iOS 通过有alpha值的图片创建蒙版

@interface ViewController ()

@property (nonatomic, weak) IBOutlet UIImageView *imageView;
@end

@implementation ViewController

- (void)viewDidLoad
{
  [super viewDidLoad];

  //create mask layer
  CALayer *maskLayer = [CALayer layer];
  maskLayer.frame = self.layerView.bounds;
  UIImage *maskImage = [UIImage imageNamed:@"Cone.png"];　　// 带有alpha值的图片
  maskLayer.contents = (__bridge id)maskImage.CGImage;

  //apply mask to image layer?
  self.imageView.layer.mask = maskLayer;　　// 需要蒙版效果的图片
}
@end