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# AVMusicUserEvent

An object that represents a custom user message.

```
class AVMusicUserEvent
```

## Overview

When playback of an [`AVMusicTrack`](/documentation/AVFAudio/AVMusicTrack) reaches this event, the system calls the track’s callback. You can’t modify the size and contents of an [`AVMusicUserEvent`](/documentation/AVFAudio/AVMusicUserEvent) once you create it.

## Topics

### Creating a User Event

[`init(data:)`](/documentation/AVFAudio/AVMusicUserEvent/init(data:))

Creates a user event with the data you specify.

### Inspecting a User Event

[`sizeInBytes`](/documentation/AVFAudio/AVMusicUserEvent/sizeInBytes)

The size of the data that the user event represents.


---

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