drawRect drawing 'transparent' text?

Question

I am looking to draw a UILabel (preferable through subclassing) as a transparent label, but with solid background. I draw up an quick example (sorry, it's ugly, but it gets the points across :)).

Basically I have a UILabel and I would like the background to be a set colour, and the text should be transparent. I do not want to colour the text with the views background, but instead have it be 100% transparent, since I have a texture in the background that I want to make sure lines up inside and outside of the label.

I've been spending the night browsing SO and searching on Google, but I have found no helpful sources. I don't have much experience with CG drawing, so I would appreciate any links, help, tutorial or sample code (maybe Apple has some I need to have a look at?).

Thanks a bunch!

Answer 1

I've rewritten it as a UILabel subclass using barely any code and posted it on GitHub

The gist of it is you override drawRect but call [super drawRect:rect] to let the UILabel render as normal. Using a white label color lets you easily use the label itself as a mask.

- (void)drawRect:(CGRect)rect
{
    CGContextRef context = UIGraphicsGetCurrentContext();

    // let the superclass draw the label normally
    [super drawRect:rect];

    CGContextConcatCTM(context, CGAffineTransformMake(1, 0, 0, -1, 0, CGRectGetHeight(rect)));

    // create a mask from the normally rendered text
    CGImageRef image = CGBitmapContextCreateImage(context);
    CGImageRef mask = CGImageMaskCreate(CGImageGetWidth(image), CGImageGetHeight(image), CGImageGetBitsPerComponent(image), CGImageGetBitsPerPixel(image), CGImageGetBytesPerRow(image), CGImageGetDataProvider(image), CGImageGetDecode(image), CGImageGetShouldInterpolate(image));

    CFRelease(image); image = NULL;

    // wipe the slate clean
    CGContextClearRect(context, rect);

    CGContextSaveGState(context);
    CGContextClipToMask(context, rect, mask);

    CFRelease(mask);  mask = NULL;

    [self RS_drawBackgroundInRect:rect];

    CGContextRestoreGState(context);

}