---
title: AudioUnitPropertyListenerProc
framework: audiotoolbox
role: symbol
role_heading: Type Alias
path: audiotoolbox/audiounitpropertylistenerproc
---

# AudioUnitPropertyListenerProc

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

## Declaration

```swift
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

Return Value A result code.

## Discussion

Discussion If you named your callback function MyAudioUnitPropertyListenerProc, you would declare it like this: Discussion You register your AudioUnitPropertyListenerProc callback function using the AudioUnitAddPropertyListener(_:_:_:_:) function.

## See Also

### Audio Unit Types

- [ScheduledAudioFileRegion](audiotoolbox/scheduledaudiofileregion.md)
- [ScheduledAudioSlice](audiotoolbox/scheduledaudioslice.md)
- [ScheduledAudioFileRegionCompletionProc](audiotoolbox/scheduledaudiofileregioncompletionproc.md)
- [ScheduledAudioSliceCompletionProc](audiotoolbox/scheduledaudioslicecompletionproc.md)
- [MIDIChannelNumber](audiotoolbox/midichannelnumber.md)
- [AUAudioObjectID](audiotoolbox/auaudioobjectid.md)
- [AUMIDICIProfileChangedBlock](audiotoolbox/aumidiciprofilechangedblock.md)
- [AUAudioChannelCount](audiotoolbox/auaudiochannelcount.md)
- [AUAudioFrameCount](audiotoolbox/auaudioframecount.md)
- [AUAudioUnitStatus](audiotoolbox/auaudiounitstatus.md)
- [AUEventListenerProc](audiotoolbox/aueventlistenerproc.md)
- [AUEventListenerRef](audiotoolbox/aueventlistenerref.md)
- [AUEventSampleTime](audiotoolbox/aueventsampletime.md)
- [AUImplementorValueObserver](audiotoolbox/auimplementorvalueobserver.md)
- [AUImplementorValueProvider](audiotoolbox/auimplementorvalueprovider.md)