Downloading DASH with external subtitles

[AGasztold]

Hello,

I need to download external subtitles (VTT format) simultaneously with the video/audio tracks.
I have added subtitles to
MediaItem.PlaybackProperties List<Subtitle> subtitles (This mediaItem is passed to DownloadHelper.forMediaItem(..)), so it is possible to select Text track inside TrackSelectionDialog for download.
Unfortunatelly, streamKeys are empty in this case ->
public DownloadRequest getDownloadRequest(String id, @Nullable byte[] data) { ... streamKeys.addAll(mediaPreparer.mediaPeriods[periodIndex].getStreamKeys(allSelections)); ... } getStreamKeys returns empty list for MergingMediaPeriod (there is no problem for DashMediaPeriod - as it has its own implementation of getStreamKeys).
Download attempt for DASH was OK, but without subtitles.
The question is -> what is the best way to add external subtitles for download?
I will appreciate Your help.

[christosts]

@ojw28 can you take a look?

[ojw28]

This is not something we support directly, so for now you have to download the subtitle file yourself, and also specify it at playback time (presumably using a file:/// URI that points to where the file has been downloaded).

Providing direct support is tracked by #5615, so closing as a duplicate of that issue.