Mask a UIView with a cut-out circle

Question

I’ve created a UIView with a semi-transparent black background to sit across my main app’s view. What I’m aiming for here is to create a cut-out circle shape somewhere in that UIView where the semi-transparent black background is not seen, giving the effect of a black mask over the whole image except the circular area (this is to highlight a button in that area).

I’ve managed to use CAShapeLayer to create a circular mask, but this has had the opposite effect (i.e. the rest of the view is clear, and INSIDE the circle is the semi-transparent black. What I’d like is everything outside of the circle to keep the semi-transparent black, then what’s inside to be clear. Here’s the code I used, how would I go about making it work in the opposite way? My blackMask is the semi-transparent black view, and my buttonCircle is the circle I’d like to be kept clear.

Perhaps I need to invert the path, somehow?

CAShapeLayer *maskLayer = [[CAShapeLayer alloc] init];
CGRect maskRect = buttonCircle.frame;
CGPathRef path = CGPathCreateWithEllipseInRect(maskRect, NULL);
maskLayer.path = path;
CGPathRelease(path);
blackMask.layer.mask = maskLayer;

EDIT: Trying this now, not seeing any mask at all with this one:

UIView *circularMaskView = [[UIView alloc] initWithFrame:buttonCircle.frame];
circularMaskView.backgroundColor = [UIColor greenColor];
CAShapeLayer *maskLayer = [[CAShapeLayer alloc] init];

CGPathRef path = CGPathCreateWithEllipseInRect(buttonCircle.frame, NULL);
maskLayer.path = path;
CGPathRelease(path);

circularMaskView.layer.mask = maskLayer;

[blackMask addSubview:circularMaskView];

Answer 1

So, here’s what I did. I created a custom UIView subclass called BlackoutView, like so:

BlackoutView.h

#import <UIKit/UIKit.h>

@interface BlackoutView : UIView

@property (nonatomic, retain) UIColor *fillColor;
@property (nonatomic, retain) NSArray *framesToCutOut;

@end

BlackoutView.m

#import "BlackoutView.h"

@implementation BlackoutView

- (void)drawRect:(CGRect)rect
{
    [self.fillColor setFill];
    UIRectFill(rect);

    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextSetBlendMode(context, kCGBlendModeDestinationOut);

    for (NSValue *value in self.framesToCutOut) {
        CGRect pathRect = [value CGRectValue];
        UIBezierPath *path = [UIBezierPath bezierPathWithOvalInRect:pathRect];
        [path fill];
    }

    CGContextSetBlendMode(context, kCGBlendModeNormal);
}

@end

I then instantiate it as normal, and set the properties to the colour of the mask I want to use, and the frames to be cut out of the mask:

[blackMask setFillColor:[UIColor colorWithWhite:0.0f alpha:0.8f]];
[blackMask setFramesToCutOut:@[[NSValue valueWithCGRect:buttonCircleA.frame],
                               [NSValue valueWithCGRect:buttonCircleB.frame]]];

This could be improved by allowing me to cut out other shapes besides ovals, but it’s fine for my purposes here and would be easily adapted to do so later. Hopefully this helps others!