第一种方法:通过设置layer的属性

最简单的一种,但是很影响性能,一般在正常的开发中使用很少.

 UIImageView *imageView = [[UIImageView alloc]initWithFrame:CGRectMake(100, 100, 100, 100)]; 
//只需要设置layer层的两个属性 
//设置圆角
imageView.layer.cornerRadius = imageView.frame.size.width / 2;  
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

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

 imageView.image = [UIImage imageNamed:@"1"]; 
//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>

#import "ViewController.h"

@interface ViewController () 
@end

@implementation ViewController
 - (void)viewDidLoad {
    [super viewDidLoad];
    UIImageView *imageView = [[UIImageView alloc]initWithFrame:CGRectMake(100, 100, 100, 100)];
    imageView.image = [UIImage imageNamed:@"1"];
    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];
}

这三种方法中第三种最好，对内存的消耗最少啊，而且渲染快速。