如何高性能的给 UIImageView 加个圆角?

强制Core Animation提前渲染屏幕的离屏绘制, 而离屏绘制就会给性能带来负面影响

self.view.layer.cornerRadius = 5;
self.view.layer.masksToBounds = YES;

- (UIImage *)circleImage
{
// NO代表透明
UIGraphicsBeginImageContextWithOptions(self.size, NO, 0.0);

// 获得上下文
CGContextRef ctx = UIGraphicsGetCurrentContext();

// 添加一个圆
CGRect rect = CGRectMake(0, 0, self.size.width, self.size.height);
CGContextAddEllipseInRect(ctx, rect);

// 裁剪
CGContextClip(ctx);

// 将图片画上去
[self drawInRect:rect];

UIImage *image = UIGraphicsGetImageFromCurrentImageContext();

// 关闭上下文
UIGraphicsEndImageContext();

return image;
}

UIImageView *imageView = [[UIImageView alloc] initWithFrame:CGRectMake(0, 0, 100, 100)];
imageView.center = CGPointMake(200, 300);
UIImage *anotherImage = [UIImage imageNamed:@"image"];
UIGraphicsBeginImageContextWithOptions(imageView.bounds.size, NO, 1.0);
[[UIBezierPath bezierPathWithRoundedRect:imageView.bounds
cornerRadius:50] addClip];
[anotherImage drawInRect:imageView.bounds];
imageView.image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
[self.view addSubview:imageView];

下面这篇博客写的非常不错，推荐看看：对于图片的处理
http://www.jianshu.com/p/4e22c6ac114d