如何在到期后使用OAuth 2.0以及iOS应用程序中的Youtube API获取新的访问令牌

I am using the Youtube Api in my iPad application. i managed to get authenticated and get the access token using the OAuth 2.0. my problem is that the token expires after one hour and i don't know how to get a new one using the refresh-token without going through the authentication process again. i am using XCode 4.5 and iOS 5.1 & 6I am using the Youtube Api in my iPad applicati