When you are working with contact book and save contact in Core Data(Local Database), you need to sync contact everytime when contact book updates. Which means to update the phone number, name, delete the contact, create new contact etc.

Sometimes when we have a bunch of contacts in our contact book and try to store it in Core Data we face many issues like Threading issue, UI stuck issue, Store data in background issue, etc.

Here is full article where you can find the easiest way to sync contacts with Core Data,  you can easily get the array of deleted, updated and newly added contact list.