view添加阴影无效

需求：需要给cell里的imageview添加阴影

问题：按照标准的代码添加阴影，然并卵：代码如下：

    imageview.layer.shadowColor = [[UIColor blackColor] CGColor];
    imageview.layer.shadowOffset = CGSizeMake(4.0f, 4.0f);
    imageview.layer.shadowRadius = 4.0;
    imageview.layer.shadowOpacity = 0.5;

后谷歌说要加一句：

imageview.layer.masksToBounds = NO;，因为阴影是在imagview的layer外面画的

这样阴影出来了，然而，由于允许子元素超出父元素，所以图片的大小就不一样了

因此歪果仁提出了一个办法，创建两层view，内层view是imageview，不允许超出边界，外层view是shadowview，ref:

http://www.innofied.com/implementing-shadow-ios/(这个是为了解决圆角view＋阴影的问题)

代码：

CALayer *sublayer = [[CALayer layer]initWithLayer:self.imageView.layer];

    UIView * shadow = [[UIView alloc] initWithFrame:self.imageView
                       .frame];
    shadow.userInteractionEnabled = NO; // Modify this if needed
    shadow.layer.shadowColor = [[UIColor blackColor] CGColor];
    shadow.layer.shadowOffset = CGSizeMake(4.0f, 4.0f);
    shadow.layer.shadowRadius = 4.0;
    shadow.layer.masksToBounds = NO;   //关键
    shadow.clipsToBounds = NO;
    shadow.layer.shadowOpacity = 0.5;
    [ self.imageView.superview insertSubview:shadow belowSubview: self.imageView];
    [shadow addSubview:self.imageView];