Draw a simple circle uiimage

Question

I try to make a 20x20 UIImage with a simple blue circle. I try with this function, but the result is a blue circle in a black square. How do I remove the black square around the circle?

Function:

+ (UIImage *)blueCircle {
    static UIImage *blueCircle = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        UIGraphicsBeginImageContextWithOptions(CGSizeMake(20.f, 20.f), YES, 0.0f);
        CGContextRef ctx = UIGraphicsGetCurrentContext();
        CGContextSaveGState(ctx);

        CGRect rect = CGRectMake(0, 0, 20, 20);
        CGContextSetFillColorWithColor(ctx, [UIColor cyanColor].CGColor);
        CGContextFillEllipseInRect(ctx, rect);

        CGContextRestoreGState(ctx);
        blueCircle = UIGraphicsGetImageFromCurrentImageContext();
        UIGraphicsEndImageContext();

    });
    return blueCircle;
}

actual result:

Answer 1

You need to include alpha channel into your bitmap: UIGraphicsBeginImageContextWithOptions(..., NO, ...) if you want to see what is behind the corners.