设置控件圆角，只设置上面两个角的圆角（任意角）
http://blog.csdn.net/shuilanjuedemaomn/article/details/45644529
代码（通过贝塞尔曲线重绘layer层）
- UIImageView *picImageView = [[UIImageView alloc] initWithFrame:CGRectMake(100, 100, 100, 100)];
- picImageView.backgroundColor = [UIColor greenColor];
- [self.view addSubview:picImageView];
- UIBezierPath *maskPath = [UIBezierPath bezierPathWithRoundedRect:picImageView.bounds byRoundingCorners:UIRectCornerTopLeft|UIRectCornerTopRight cornerRadii:CGSizeMake(10, 10)]; // UIRectCornerBottomRight通过这个设置
- CAShapeLayer *maskLayer = [CAShapeLayer layer];
- maskLayer.frame = picImageView.bounds;
- maskLayer.path = maskPath.CGPath;
- picImageView.layer.mask = maskLayer;