直接上代碼：

@property (weak, nonatomic) IBOutlet UIImageView *userHead;
@property (nonatomic,strong) CAShapeLayer * userMaskLayer;

self.userHead.layer.mask = [self userHeaderMask:self.userHead];

• (CALayer *)userHeaderMask:(UIView *)originalView{
  CALayer * userMask = [CALayer layer];
  userMask.frame = originalView.bounds;

  CGFloat x = originalView.frame.size.width/2;
  CGFloat y = originalView.frame.size.height/2;

  self.userMaskLayer = [CAShapeLayer layer];
  self.userMaskLayer.bounds = originalView.bounds;
  self.userMaskLayer.path = [UIBezierPath bezierPathWithArcCenter:CGPointMake(x, y) radius:50 startAngle:0 endAngle:2*M_PI clockwise:YES].CGPath;
  self.userMaskLayer.position = CGPointMake(50, 50);
  [userMask addSublayer:self.userMaskLayer];

  return userMask;
  }

原理：通過設置UIImageView的layer的mask屬性。

一般我們都是用
self.userHead.layer.cornerRadius = 50;
self.userHead.layer.masksToBounds = YES;
這種方法來設置圖片圓角，這樣是改變了view的layer屬性。