# How to implement preview thumbnails on iOS

This article describes how you can enable preview thumbnails, which THEOplayer implements through the TextTrack API. The thumbnails are loaded as a separate text track, by side-loading them as a text track in your source description.

A demo of the preview thumbnails on the THEOplayer Web SDK is available at <https://demo.theoplayer.com/preview-thumbnails>.

## Usage

You can add or edit your source description as follows.

```swift
player.source = SourceDescription(
    source: TypedSource(
        src: "https://cdn.theoplayer.com/video/big_buck_bunny/big_buck_bunny.m3u8",
        type: "application/x-mpegurl"
    ),
    textTracks: [
        TextTrackDescription(
            src: "https://cdn.theoplayer.com/dash/theoplayer/thumbnails/big_buck_bunny_thumbnails.vtt",
            srclang: "en",
            isDefault: true,
            kind: .metadata,
            label: "thumbnails"
        )
    ]
)
```

## Related articles

* [How to insert subtitles](https://optiview.dolby.com/docs/theoplayer/how-to-guides/ios/text-tracks/insert-subtitles.md)