game center authentication in ios6

game center authentication in ios6

In ios6, I use the following code to authenticate on game center:
[GKLocalPlayer localPlayer].authenticateHandler = ^(UIViewController*
viewController, NSError* error)
{
if (error != nil)
o
return;
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else
{
if (viewController != nil)
{
[rootViewController presentModalViewController:viewController
animated:YES];
}
}
The code above is bind to a button. Things works fine if the users
correctly entered their username and password. However, if the user
canceled the viewController and then tab the button again, the code block
is never entered again. I want to show the login viewController after each
time the user tabs the button (or at least for the first two or three
times). How could I update my code?
Thanks for your reply