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# AVAudioSequencerUserCallback

A callback the sequencer calls asynchronously during playback when it encounters a user event.

```
typealias AVAudioSequencerUserCallback = (AVMusicTrack, Data, AVMusicTimeStamp) -> Void
```

## Parameters

`track`

The track that contains the user event.

`userData`

The data used to initialize the user event.

`timeStamp`

The beat location of the event.

## Discussion

The sequencer delivers this callback asynchronously to the rendering thread on an internal queue. The `userData` this returns is unique to each [`AVMusicUserEvent`](/documentation/AVFAudio/AVMusicUserEvent) instance.

---

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