ios - UIView drawRect: Draw the inverted pixels, make a hole, a window, negative space

Question

With the code below I am drawing a rounded rectangle. It draws a nice solid light gray filled rounded rectangle (at the size of "self"). I actually want to draw the pixel inverse of this, that is: not a solid rounded rectangle, but a window or hole in the shape of this round rectangle in a solid light gray rectangle.

Is there a reverse clip method that I need to use? Or do I need to use a bezier path? Excuse if this is very basic, can't find the info though.

Thanks for reading!

- (void)drawRect:(CGRect)rect
{

    // get the context
    CGContextRef context = UIGraphicsGetCurrentContext

    CGContextSaveGState(context);    

    //draw the rounded rectangle
    CGContextSetStrokeColorWithColor(context, [[UIColor blackColor] CGColor]);
    CGContextSetRGBFillColor(context, 0.8, 0.8, 0.8, 1.0);
    CGContextSetLineWidth(context, _lineWidth);

    CGRect rrect = CGRectMake(CGRectGetMinX(rect), CGRectGetMinY(rect), CGRectGetWidth(rect), CGRectGetHeight(rect));
    CGFloat radius = _cornerRadius;

    CGFloat minx = CGRectGetMinX(rrect), midx = CGRectGetMidX(rrect), maxx = CGRectGetMaxX(rrect);
    CGFloat miny = CGRectGetMinY(rrect), midy = CGRectGetMidY(rrect), maxy = CGRectGetMaxY(rrect);

    CGContextMoveToPoint(context, minx, midy);
    // Add an arc through 2 to 3
    CGContextAddArcToPoint(context, minx, miny, midx, miny, radius);
    // Add an arc through 4 to 5
    CGContextAddArcToPoint(context, maxx, miny, maxx, midy, radius);
    // Add an arc through 6 to 7
    CGContextAddArcToPoint(context, maxx, maxy, midx, maxy, radius);
    // Add an arc through 8 to 9
    CGContextAddArcToPoint(context, minx, maxy, minx, midy, radius);
    // Close the path
    CGContextClosePath(context);

    // Fill the path
    CGContextDrawPath(context, kCGPathFill);

    CGContextRestoreGState(context);

}

Answer 1

Here's yet another approach, using just UI object calls:

- (void)drawRect:(CGRect)rect
{
    [[UIColor lightGrayColor] setFill];
    CGRect r2 = CGRectInset(rect, 10, 10);
    UIBezierPath* p = [UIBezierPath bezierPathWithRoundedRect:r2 cornerRadius:15];
    [p appendPath: [UIBezierPath bezierPathWithRect:rect]];
    p.usesEvenOddFillRule = YES;
    [p fill];
}

Yields this:

The white is the background of the window; the grey is the UIView. As you can see, we're seeing right thru the view to whatever is behind it, which sounds like what you're describing.