Switching subtitles of hls on tvOS/swift - swift

Question

I am using AVplayer of swift for my tvOSApp.
When I use default player, subtitles of the video will be shown on "subtitle" bar on top and can easily switch subtitles.
So, how can I switch subtitles like that, without using the default playback controls?
I have buttons for switching subtitles on the screen and I'd like to switch during target method of these.
Here is my code for AVPlayer.
let asset = AVAsset(url: URL(string: videoUrl)!)
let playerItem = AVPlayerItem(asset: asset)
let player = AVPlayer(playerItem: playerItem)
player.play()
self.bgMovieLayer = AVPlayerLayer(player: player)
self.bgMovieLayer.frame = view.bounds
self.bgMovieLayer.videoGravity = .resizeAspectFill
self.bgMovieLayer.zPosition = -1
self.view.layer.insertSublayer(self.bgMovieLayer, at: 0)
I am using the video below for a test.
http://184.72.239.149/vod/smil:BigBuckBunny.smil/playlist.m3u8
I read the documents regarding AVPlayer/AVPlayerItem and came into no answer. I would appreciate if someone can help this.

Answer 1

You have to load availableMediaCharacteristics first and after that look for AVMediaCharacteristic.legible.
Something like this:
let asset:AVAsset
let key = #keyPath(AVAsset.availableMediaCharacteristicsWithMediaSelectionOptions)
asset.loadValuesAsynchronously(forKeys: [key]) {
var error: NSError? = nil
let status = asset.statusOfValue(forKey: key, error: &error)
switch status {
case .loaded:
// Sucessfully loaded, continue processing
case .failed:
// Examine NSError pointer to determine failure
case .cancelled:
// Loading cancelled
default:
// Handle all other cases
}
}
let subtitlesGroup = asset.mediaSelectionGroup(forMediaCharacteristic: AVMediaCharacteristic.legible)