Admob swift loadRequest问题

Question

编译器每次都显示此消息

要在此设备上投放测试广告，请致电：request.testDevices = @ [GAD_SIMULATOR_ID];

广告没有出现

override func viewDidLoad() {
    super.viewDidLoad()

    var interstitial = GADInterstitial()
    interstitial.delegate = self
    interstitial.adUnitID = "ca-app-pub-xxxxxxxxx/xxxxxxxx"

    var Request  = GADRequest()
    Request.testDevices = [ GAD_SIMULATOR_ID ] // not working i don't know why ?!
    interstitial.loadRequest(Request)