapInfo = CGBitmapInfo(rawValue: CGImageAlphaInfo.PremultipliedFirst.rawValue | CGBitmapInfo.ByteOrder32Little.rawValue)
let ctx = CGBitmapContextCreate(nil, Int(size.width*scale), Int(size.height*scale),
8, 4 * Int(size.width * scale), CGColorSpaceCreateDeviceRGB(), bitmapInfo.rawValue)
CGContextScaleCTM(ctx, scale, scale)
// iOS draws upsidedown from Core Graphics, so you'll probably want to flip your context:
CGContextConcatCTM(ctx, CGAffineTransformMake(1, 0, 0, -1, 0, size.height))
// Everything we want to draw should be opaque
// Everything we want to clip should be translucent
CGContextSetGrayFillColor(ctx, 1, 1) // white
CGContextFillRect(ctx, blurView.bounds)
// And now we clear all our circles
CGContextSetBlendMode(ctx, .Clear)
CGContextSetGrayFillColor(ctx, 1, 0) // clear
for f in frames {
CGContextFillEllipseInRect(ctx, f)
}
// Make the masking image
let maskImage = CGBitmapContextCreateImage(ctx)
// For your playground amusement (you can quicklook this line)
UIImage(CGImage: maskImage)
// Create the masking layer
let mask = CALayer()
mask.frame = blurView.layer.frame
mask.contents = maskImage
// And apply it
blurView.layer.mask = mask
// And the final so you can quicklook it
blurView