iOS CoreAnimation ShadowPath阴影不显示

在学习CoreAnimation，有个例子中阴影不显示，去掉ShadowPath设置后能正常显示，加上不显示，麻烦看下代码：

    self.IV1 = ({
        UIImageView *var = [UIImageView new];
        [self.view addSubview:var];
        [var mas_makeConstraints:^(MASConstraintMaker *make) {
            make.size.mas_equalTo(CGSizeMake(200, 200));
            make.centerY.mas_equalTo(var.superview).multipliedBy(2/3.0);
            make.centerX.mas_equalTo(var.superview);
        }];
        var.backgroundColor = [UIColor redColor];
        
        var.layer.shadowOpacity = 1.0f;
        //square shadow
        CGMutablePathRef squarePath = CGPathCreateMutable();
        CGPathAddRect(squarePath, NULL, var.frame);
        var.layer.shadowPath = squarePath;
        CGPathRelease(squarePath);
        
        var;
    });