Drawing multiple arcs using Core Graphics - core-graphics

Question

I'm trying to draw two concentric circles using Core Graphics. I would expect the following code to draw a blue circle and a smaller white circle inside, however it just draws the blue circle. What am I missing?
CGContextRef c = UIGraphicsGetCurrentContext();
CGContextAddArc(c, self.bounds.size.width/2, self.bounds.size.height/2, 100, 0, M_PI * 2, true);
CGContextSetFillColor(c, CGColorGetComponents([[UIColor blueColor] CGColor]));
CGContextFillPath(c);
CGContextAddArc(c, self.bounds.size.width/2, self.bounds.size.height/2, 90, 0, M_PI * 2, true);
CGContextSetFillColor(c, CGColorGetComponents([[UIColor whiteColor] CGColor]));
CGContextFillPath(c);

Answer 1

You are setting the fill color in an unnecessarily indirect way. Do this instead:
CGContextAddArc(c, self.bounds.size.width/2, self.bounds.size.height/2, 100, 0, M_PI * 2, true);
CGContextSetFillColorWithColor(c, [[UIColor blueColor] CGColor]);
CGContextFillPath(c);
CGContextAddArc(c, self.bounds.size.width/2, self.bounds.size.height/2, 90, 0, M_PI * 2, true);
CGContextSetFillColorWithColor(c, [[UIColor whiteColor] CGColor]);
CGContextFillPath(c);
Or, even better, just use UIKit's drawing methods directly:
[[UIColor blueColor] setFill];
[[UIBezierPath bezierPathWithArcCenter:CGPointMake(self.bounds.size.width/2, self.bounds.size.height/2) radius:100 startAngle:0 endAngle:M_PI * 2 clockwise:YES] fill];
[[UIColor whiteColor] setFill];
[[UIBezierPath bezierPathWithArcCenter:CGPointMake(self.bounds.size.width/2, self.bounds.size.height/2) radius:90 startAngle:0 endAngle:M_PI * 2 clockwise:YES] fill];
Your code is failing because [[UIColor whiteColor] CGColor] returns a color in the "gray" colorspace, which has only two components, a "gray" value (0 for black, 1 for white) and an alpha value. In this case, the context is RGBA, so CGContextSetFillColor expects to see 4 components, three for RGB and one for alpha.
The documentation for CGContextSetFillColor points out:
Note that the preferred API to use is now
CGContextSetFillColorWithColor.