How to subclass CALayer for use as another CALayer mask?

Question

I' m trying to subclass CALayer for use as a mask on another layer.

I want to use my CALayer subclass in place of a CAGradientLayer and use it for a gradient mask as explained here.

However I wish to use a custom CALayer that uses an internal CGGradient for drawing instead, as this should produce smoother results than CAGradientLayer (see here). I don't care about performance, I want better gradient quality.

I'm following this example for creating my CGGradient and storing it on the CALayer.. However, I cannot get the mask to draw.

I don't know where to put the draw code: CALayer's display nor drawInContext:(CGContextRef)ctx nor drawInContext:(CGContextRef)ctx seem to be called when it is used as a mask.

Bear with me as I'm new with CoreAnimation. So, how to fix this so my CALayer subclass works as a replacement for CAGradientLayer but draws using a CGGradient?

---

My current code here:

@interface CANiceGradientLayer : CALayer

@property (nonatomic) CGGradientRef gradient;
@property (atomic) CGPoint startPoint;
@property (atomic) CGPoint endPoint;

@end

@implementation CANiceGradientLayer

- (instancetype)initWithGradientRef:(CGGradientRef)gradient startPoint:(CGPoint)startPoint endPoint:(CGPoint)endPoint
{
    if ( !(self = [super init]) )
    {
        return nil;
    }

    self.gradient = CGGradientRetain(gradient);
    self.startPoint = startPoint;
    self.endPoint = endPoint;

    return self;
}

- (void)dealloc
{
    CGGradientRelease(self.gradient);
}

- (void)display
{
    NSLog(@"display");
}

- (void)drawInContext:(CGContextRef)ctx
{
    NSLog(@"drawInContext:");
    CGContextDrawLinearGradient(ctx, self.gradient, self.startPoint, self.endPoint, kCGGradientDrawsBeforeStartLocation | kCGGradientDrawsAfterEndLocation);;
}

- (void)renderInContext:(CGContextRef)ctx
{
    NSLog(@"renderInContext:");
}

@end

And here is how I create it:

size_t num_locations = 2;
CGFloat locations[2] = { 0.0, 1.0 };
CGFloat components[8] = { 1.0, 1.0, 1.0, 1.0,  // Start color
                          1.0, 1.0, 1.0, 0.0 }; // End color

CGColorSpaceRef rgbColorspace = CGColorSpaceCreateDeviceRGB();
CGGradientRef gradient = CGGradientCreateWithColorComponents(rgbColorspace, components, locations, num_locations);


self.collectionViewTickerMaskLayer = [[CANiceGradientLayer alloc] initWithGradientRef:gradient startPoint:CGPointZero endPoint:CGPointZero];


self.collectionViewTickerMaskLayer.anchorPoint = CGPointZero;
view.layer.mask = self.collectionViewTickerMaskLayer;

If I use a CAGradientLayer instead, it works fine (but the gradient looks bad).

Answer 1

Call -setNeedsDisplay on your layer subclass before setting it to the mask and you'll only need to override -drawInContext: of the CALayer subclass. That will help you get the method to be called (CALayer calls -drawInContext: using its default implementation of -display which is called after -setNeedsDisplay). You may also have to set the frame of the layer:

self.collectionViewTickerMaskLayer.frame = view.layer.bounds;
self.collectionViewTickerMaskLayer.autoresizingMask = kCALayerWidthSizable | kCALayerHeightSizable;

You have a logic issue in this code. You are using an endpoint which is CGPointZero, so even if the methods were called, no gradient would even be drawn to mask your layer.