Fix caption auto-selection not reflected in player GUI

[karyogamy]

What is it?

• Bugfix (user facing)
• Feature (user facing)
• Codebase improvement (dev facing)
• Meta improvement to the project (dev facing)

Description of the changes in your PR

This PR fixes an issue introduced by #8020, where CustomTrackSelector was replaced by DefaultTrackSelector, delegating preferred caption language selection to ExoPlayer instead. When transitioning from a video with manual subtitles selected, e.g. english, to a video with only auto-generated subtitles, e.g. english (auto-generated), ExoPlayer would apply the previously selected language as preferred language and use the auto-generated subtitles instead of disabling captions. This is not reflected in the GUI since it is still trying to find that previously selected language in the list of available languages for the second video, which isn't there, so the GUI shows No Captions. This is fixed by looking at the currently select tracks onTracksInfoChanged, and show the ExoPlayer's selected tracks instead.

This change also fixes an unreported caption bug when selecting language variants, where a video has one caption language with multiple formats in parentheses, e.g. english (auto-generated) and english (united kingdom) in this order. When any variant of a language is selected (e.g. english (auto-generated)), the selected caption would jump to the last variant of a language group instead, in this case, english (united kingdom). This happens because of parentheses detection logic when setting userPreferredLanguage, and is also fixed by delegating preferred language selection to ExoPlayer.

Fixes the following issue(s)

• Fixes ExoPlayer 2.17.1 update and MediaSource management rework #8020 (comment)

APK testing

The APK can be found by going to the "Checks" tab below the title. On the left pane, click on "CI", scroll down to "artifacts" and click "app" to download the zip file which contains the debug APK of this PR.

Due diligence

• I read the contribution guidelines.

[Redirion]

I have tested a bit, so far so good. But now having english (united kingdom) and choosing a video with english (automatic) or vise versa unselects the captions.

I preferred the previous behaviour actually. If I understood correctly you wanted to make sure that if both (united kingdom) and (automatic) are available it would keep the previous. But this now feels like a downgrade when not both are available at the same time.

[karyogamy]

But now having english (united kingdom) and choosing a video with english (automatic) or vise versa unselects the captions.

I think this is the default behavior for Youtube browser player, probably why it is replicated on ExoPlayer as default behavior.
This was remedied by that auto-generated caption seeking logic that converts english (united kingdom) to english (automatic), but as mentioned, it has a bug that breaks selection on multiple language variants, which is problematic for videos like this that has 6 variants of Chinese captions.

This is now fixed by using multiple preferred languages in ExoPlayer caption support, though it requires parsing for the "stem" of language descriptor: e.g. english (united kingdom) -> english. So when english (united kingdom) is selected, both english (united kingdom) and english will be applied, in that order, and ExoPlayer will try to find a caption in the same order when the next video starts playing. Proper support of this should probably be included in the extractor, though that is out of scope on my side time-wise, at least in the near future.

[Redirion]

Tested again.

I just tested a video with english (united kingdom) and then switched to another video which has both english (automatisch erzeugt) and english (united states).

It appears to be random / nondeterministic. Sometimes cc even get's competely unselected. I don't know what's happening there.

[karyogamy]

I just tested a video with english (united kingdom) and then switched to another video which has both english (automatisch erzeugt) and english (united states).

Thanks for testing, not sure how I missed this one.
It seems to be caused by auto-generated tracks sharing the same priority inside DefaultTrackSelector as manual tracks since both tracks have the same stems (english) with no track having an exactly matching language name as the preferred. Thankfully there is a way to distinguish auto-generated tracks in the resolver and tracks are now given different roles that ExoPlayer can use in track selection. The updated track selection order is left as a comment.

Sometimes cc even get's competely unselected.

I'm not able to reprod this one. Could you test the updated apk and provide some steps if this still happens?

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

0.0% Coverage
0.0% Duplication

[Redirion]

Tested again, working now as expected. Thank you!