Quartz and clipping area - ios

Question

I have a question about Quartz and clipping area:
I would like to have a rectangle A
inside this rectangle I would like to have a rectangle B
The filling of B dveve also cut in A: I would like that A was pierced by B. What is the best way to do this in quartz? I did not really understand how the clipping

Answer 1

If I understand you correctly, you want to draw a smaller rectangle inside a larger rectangle so that the inner rectangle is transparent. You can achieve this by drawing a CAShapeLayer with a path that contains both rectangles as subpaths. Don't forget to set the layer's fill rule to kCAFillRuleEvenOdd.
Try something like this:
CGRect rectA = CGRectMake(100, 100, 200, 200);
CGRect rectB = CGRectMake(150, 150, 100, 100);
UIBezierPath *path=[[UIBezierPath alloc] init];
// Add sub-path for rectA
[path moveToPoint:CGPointMake(rectA.origin.x, rectA.origin.y)];
[path addLineToPoint:CGPointMake(rectA.origin.x+rectA.size.width, rectA.origin.y)];
[path addLineToPoint:CGPointMake(rectA.origin.x+rectA.size.width, rectA.origin.y+rectA.size.height)];
[path addLineToPoint:CGPointMake(rectA.origin.x, rectA.origin.y+rectA.size.height)];
[path closePath];
// Add sub-path for rectB
[path moveToPoint:CGPointMake(rectB.origin.x, rectB.origin.y)];
[path addLineToPoint:CGPointMake(rectB.origin.x+rectB.size.width, rectB.origin.y)];
[path addLineToPoint:CGPointMake(rectB.origin.x+rectB.size.width, rectB.origin.y+rectB.size.height)];
[path addLineToPoint:CGPointMake(rectB.origin.x, rectB.origin.y+rectB.size.height)];
[path closePath];
// Create CAShapeLayer with this path
CAShapeLayer *pathLayer = [CAShapeLayer layer];
[pathLayer setFillRule:kCAFillRuleEvenOdd]; /* <- IMPORTANT! */
[pathLayer setPath:path.CGPath];
[pathLayer setFillColor:[UIColor blackColor].CGColor];
// Add the CAShapeLayer to a view
[someView.layer addSublayer:pathLayer];

Answer 2

I solved with this simple way:
- (void)drawRect:(CGRect)rect
{
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextSetRGBFillColor(context, 0.0, 0.0, 0.0, 0);
CGContextFillRect(context,self.bounds);
CGContextAddRect(context, self.bounds);
//Add cropped rectangle:
CGContextAddRect(context, _croppedRegion);
//Clip:
CGContextEOClip(context);
CGContextSetRGBFillColor(context, 255.0, 255.0, 255.0, 0.5);
CGContextFillRect(context, self.bounds);
}