如何检测设备上使用的iCloud帐号是否已更改？
用户从“设置”> iCloud和另一个用户登录他/她的帐户.
打开应用程序时如何检测到此更改？
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
in Settings,the ubiquityIdentityToken method returns nil. To enable
your app to detect when a user signs out and signs back in,register
for changes in iCloud account availability. – Apple Documentation
http://developer.apple.com/library/mac/#documentation/General/Conceptual/iCloudDesignGuide/Chapters/iCloudFundametals.html