Invert CALayer mask that is based on a stroke (no fill)

You can achieve this by drawing with transparent colors to a non-opaque layer. This can be done by using another blend mode for drawing. Unfortunately CAShapeLayer doesn't support this. Thus, you must wrote your own shape layer class:

@interface ShapeLayer : CALayer@property(nonatomic) CGPathRef path;@property(nonatomic) CGColorRef fillColor;@property(nonatomic) CGColorRef strokeColor;@property(nonatomic) CGFloat lineWidth;@end@implementation ShapeLayer@dynamic path;@dynamic fillColor;@dynamic strokeColor;@dynamic lineWidth;- (void)drawInContext:(CGContextRef)inContext {    CGContextSetGrayFillColor(inContext, 0.0, 1.0);    CGContextFillRect(inContext, self.bounds);    CGContextSetBlendMode(inContext, kCGBlendModeSourceIn);    if(self.strokeColor) {        CGContextSetStrokeColorWithColor(inContext, self.strokeColor);    }    if(self.fillColor) {        CGContextSetFillColorWithColor(inContext, self.fillColor);    }    CGContextSetLineWidth(inContext, self.lineWidth);    CGContextAddPath(inContext, self.path);    CGContextDrawPath(inContext, kCGPathFillStroke);}@end

Creating a layer with a transparent path:

ShapeLayer *theLayer = [ShapeLayer layer];theLayer.path = ...;theLayer.strokeColor = [UIColor clearColor].CGColor;theLayer.fillColor = [UIColor colorWithWhite:0.8 alpha:0.5];theLayer.lineWith = 3.0;theLayer.opaque = NO; // Important, otherwise you will get a black rectangle

I've used this code to draw a semi-transparent circle with transparent border in front of a green background:

Edit: Here is the corresponding code for the layer in Swift:

public class ShapeLayer: CALayer {    @NSManaged var path : CGPath?    @NSManaged var fillColor : CGColor?    @NSManaged var strokeColor : CGColor?    @NSManaged var lineWidth : CGFloat    override class func defaultValue(forKey inKey: String) -> Any? {        return inKey == "lineWidth" ? 1.0 : super.defaultValue(forKey: inKey)    }    override class func needsDisplay(forKey inKey: String) -> Bool {        return inKey == "path" || inKey == "fillColor" || inKey == "strokeColor" || inKey == "lineWidth" || super.needsDisplay(forKey: inKey)    }    override public func draw(in inContext: CGContext) {        if let thePath = path {            inContext.setFillColor(gray: 0.0, alpha: 1.0)            inContext.fill(self.bounds)            inContext.setBlendMode(.sourceIn)            if let strokeColor = self.strokeColor {                inContext.setStrokeColor(strokeColor)            }            if let fillColor = self.fillColor {                inContext.setFillColor(fillColor)            }            inContext.setLineWidth(self.lineWidth)            inContext.addPath(thePath)            inContext.drawPath(using: .fillStroke)        }    }}

Note: By marking the properties with @NSManaged you can easily make the properties animatable by implementing needsDisplay(forKey inKey:) in Swift or needsDisplayForKey: in Objective C, respectively. I've adapted the Swift code accordingly.

But even if you don't need animations, it is better to mark the properties with @NSManaged, because QuartzCore makes copies of layers and should also copy all properties with it. @NSManaged in Swift is the counterpart to @dynamic in Objective C, because it avoids the creation of a property implementation. Instead CALayer gets and sets property values with value(forKey:) and setValue(_:forKey:), respectively.