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圆角的设置在iOS中随处可见，开发的时候也很方便，但是有的时候如果一个页面有大量的需要设置圆角的图片，容易产生性能问题，UIImageView ios9.0之前设置圆角是会产生离屏渲染的，9.0之后不会产生离屏渲染
因此需要日常设置圆角的方法上加一些改动：
1.最简单的图片圆角设置:
self.imageView=[[UIImageView alloc]initWithFrame:CGRectMake(100,200, 100, 100)];
[self.imageView setImage:[UIImage imageNamed:@"FlyElephant.jpg"]];
self.imageView.layer.cornerRadius=50;
self.imageView.layer.masksToBounds=YES;
[self.view addSubview:self.imageView];
2.设置Rasterize栅格化处理，会将图片放在缓存区，不会不断的进行图片渲染:
self.imageView=[[UIImageView alloc]initWithFrame:CGRectMake(100,200, 100, 100)];
[self.imageView setImage:[UIImage imageNamed:@"dress3.jpg"]];
self.imageView.layer.cornerRadius=50;
self.imageView.layer.shouldRasterize = YES;
self.imageView.clipsToBounds=YES;
self.imageView.layer.rasterizationScale=[UIScreen mainScreen].scale; //不设置会模糊，不相信可以自己尝试
[self.view addSubview:self.imageView];
3.UIBezierPath贝塞尔曲线绘制（推荐）
self.imageView=[[UIImageView alloc]initWithFrame:CGRectMake(100,200, 100, 100)];
UIImage *anotherImage = [UIImage imageNamed:@"FlyElephant.jpg"];
//注意第三个选项的设置
UIGraphicsBeginImageContextWithOptions(self.imageView.bounds.size, NO, [UIScreen mainScreen].scale);
//在绘制之前先裁剪出一个圆形
[[UIBezierPath bezierPathWithRoundedRect:self.imageView.bounds
cornerRadius:50] addClip];
//图片在设置的圆形里面进行绘制
[anotherImage drawInRect:self.imageView.bounds];
//获取图片
self.imageView.image = UIGraphicsGetImageFromCurrentImageContext();
//结束绘制
UIGraphicsEndImageContext();
[self.view addSubview:self.imageView];
参考资料:http://stackoverflow.com/questions/11049016/cliptobounds-and-maskstobounds-performance-issue
http://stackoverflow.com/questions/17593524/using-cornerradius-on-a-uiimageview-in-a-uitableviewcell
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原文地址：http://www.cnblogs.com/xiaofeixiang/p/5123169.html
