GameCenter login alert
问题 In a game I am developing using GameCenter, I want to handle the following scenario: the user starts up the game. He is shown the system alert that prompts him to log on GameCenter. He ignores it for now. after a while, the user wants to log in to GameCenter and clicks on(for instance) the Leaderboards menu item. He choses cancel instead of Log in, for now. the process repeats several times. Eventually the user DOES want to log in to GameCenter. He clicks the Leaderboard menu item one more