1.
/// 如果JY_WINDOW有rootViewController， 并且rootViewController类型是AdvertisementViewController，就执行stopPlay()
if let vc = JY_WINDOW?.rootViewController as? AdvertisementViewController {
vc.stopPlay()
}
2.详情
https://www.jianshu.com/p/e1fe08c5db1a
Swift 基本语法03-"if let"和"guard let"
原文：https://www.cnblogs.com/qingzZ/p/9273122.html