UIImageView *imageView = [[UIImageView alloc]initWithFrame:CGRectMake(100, 100, 100, 100)];
  //只需要設置layer層的兩個屬性
  //設置圓角
  imageView.layer.cornerRadius = imageView.frame.size.width / 2;
  //將多余的部分切掉
  imageView.layer.masksToBounds = YES;
  [self.view addSubview:imageView];

 UIImageView *imageView = [[UIImageView alloc]initWithFrame:CGRectMake(100, 100, 100, 100)];
  imageView.image = [UIImage imageNamed:@"1"];
  //開始對imageView進行畫圖
  UIGraphicsBeginImageContextWithOptions(imageView.bounds.size, NO, 1.0);
  //使用貝塞爾曲線畫出一個圓形圖
  [[UIBezierPath bezierPathWithRoundedRect:imageView.bounds cornerRadius:imageView.frame.size.width] addClip];
  [imageView drawRect:imageView.bounds];
  imageView.image = UIGraphicsGetImageFromCurrentImageContext();
   //結束畫圖
  UIGraphicsEndImageContext();
  [self.view addSubview:imageView];

#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];
  //設置大小
  maskLayer.frame = imageView.bounds;
  //設置圖形樣子
  maskLayer.path = maskPath.CGPath;
  imageView.layer.mask = maskLayer;
  [self.view addSubview:imageView];
}