How to check when GameKit UI displays

I've got very simple game that I'm trying to connect to Game Center.

After calling:

GKLocalPlayer *localPlayer = [GKLocalPlayer localPlayer];
[localPlayer authenticateWithCompletionHandler:^(NSError *error) {

}];

First time, Game Center dialog comes up asking for username and password and afte开发者_如何学Gor that the whole Game Center windows slides up to set up account.

The problem is that my game is still running underneath that window and I just can not find any notifications to pause it.

viewWillDisappear, viewDidDisappear in UIViewController don't get called; applicationWillResignActive in AppDelegate is not called either.

Is there any way to detect that Game Center windows shows up?

---

Could you start the game after the user is authenticated?

Here would be an example from Apple's Docs (http://developer.apple.com/library/ios/#documentation/NetworkingInternet/Conceptual/GameKit_Guide/Users/Users.html)

- (void) authenticateLocalPlayer
{
    GKLocalPlayer *localPlayer = [GKLocalPlayer localPlayer];
    [localPlayer authenticateWithCompletionHandler:^(NSError *error) {
         if (localPlayer.isAuthenticated)
         {
             // Start Game
         }
     }];
}