iAd: How to catch no Ad found event?

How do I catch the ad not found event from iAd f开发者_StackOverflow中文版ramework? if the answer is using didFailToReceiveAdWithError then what is the error type

thanks -Z

---

I guess that the error type is ADErrorInventoryUnavailable, which indicates that iAd don't have ads for your application. More info can be found in the header for ADBannerView to see the errors that the iAd framework will post.