ios之检测系统警报 View 何时出现/将关闭

我想检测系统 alertView 何时出现在屏幕上，例如应用内购买确认提醒。 我正在尝试将键值观察器添加到 UIApplication，但它不起作用。

UIApplication.sharedApplication().addObserver(self, forKeyPath: "windows", options: NSKeyValueObservingOptions.allZeros, context: &myContext) 

请您参考如下方法:

这不是我想要的，但它对我有用。

NotificationCenter.addObserver(self, selector: "applicationWillResignActive", name: UIApplicationWillResignActiveNotification, object: nil) 
NotificationCenter.addObserver(self, selector: "applicationDidBecomeActive", name: UIApplicationDidBecomeActiveNotification, object: nil) 
 
func applicationDidBecomeActive() 
{ 
    if Library.isPurchasing 
    { 
        Application.networkActivityIndicatorVisible = true 
    } 
} 
 
func applicationWillResignActive() 
{ 
    if Library.isPurchasing 
    { 
        Application.networkActivityIndicatorVisible = false 
    } 
}