1.使用layer去裁剪 这两行代码在iOS9之前滚动tableview的时候会出现卡顿的显现，iOS9已经修复了卡顿现象，建议使用第二种方法

    UIImage *image = [UIImage imageNamed:@"avar"];
    UIImageView *imageView = [[UIImageView alloc] initWithImage:image];
    imageView.layer.cornerRadius = image.size.width/2;
    imageView.layer.masksToBounds = YES;```

2.使用上下文裁剪图片

<pre>        UIGraphicsBeginImageContextWithOptions(image.size, NO, 0);
        //描述裁剪区域
        UIBezierPath *path = [UIBezierPath bezierPathWithOvalInRect:CGRectMake(0, 0, image.size.width, image.size.height)];
        //设置裁剪区域
        [path addClip];
        //画图片
        [image drawAtPoint:CGPointZero];
        //取出图片
        image = UIGraphicsGetImageFromCurrentImageContext();
        //关闭上下文
        UIGraphicsEndImageContext();
        imageView.image = image;</pre>