I have an iPad-appliction that syncs contact with the contacts on server side.

How do I detect only that ABAdressbook-Entries that have changed? It is possible, that there occur changes on server-side, in my application, or externally on the ipad.

When I use 

void ABAddressBookRegisterExternalChangeCallback (
   ABAddressBookRef addressBook,
   ABExternalChangeCallback callback,
   void *context
);

I get the callback of external changes, but without any information about what changed. How do I get that information?

When I use the NSString * const kABModificationDateProperty I don't know what to compare with.

ios ios4 abaddressbook
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