CAlayer

//CALayer 蒙版图层真正厉害的地方在于蒙版图层不局限于静态图。任何有图层构成的都可以作为mask属性，这意味着你的蒙版可以通过代码甚至是动画实时生成！

#import "ViewController.h"

@interface ViewController ()

@property (nonatomic,strong) UIImageView *layerView1;

@end

@implementation ViewController

- (void)viewDidLoad {

[super viewDidLoad];

_layerView1 = [[UIImageView alloc]initWithFrame:CGRectMake(0, 0, 100, 100)];

_layerView1.backgroundColor = [UIColor whiteColor];

self.layerView1.layer.shadowOpacity = 0.5f;

//给ImageView添加图像

_layerView1.image = [UIImage imageNamed:@"2"];

//创建maskLayer

CALayer *maskLayer = [CALayer layer];

//给maskLayer 设定frame

maskLayer.frame = self.layerView1.bounds;

UIImage *maskImage = [UIImage imageNamed:@"1"];

//给图层的contents添加内容

maskLayer.contents = (__bridge id)maskImage.CGImage;

self.layerView1.layer.mask = maskLayer;

[self.view addSubview:_layerView1];

}