<!--
{
  "availability" : [
    "iOS: -",
    "iPadOS: -",
    "macCatalyst: -",
    "macOS: -",
    "tvOS: -",
    "visionOS: -"
  ],
  "documentType" : "symbol",
  "framework" : "AudioToolbox",
  "identifier" : "/documentation/AudioToolbox/AudioUnitPropertyListenerProc",
  "metadataVersion" : "0.1.0",
  "role" : "Type Alias",
  "symbol" : {
    "kind" : "Type Alias",
    "modules" : [
      "Audio Toolbox"
    ],
    "preciseIdentifier" : "c:@T@AudioUnitPropertyListenerProc"
  },
  "title" : "AudioUnitPropertyListenerProc"
}
-->

# AudioUnitPropertyListenerProc

Called by the system when the value of a specified audio unit property has changed.

```
typealias AudioUnitPropertyListenerProc = (UnsafeMutableRawPointer, AudioUnit, AudioUnitPropertyID, AudioUnitScope, AudioUnitElement) -> Void
```

## Parameters

`inRefCon`

Custom data that you provided when registering your callback with the audio unit.

`inUnit`

The audio unit upon which the specified property value has changed.

`inID`

The property whose value has changed.

`inScope`

The scope of the property whose value has changed.

`inElement`

The element ID on the scope of the property whose value has changed.

## Return Value

A result code.

## Discussion

If you named your callback function `MyAudioUnitPropertyListenerProc`, you would declare it like this:

### Discussion

You register your `AudioUnitPropertyListenerProc` callback function using the [`AudioUnitAddPropertyListener(_:_:_:_:)`](/documentation/AudioToolbox/AudioUnitAddPropertyListener(_:_:_:_:)) function.

---

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)