Adding a mask with CALayers - iphone

Question

I seem to be having difficulties adding a mask via CALayers. I'm simply trying to mask a UIImageView. Here's my code:
CALayer *maskLayer = [CALayer layer];
UIImage *mask = [UIImage imageNamed:#"mask.png"];
maskLayer.contents = mask;
UIImageView *viewToMask = [[UIImageView alloc] initWithFrame:CGRectMake(0, 0, 1024, 768)];
viewToMask.image = [UIImage imageNamed:#"picture.png"];
viewToMask.layer.mask = maskLayer;
[self.view addSubview:viewToMask];
Mask.png is black with a transparent circle punched through it (is this correct way to mask?). I'm not sure where this is failing, perhaps at maskLayer.contents since its supposed to be a CGImageRef but I get errors when I set it as mask.CGImage, or through a local variable CGImageRef = mask.CGImage. Anyway, the way its set now doesn't give errors, so I hope its fine.
Does anyone know what's going on, or how to properly set masks with CALayers? Thanks

Answer 1

Try
maskLayer.contents = (id)mask.CGImage;
Yes, the cast sucks, but it's necessary.
I think you'll also need to say
maskLayer.bounds = (CGRect){CGPointZero, mask.size};

Answer 2

try this:
CALayer *maskLayer = [CALayer layer];
UIImage *mask = [UIImage imageNamed:#"mask.png"];
maskLayer.contents = (id)mask.CGImage;
// maskLayer.contentsGravity = kCAGravityCenter;
maskLayer.frame = CGRectMake(0.0, 0.0,1024,768);
UIImageView *viewToMask = [[UIImageView alloc] initWithFrame:CGRectMake(0, 0, 1024, 768)];
viewToMask.image = [UIImage imageNamed:#"picture.png"];
viewToMask.layer.mask = maskLayer;
[self.view addSubview:viewToMask];
you also need to set mask frame