[iOS] 取得目前正顯示的view

目的是為了寫一個簡易的show alert message的method

//程式碼
extension UIApplication {
    class func topViewController(base base: UIViewController? = UIApplication.sharedApplication().keyWindow?.rootViewController) -> UIViewController? {
        if let nav = base as? UINavigationController {
            return topViewController(base: nav.visibleViewController)
        }
        if let tab = base as? UITabBarController {
            if let selected = tab.selectedViewController {
                return topViewController(base: selected)
            }
        }
        if let presented = base?.presentedViewController {
            return topViewController(base: presented)
        }
        return base
    }
}

//用法
let presentedView = UIApplication.topViewController()
AlertMessageUtils.showSimpleAlert("系統通知", message: "電量\(values[0])%", view: presentedView!)

參考︰Find the current top view controller for your iOS application https://gist.github.com/snikch/3661188