如何高性能的给UIImageView加个圆角？（不准说layer.cornerRadius!）

利用UIBezierPath这个类绘制圆形图片，如下：

- (UIImage*)drawImageWithSize:(CGSize)size imageName:(NSString *)imageName

{

    // 申请一块特定大小的画布

    UIGraphicsBeginImageContext(size);

    // 画出一个与画布等大的圆

    UIBezierPath *path = [UIBezierPathbezierPathWithOvalInRect:CGRectMake(0,0, size.width, size.height)];

    // 将这个圆以外的部分设置为无效区

    [path addClip];

    UIImage *oldImage = [UIImageimageNamed:imageName];

    [oldImage drawInRect:CGRectMake(0,0, size.width, size.height)];

    UIImage *newImage =UIGraphicsGetImageFromCurrentImageContext();

    UIGraphicsEndImageContext();

    return newImage;

}

imageView.image =newImage;