GADBannerViewDelegate不会在Swift中触发adViewDidReceiveAd

我在另一个名为工具的类中使用GADBannerView。但是adViewDidReceiveAd委托方法不会在Tools类中触发。我该怎么办？

I use GADBannerView in another class that named Tools. But adViewDidReceiveAd delegate method doesn't fire in Tools class. What can I do?

class Tools: NSObject, GADBannerViewDelegate {

    var viewController: UIViewController!

    func showAds(viewController: UIViewController) -> Void {

        self.viewController = viewController

        let request: GADRequest = GADRequest()
        request.testDevices = [kGADSimulatorID]

        let bannerView = GADBannerView(adSize: kGADAdSizeSmartBannerPortrait)
        bannerView.delegate = self
        bannerView.rootViewController = viewController
        bannerView.adUnitID = "ca-app-pub-xxx-xxx"
        bannerView.loadRequest(request)

    }

    func adViewDidReceiveAd(bannerView: GADBannerView!) {
        print("adViewDidReceiveAd");
        viewController.view.addSubview(bannerView)
    }
}

然后，我在ViewController中调用了工具

And, I called Tools in ViewController

override func viewDidLoad() {
    super.viewDidLoad()        
    Tools().showAds(self);
}