from http://stackoverflow.com/a/14485362/2138564

addRoundCorners.m

-(void)addRoundedCorners:(UIRectCorner)corners withRadii:(CGSize)radii {
    CAShapeLayer *maskLayer = [CAShapeLayer layer];
    maskLayer.frame = self.bounds;
    
    UIBezierPath *roundedPath = [UIBezierPath bezierPathWithRoundedRect: maskLayer.bounds byRoundingCorners:corners cornerRadii:radii];
//    maskLayer.fillColor = [[UIColor yellowColor] CGColor];
    maskLayer.backgroundColor = [[UIColor clearColor] CGColor];
    maskLayer.path = [roundedPath CGPath];
    self.layer.mask = maskLayer;
}