ios 将图片做成圆形

UIImageView * imageView = [[UIImageView alloc] initWithImage:[UIImage imageNamed:@"oiuyfdsa.png"]];  
imageView.frame = CGRectMake(20.f, 20.f, 100.f, 100.f);  
imageView.layer.masksToBounds = YES;  
imageView.layer.cornerRadius = 50;