NSUbiquityIdentityDidChangeNotification and SIGKILL

Short Answer

To be notified in iOS when a user logs in or out of iCloud while using your app, use CKAccountChangedNotification, not NSUbiquityIdentityChanged.

Long Answer

I've been trying to get this to work as well. I remembered something from one of the talks at WWDC16 that there was something like this that they recommended to use. However, from the sample code they provide, I've only been able to find NSUbiquityKeyIdentityChanged, which I haven't been able to get to work.

So I went back to the video (it's from 2015). That's where I saw them refer to CKAccountChangedNotification – and it works exactly as expected:

• Launch your app on the simulator from Xcode
• Exit to the Settings app on the simulator
• Log in (or out) of iCloud account
• Go back into your app (tap icon on simulator home screen)
  • A notification is received.
• Exit to Settings app again
• Log back out (or in) to iCloud account
• Go back into your app again
  • Another notification is received.

In Swift 3.0 there was another renaming:

Now the NSUbiquityIdentityDidChangeNotification has changed into NSNotification.Name.NSUbiquityIdentityDidChange.

So the full registering is the following:

// Register for iCloud availability changesNotificationCenter.default.addObserver(self, selector: #selector(...), name: NSNotification.Name.NSUbiquityIdentityDidChange, object: nil)

On iOS 10 I found that NSUbiquityIdentityDidChangeNotification was never sent. Provided I had a CKContainer (as per the docs), CKAccountChangedNotification was sent in very limited circumstances.

Built with xCode 9.1 then tested on iOS 10.02 iPhone 6+ and iOS 11.0.3 iPhone SE

CKAccountChangedNotification was sent if

• User logged into iCloud account, or
• User enabled iCloud Drive in iOS 11. This always resulted in iCloud Drive->App being enabled. However, fetching the account status afterwards yielded NoAccount!
• User enabled iCloud Drive in iOS 10. The subsequent state of iCloud Drive->App was whatever it was when I disabled iCloud Drive. The account status was appropriate. However, if iCloud Drive->App was disabled at this point, enabling it did not produce a termination or a notification.

Application was terminated if

• User logged out of iCloud regardless of iCloud Drive status
• User disabled iCloud Drive->App
• User disabled iCloud Drive (even if iCloud Drive->App already disabled)
• User started the app with iCloud Drive enabled, then enabled iCloud Drive->App