解决方法
只需添加名为NSUbiquityIdentityDidChangeNotification的通知的观察员
[[NSNotificationCenter defaultCenter] addObserver: self selector: @selector (iCloudAccountAvailabilityChanged:) name: NSUbiquityIdentityDidChangeNotification object: nil];
If a user signs out of iCloud,such as by turning off Documents & Data@H_301_14@ in Settings,the ubiquityIdentityToken method returns nil. To enable@H_301_14@ your app to detect when a user signs out and signs back in,register@H_301_14@ for changes in iCloud account availability. – Apple Documentation