//第一种方法:最常用的方法，但是性能最差

UIImageView *imageView = [[UIImageView alloc]initWithFrame:CGRectMake(100, 100, 100, 100)];

imageView.image = [UIImage imageNamed:@"晚起的蚂蚁.jpg"];

//只需要设置layer层的两个属性

//设置圆角

imageView.layer.cornerRadius = imageView.frame.size.width / 2;

//将多余的部分切掉

imageView.layer.masksToBounds = YES;

[self.view addSubview:imageView];

//第二种方法:使用贝塞尔曲线UIBezierPath和Core Graphics框架画出一个圆角，性能比第一种高很多（有人说100多倍，具体是多少我也不知道）

UIImageView *imageView = [[UIImageView alloc]initWithFrame:CGRectMake(100, 100, 100, 100)];

imageView.image = [UIImage imageNamed:@"晚起的蚂蚁.jpg"];

//开始对imageView进行画图

UIGraphicsBeginImageContextWithOptions(imageView.bounds.size, NO, 1.0);

//使用贝塞尔曲线画出一个圆形图

[[UIBezierPath bezierPathWithRoundedRect:imageView.bounds cornerRadius:imageView.frame.size.width] addClip];

[imageView drawRect:imageView.bounds];

imageView.image = UIGraphicsGetImageFromCurrentImageContext();

//结束画图

UIGraphicsEndImageContext();

[self.view addSubview:imageView];

//第三种方法:使用CAShapeLayer和UIBezierPath设置圆角

//首先需要导入<AVFoundation/AVFoundation.h>

//对内存的消耗最少啊，而且渲染快速。

UIImageView *imageView = [[UIImageView alloc]initWithFrame:CGRectMake(100, 100, 100, 100)];

imageView.image = [UIImage imageNamed:@"晚起的蚂蚁.jpg"];

UIBezierPath *maskPath = [UIBezierPath bezierPathWithRoundedRect:imageView.bounds byRoundingCorners:UIRectCornerAllCorners cornerRadii:imageView.bounds.size];

CAShapeLayer *maskLayer = [[CAShapeLayer alloc]init];

//设置大小

maskLayer.frame = imageView.bounds;

//设置图形样子

maskLayer.path = maskPath.CGPath;

imageView.layer.mask = maskLayer;

[self.view addSubview:imageView];