picture.png
如图：
用 UIBezierPath 类的方法可以实现这个效果：
UIBezierPath *path = [UIBezierPath bezierPathWithRoundedRect:self.bounds
byRoundingCorners:(UIRectCornerTopLeft | UIRectCornerBottomLeft)
cornerRadii:CGSizeMake(5, 5)];
CAShapeLayer *maskLayer = [[CAShapeLayer alloc] init];
maskLayer.frame = self.bounds;
maskLayer.path = path.CGPath;
self.layer.mask = maskLayer;