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# AudioSessionPropertyListener

Invoked when an audio session property changes in iOS.

```
typealias AudioSessionPropertyListener = (UnsafeMutableRawPointer?, AudioSessionPropertyID, UInt32, UnsafeRawPointer?) -> Void
```

## Parameters

`inClientData`

Data that you specified in the `inClientData` parameter of the [`AudioSessionAddPropertyListener(_:_:_:)`](/documentation/AudioToolbox/AudioSessionAddPropertyListener(_:_:_:)) function. Can be `NULL`.

`inID`

The identifier for the audio session property whose value just changed. See [Audio Session Property Identifiers](/documentation/AudioToolbox/1618455-audio-session-property-identifie).

`inDataSize`

The size, in bytes, of the value of the changed property.

`inData`

The new value of the changed property.

## Discussion

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

### Discussion

You can register one or more property listener callbacks with your application’s audio session object by calling the [`AudioSessionAddPropertyListener(_:_:_:)`](/documentation/AudioToolbox/AudioSessionAddPropertyListener(_:_:_:)) function.

---

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