iOS——CALayer的shadow无效问题

CALayer *sublayer = [CALayer layer];
    sublayer.backgroundColor = [UIColor purpleColor].CGColor;
    sublayer.shadowOffset = CGSizeMake(, );
    sublayer.shadowRadius = 5.0;
    sublayer.shadowColor = [UIColor blackColor].CGColor;
    sublayer.shadowOpacity = 0.8;
    sublayer.frame = CGRectMake(, , , );
    sublayer.borderColor = [UIColor blackColor].CGColor;
    sublayer.borderWidth = 2.0;
    sublayer.cornerRadius = 10.0;
    [self.view.layer addSublayer:sublayer];

    CALayer *imageLayer = [CALayer layer];
    imageLayer.frame = sublayer.bounds;
    imageLayer.cornerRadius = 10.0;
    imageLayer.contents = (id)[UIImage imageNamed:@"snaguosha.png"].CGImage;
    imageLayer.masksToBounds = YES;
    [sublayer addSublayer:imageLayer];

    CALayer *customDrawn = [CALayer layer];
    customDrawn.delegate = self;
    customDrawn.backgroundColor = [UIColor greenColor].CGColor;
    customDrawn.frame = CGRectMake(, , , );
    customDrawn.shadowOffset = CGSizeMake(, );
    customDrawn.shadowRadius = 5.0;
    customDrawn.shadowColor = [UIColor blackColor].CGColor;
    customDrawn.shadowOpacity = ;
    customDrawn.cornerRadius = 10.0;
    customDrawn.borderColor = [UIColor blackColor].CGColor;
    customDrawn.borderWidth = 2.0;
    customDrawn.masksToBounds = YES;
    [self.view.layer addSublayer:customDrawn];
    [customDrawn setNeedsDisplay];

以上代码预览如下：

可以看到下方的部分没有阴影，Why？看了半天也没看出什么不一样，后来发现是因为customDrawn.masksToBounds = YES;这句代码导然不存在，如果去掉这行代码会是这样的：

阴影出来了，但是圆角没了。解决办法很简单，像上面图片的方式，里面套一层，masksToBounds作用在里层，外层显示阴影