Address Book callback not called

I have an iPhone app that makes use of the AddressBook.framework and uses Core Data to store these contacts. In order to make sure I update my own database when the Address Book is updated (whether via MobileMe or editing within my own app), I am subscribing to the notification as to when the Address Book is updating. I call this on startup:

ABAddressBookRef book = ABAddressBookCreate();
ABAddressBookRegisterExternalChangeCallback(book, addressBookChanged, self);

Which (supposedly) calls this on any editing. I have an ABPersonViewController which allows editing, and addressBookChanged never seems to get called.

void addressBookChanged(ABAddressBookRef reference, CFDictionaryRef dictionary, void *context) {
    // The contacts controller we need to call
    ContactsController *contacts = (ContactsController *)context;

    // Sync with the Address Book
    [contacts synchronizeWithAddressBook:reference];
}

Is there any reason for it to开发者_StackOverflow中文版 not be called?

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It turns out the reason it wasn't getting called was a simple mistake. I did:

CFRelease(book);

After setting the notification. I removed that and it's now getting called.