//创建一个图层
CALayer *layer = [CALayer layer];
layer.backgroundColor = [UIColor redColor].CGColor;
layer.frame = CGRectMake(50, 50, 100, 100);
layer.cornerRadius = 14;
layer.masksToBounds = YES;
//必须强制转成 id 类型
layer.contents = (id)[UIImage imageNamed:@"1.png"].CGImage;