iOS——CALayer的shadow无效问题

CALayer *sublayer = [CALayer layer];
    sublayer.backgroundColor = [UIColor purpleColor].CGColor;
    sublayer.shadowOffset = CGSizeMake(0, 3);
    sublayer.shadowRadius = 5.0;
    sublayer.shadowColor = [UIColor blackColor].CGColor;
    sublayer.shadowOpacity = 0.8;
    sublayer.frame = CGRectMake(30, 30, 128, 192);
    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(30, 250, 280, 200);
    customDrawn.shadowOffset = CGSizeMake(0, 3);
    customDrawn.shadowRadius = 5.0;
    customDrawn.shadowColor = [UIColor blackColor].CGColor;
    customDrawn.shadowOpacity = 1;
    customDrawn.cornerRadius = 10.0;
    customDrawn.borderColor = [UIColor blackColor].CGColor;
    customDrawn.borderWidth = 2.0;
    customDrawn.masksToBounds = YES;
    [self.view.layer addSublayer:customDrawn];
    [customDrawn setNeedsDisplay];