How to draw CALayer border around its mask?

Question

So, I have a CALayer, which has a mask & I want to add border around this layer\'s mask. For example, I have set triangle mask to the layer and I want to have b

Answer 1

In a general case you cannot easily set a border around a mask. That's like asking to put a border around the transparent pixels of an image. Perhaps it may be done using image filters. In some more specific case, if you are using plain CAShapeLayer then here is a sample of code that does that:

[CATransaction begin];
[CATransaction setDisableActions:YES];

CALayer *hostLayer = [CALayer layer];
hostLayer.backgroundColor = [NSColor blackColor].CGColor;
hostLayer.speed  = 0.0;
hostLayer.timeOffset = 0.0;

CALayer *maskedLayer = [CALayer layer];
maskedLayer.backgroundColor = [NSColor redColor].CGColor;
maskedLayer.position = CGPointMake(200, 200);
maskedLayer.bounds   = CGRectMake(0, 0, 200, 200);

CAShapeLayer *mask = [CAShapeLayer layer];
mask.fillColor = [NSColor whiteColor].CGColor;
mask.position = CGPointMake(100, 100);
mask.bounds   = CGRectMake(0, 0, 200, 200);

CGMutablePathRef path = CGPathCreateMutable();
CGPathMoveToPoint(path, NULL, 100, 100);
for (int i=0;  i<20;  i++) {
    double x = arc4random_uniform(2000) / 10.0;
    double y = arc4random_uniform(2000) / 10.0;
    CGPathAddLineToPoint(path, NULL, x, y);
}
CGPathCloseSubpath(path);

mask.path = path;

CGPathRelease(path);

maskedLayer.mask = mask;

CAShapeLayer *maskCopy = [NSKeyedUnarchiver unarchiveObjectWithData:[NSKeyedArchiver archivedDataWithRootObject:mask]];
maskCopy.fillColor = NULL;
maskCopy.strokeColor = [NSColor yellowColor].CGColor;
maskCopy.lineWidth = 4;
maskCopy.position = maskedLayer.position;

// Alternately, don't set the position and add the copy as a sublayer
// maskedLayer.sublayers = @[maskCopy];

hostLayer.sublayers = @[maskedLayer,maskCopy];

_contentView.layer = hostLayer;
_contentView.wantsLayer = YES;

[CATransaction commit];

It basically creates an arbitrary path and sets it as the mask. It then takes a copy of this layer to stroke the path. You might need to tweak things to get the exact effect you are looking for.