<!--
{
  "availability" : [
    "iOS: 2.0.0 - 9.0.0",
    "iPadOS: 2.0.0 - 9.0.0",
    "macCatalyst: 13.1.0 - 13.1.0"
  ],
  "documentType" : "symbol",
  "framework" : "AddressBook",
  "identifier" : "/documentation/AddressBook/ABExternalChangeCallback",
  "metadataVersion" : "0.1.0",
  "role" : "Type Alias",
  "symbol" : {
    "kind" : "Type Alias",
    "modules" : [
      "Address Book"
    ],
    "preciseIdentifier" : "c:@T@ABExternalChangeCallback"
  },
  "title" : "ABExternalChangeCallback"
}
-->

# ABExternalChangeCallback

Prototype for a function callback invoked on an address book when the Address Book database is modified by another address book instance.

```
typealias ABExternalChangeCallback = (ABAddressBook?, CFDictionary?, UnsafeMutableRawPointer?) -> Void
```

## Parameters

`addressBook`

An address book used to interact with the Address Book database.

`info`

Always `NULL`.

`context`

The object to pass to the callback function.

## Discussion

If you name your callback function `MyAddressBookExternalChangeCallback`, you declare it like this:

### Discussion

Use [`ABAddressBookRegisterExternalChangeCallback(_:_:_:)`](/documentation/AddressBook/ABAddressBookRegisterExternalChangeCallback(_:_:_:)) to register and [`ABAddressBookUnregisterExternalChangeCallback(_:_:_:)`](/documentation/AddressBook/ABAddressBookUnregisterExternalChangeCallback(_:_:_:)) to unregister the callback function.

You can register for a callback with different contexts or callback functions. The run loop on the thread that registered the callback invokes the callback.

The `addressBook` object does not take any action to flush or synchronize cached state with the Address Book database. If you want to ensure that `addressBook` doesn’t contain stale values, use [`ABAddressBookRevert(_:)`](/documentation/AddressBook/ABAddressBookRevert(_:)).

---

Copyright &copy; 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)