How do I cut a notch out of a CALayer?

Question

I have a set of views in a carousel, each using a CAGradientLayer as a background. The carousel sits over a textured background. I've been asked for the background to poke up in a triangle to show the selected view. I can't just use a triangular image with the background texture, as it won't necessarily match up with the main background. I'd like to cut a notch out of the background of the current view, so that the textured background is visible through the notch.

How should I go about this? Is it possible to make a polygonal layer?

Answer 1

I found I was able to do it using a CAShapeLayer:

CAShapeLayer *mask = [[[CAShapeLayer alloc] init] autorelease];
mask.frame = backgroundLayer.bounds;
mask.fillColor = [[UIColor blackColor] CGColor];

CGFloat width = backgroundLayer.frame.size.width;
CGFloat height = backgroundLayer.frame.size.height;

CGMutablePathRef path = CGPathCreateMutable();

CGPathMoveToPoint(path, nil, 0, 0); 
CGPathAddLineToPoint(path, nil, width, 0);
CGPathAddLineToPoint(path, nil, width, height);
CGPathAddLineToPoint(path, nil, (width/2) + 5, height);
CGPathAddLineToPoint(path, nil, width/2, height - 5);
CGPathAddLineToPoint(path, nil, (width/2) - 5, height);
CGPathAddLineToPoint(path, nil, 0, height);
CGPathAddLineToPoint(path, nil, 0, 0);
CGPathCloseSubpath(path);

mask.path = path;
CGPathRelease(path);

backgroundLayer.mask = mask;