Swift version of this https://gist.github.com/Raiden996/74ea67db8f17673f1b8b

custom_camera_cropping_overlay.swift

    // Discussion on how to do custom cropping overlay on UIImageViewPicker
    // https://gist.github.com/Raiden996/74ea67db8f17673f1b8b
    
    func navigationController(navigationController: UINavigationController, willShowViewController viewController: UIViewController, animated: Bool) {
        println("viewcontrollers count: \(navigationController.viewControllers.count)")

        if navigationController.viewControllers.count == 3 {
            println("viewcontrollers name: \(navigationController.viewControllers[2].classString())")
            if (navigationController.viewControllers[2].classString() == "PUUIImageViewController") || (navigationController.viewControllers[2].classString() == "PLUIImageViewController") {
                addOverlayToImagePicker(viewController)
            }
        }
    }
    
    func addOverlayToImagePicker(viewController:UIViewController){
        let circleColor = UIColor.clearColor()
        let maskColor = UIColor.blackColor().colorWithAlphaComponent(0.8)
        
        let screenHeight = UIScreen.mainScreen().bounds.size.height
        let screenWidth = UIScreen.mainScreen().bounds.size.width
        
        var plCropOverlayCropView:UIView = UIView() //The default crop view, we wan't to hide it and show our circular one
        var plCropOverlayBottomBar:UIView = UIView() //On iPhone is the bar with "cancel" and "choose" button, on Ipad is an Image View with a label saying "Scale and move"
        
        if UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiom.Pad {
            plCropOverlayCropView = viewController.view.subviews[1] as! UIView//[viewController.view.subviews objectAtIndex:1];
            plCropOverlayBottomBar = viewController.view.subviews[1].subviews[1] as! UIView //[[[[viewController.view subviews] objectAtIndex:1] subviews] objectAtIndex:1];
            
            //Protect against iOS changes...
            if plCropOverlayCropView.classString() != "PLCropOverlay" {//(! [[[plCropOverlayCropView class] description] isEqualToString:@"PLCropOverlay"]){
                println("Image Picker with circle overlay: PLCropOverlay not found")
                return;
            }
            if plCropOverlayBottomBar.classString() != "UIImageView"{ //(! [[[plCropOverlayBottomBar class] description] isEqualToString:@"UIImageView"]){
                println("Image Picker with circle overlay: PLCropOverlayBottomBar not found")
                return;
            }
        } else {
            plCropOverlayCropView = viewController.view.subviews[1].subviews.first as! UIView//[[[viewController.view.subviews objectAtIndex:1] subviews] firstObject];
            plCropOverlayBottomBar = viewController.view.subviews[1].subviews[1] as! UIView//[[[[viewController.view subviews] objectAtIndex:1] subviews] objectAtIndex:1];
            
            //Protect against iOS changes...
            if plCropOverlayCropView.classString() != "PLCropOverlayCropView" {//(! [[[plCropOverlayCropView class] description] isEqualToString:@"PLCropOverlay"]){
                println("Image Picker with circle overlay: PLCropOverlay not found")
                return;
            }
            if plCropOverlayBottomBar.classString() != "PLCropOverlayBottomBar"{ //(! [[[plCropOverlayBottomBar class] description] isEqualToString:@"UIImageView"]){
                println("Image Picker with circle overlay: PLCropOverlayBottomBar not found")
                return;
            }
        }
        
        plCropOverlayCropView.hidden = true //Hide default CropView
        
        let circleLayer = CAShapeLayer()
        //Center the circleLayer frame:
        let circlePath = UIBezierPath(ovalInRect: CGRect(x: 0, y: screenHeight/2 - screenWidth/2, width: screenWidth, height: screenWidth))
        circlePath.usesEvenOddFillRule = true
        circleLayer.path = circlePath.CGPath
        circleLayer.fillColor = circleColor.CGColor
        //Mask layer frame: it begins on y=0 and ends on y = plCropOverlayBottomBar.origin.y
        let maskPath = UIBezierPath(roundedRect: CGRect(x: 0, y: 0, width: screenWidth, height: screenHeight - (plCropOverlayBottomBar.frame.size.height * 2)), cornerRadius: 0)
        maskPath.appendPath(circlePath)
        maskPath.usesEvenOddFillRule = true
        
        let maskLayer = CAShapeLayer()
        maskLayer.path = maskPath.CGPath
        maskLayer.fillRule = kCAFillRuleEvenOdd
        maskLayer.fillColor = maskColor.CGColor
        viewController.view.layer.addSublayer(maskLayer)
        
        if UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiom.Phone {
            //On iPhone add an hint label on top saying "scale and move" or whatever you want
            let cropLabel = UILabel(frame: CGRectMake(0, 10, screenWidth, 50))
            cropLabel.text = "FOO"//NSLocalizedString("IMAGE_PICKER_CROP_LABEL", caption: "")
            cropLabel.textAlignment = NSTextAlignment.Center
            cropLabel.textColor = UIColor.whiteColor()
            viewController.view.addSubview(cropLabel)
        } else {
            //On iPad re-add the overlayBottomBar with the label "scale and move" because we set its parent to hidden (it's a subview of PLCropOverlay) 
            viewController.view.addSubview(plCropOverlayBottomBar)
        }
    }