Invert a radial gradient mask in core graphics - ios

Question

I am able to mask a picture with black/white radial gradient in iOS using core graphics. But I am getting result just the opposite I want. I want the picture to be transparent from the region where gradient is applied and shall display the remaining part of the picture. That means a "![hole][1]" in the picture. But here the hole will be created by a gradient mask.
Can anyone please suggest me some way or any hint to invert the radial gradient mask, just like we do in photoshop.

Answer 1

Instead of using mask, I think the best way is to insert the layer. That's what I did for my IOS app.
- (void) addSublayer {
CGPoint center = CGPointMake(self.addButton.frame.origin.x, self.addButton.frame.origin.y);
UIBezierPath* clip = [UIBezierPath bezierPathWithArcCenter:center
radius:CGRectGetMaxX(self.addUnavailabilitySubView.frame) - self.addButton.center.x
startAngle:-1.57
endAngle:1.57
clockwise:YES];
[clip addLineToPoint:center];
[clip closePath];
CAShapeLayer *shapeLayer = [[CAShapeLayer alloc] init];
[shapeLayer setMasksToBounds:YES];
shapeLayer.frame = self.view.layer.bounds;
shapeLayer.path = clip.CGPath;
//This gradient layer can be a customised one
CALayer *gradientLayer = [CALayer layer];
gradientLayer.frame = CGRectMake(0, 0, self.view.bounds.size.height, self.view.bounds.size.width);
[self.view.layer insertSublayer:gradientLayer atIndex:0];
}
I can also share my radial gradient layer here :)
-(void)drawInContext:(CGContextRef)ctx {
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
size_t num_locations = 2;
CGFloat locations[2] = { 0.0, 1.0 }; // End color
CGFloat components[8] = {0.f, 0.f, 0.f, 0.3f, 0.f, 0.f, 0.f, 0.8f};
CGGradientRef gradient = CGGradientCreateWithColorComponents(colorSpace, components, locations, num_locations);
CGFloat myStartRadius, myEndRadius;
CGPoint radialCenter = CGPointMake(100, 100)
myStartRadius = 20;
myEndRadius = 300;
CGContextDrawRadialGradient (ctx, gradient, radialCenter,
myStartRadius, radialCenter, myEndRadius,
kCGGradientDrawsAfterEndLocation);
}