Working UIBlurEffect and UIVisualEffectView masking example on iOS10 using Swift 3

ViewController.swift

import UIKit

class ViewController: UIViewController {

  func addTheBlurView(data :Data) {

    let generalFrame = self.view.bounds;
    let parentView = UIView(frame: generalFrame)
    self.view.addSubview(parentView)

    let imageView = UIImageView(frame: parentView.bounds)
    imageView.image = UIImage(data: data)
    imageView.contentMode = .scaleAspectFill

    let maskView = UIView(frame: parentView.bounds)
    maskView.backgroundColor = UIColor.black
    maskView.layer.mask = {
      () -> CALayer in
      var roundedRect = CGRect(
          x: 0.0,
          y: 0.0,
          width: parentView.bounds.size.width * 0.5,
          height: parentView.bounds.size.width * 0.5
          );
      roundedRect.origin.x = parentView.frame.size.width / 2 - roundedRect.size.width / 2;
      roundedRect.origin.y = parentView.frame.size.height / 2 - roundedRect.size.height / 2;

      let cornerRadius = roundedRect.size.height / 2.0;

      let path = UIBezierPath(rect: parentView.bounds)
      let croppedPath = UIBezierPath(roundedRect: roundedRect, cornerRadius: cornerRadius)
      path.append(croppedPath)
      path.usesEvenOddFillRule = true

      let maskLayer = CAShapeLayer()
      maskLayer.path = path.cgPath;
      maskLayer.fillRule = kCAFillRuleEvenOdd
      return maskLayer
    }()

    let blurView = UIBlurEffect(style: .light)
    let effectView = UIVisualEffectView(effect: blurView)
    effectView.frame = generalFrame

    effectView.mask = maskView
    parentView.addSubview(imageView)
    parentView.addSubview(effectView)

  }

  override func viewDidLoad() {
    debugPrint("Running...")

    super.viewDidLayoutSubviews();

    // Lets load an image first, so blur looks cool
    let url = URL(string: "https://static.pexels.com/photos/168066/pexels-photo-168066-large.jpeg")

    URLSession.shared.dataTask(with: url!) {
      (data, response, error) in

      if error != nil {
        print(error)
        return
      }

      DispatchQueue.main.async(execute: {
        self.addTheBlurView(data: data!)
      })

    }.resume()

  }
}