func isAppAlreadyLaunchedOnce()->Bool{
    let defaults = UserDefaults.standard
    if let _ = defaults.string(forKey: "isAppAlreadyLaunchedOnce"){
        print("App already launched")
        return true
    }else{
        defaults.set(true,forKey: "isAppAlreadyLaunchedOnce")
        print("App launched first time")
        return false
    }
}

注意：在用户重新安装应用程序并首次启动后,此方法将返回false.