Question

Usually when you draw in a invalid context, you see something like invalid context 0x00, but this is not the case, as I see an address. And all my Core Graphics code is inside drawRect of a view.
Sometimes, not always, I see:
“: CGContextFillRects: invalid context 0xa88a500”
Sometimes it seems my code works but others fail.
What I want to draw is something like a mask.

Can you tell me if you see anything wrong here?:

UIGraphicsBeginImageContext(rect.size);
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextSetFillColorWithColor(context, [[UIColor blackColor] CGColor]);
CGContextFillRect(context, rect);

switch (shape) {
    case SHAPE_ELIPSE:
        //
        CGContextAddEllipseInRect(context, maskBox);
        CGContextSetFillColorWithColor(context, [[UIColor whiteColor] CGColor]);
        CGContextFillEllipseInRect(context, maskBox);
        break;
    case SHAPE_SQUARE:
        //
        CGContextAddRect(context, maskBox);
        CGContextSetFillColorWithColor(context, [[UIColor whiteColor] CGColor]);
        CGContextFillRect(context, maskBox);
        break;
    default:
        break;
}
CGContextStrokePath(context);
UIImage *backGroundImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

CGImageRef maskRef = backGroundImage.CGImage;

CGImageRef mask = CGImageMaskCreate(CGImageGetWidth(maskRef),
                                    CGImageGetHeight(maskRef),
                                    CGImageGetBitsPerComponent(maskRef),
                                    CGImageGetBitsPerPixel(maskRef),
                                    CGImageGetBytesPerRow(maskRef),
                                    CGImageGetDataProvider(maskRef), NULL, false);

UIGraphicsBeginImageContext(rect.size);
CGContextRef contextBlack = UIGraphicsGetCurrentContext();
CGContextSetFillColorWithColor(contextBlack, [[UIColor blackColor] CGColor]);
CGContextFillRect(context, rect);
UIImage *blackImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

CGImageRef masked = CGImageCreateWithMask([blackImage CGImage], mask);
CGImageRelease(mask);
[self setBackgroundColor:[UIColor colorWithPatternImage:[UIImage imageWithCGImage:masked]]];
CGImageRelease(masked);

Thanks a lot.

Answer 1

Calling UIGraphicsEndImageContext() would invalidate the context context. However, your program continues to use that variable. IOW, you should reassign context after calling UIGraphicsBeginImageContext() the second time:

context = UIGraphicsGetCurrentContext();