clear a circle/ellipse area like CGContextClearRect to clear a rect area

clearEllipseInRect.m

- (void)drawRect:(CGRect)rect
{
    CGContextRef con = UIGraphicsGetCurrentContext();
    CGContextSetFillColorWithColor(con, [UIColor grayColor].CGColor);
    CGContextFillRect(con, rect);
    
    CGContextAddEllipseInRect(con, self.circle);            // self.circle is a property of CGRect
    CGContextClip(con);
    CGContextClearRect(con, self.circle);
}