# CFNetServiceMonitorClientCallBack Defines a pointer to the callback function that is to be called when a monitored record type changes. ``` typealias CFNetServiceMonitorClientCallBack = (CFNetServiceMonitor, CFNetService?, CFNetServiceMonitorType, CFData?, UnsafeMutablePointer?, UnsafeMutableRawPointer?) -> Void ``` ## Parameters `theMonitor` CFNetServiceMonitor for which the callback is being called. `theService` CFNetService for which the callback is being called. `typeInfo` Type of record that changed. For possible values, see [`CFNetServiceMonitorType`](/documentation/CFNetwork/CFNetServiceMonitorType). `rdata` Contents of the record that changed. `error` Pointer to structure whose `error` field contains an error code if an error occurred. `info` Arbitrary pointer to the user-defined data that was specified in the `info` field of the `CFNetServiceClientContext` structure when the monitor was created by [`CFNetServiceMonitorCreate(_:_:_:_:)`](/documentation/CFNetwork/CFNetServiceMonitorCreate(_:_:_:_:)). ## Discussion If you name your callback `MyNetServiceMonitorClientCallBack`, you would declare it like this: ### Discussion The callback function will be called when the monitored record type changes or when the monitor is stopped by calling [`CFNetServiceMonitorStop(_:_:)`](/documentation/CFNetwork/CFNetServiceMonitorStop(_:_:)). --- Copyright © 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)