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app/src/main/kotlin/space/subread/overlay/Follower.kt (4962 bytes)

 package space.subread.overlay
 
 import android.media.session.MediaController
 import android.media.session.PlaybackState
 import android.os.Handler
 import android.os.SystemClock
 import space.subread.overlay.core.CueIndex
 import space.subread.overlay.core.PlayClock
 
 /**
  * Follows the media player that plays now, and says which subtitle line is on.
  *
  * Each Android media player publishes a media session: Voice, VLC, mpv, YouTube, a podcast app.
  * The session has the position, the time of that position and the speed. The follower computes
  * the position between two reports, and sleeps until the next line starts. It does not poll.
  *
  * A player that publishes no position: Android makes a position for it that starts at zero when
  * the player starts to play, and gives none while the player is paused. The follower then holds
  * the last position, so the line stays. The user sets the timing with the line buttons.
  */
 class Follower(
     private val handler: Handler,
     private val now: () -> Long = SystemClock::elapsedRealtime,
     /**
      * The index of the line that is on in [index] (-1 before the first line), true when there is
      * a player to follow, and true when that player plays.
      */
     private val onLine: (Int, Boolean, Boolean) -> Unit,
 ) {
     var index: CueIndex = CueIndex(emptyList())
         set(value) {
             field = value
             update()
         }
 
     /** Added to the position of the player, in milliseconds: a file with an intro, a player that is late. */
     var offsetMs: Long = 0
         set(value) {
             field = value
             update()
         }
 
     private var controller: MediaController? = null
     private var last: PlayClock? = null
     private val wake = Runnable { update() }
 
     private val callback = object : MediaController.Callback() {
         override fun onPlaybackStateChanged(state: PlaybackState?) = update()
         override fun onSessionDestroyed() = follow(emptyList())
     }
 
     /**
      * The sessions that are active, in the order of the system: the one that plays is first.
      * The follower keeps its player while that player is in the list and no other one plays.
      */
     fun follow(controllers: List<MediaController>) {
         val playing = controllers.firstOrNull { it.playbackState?.state == PlaybackState.STATE_PLAYING }
         val kept = controllers.firstOrNull { it.sessionToken == controller?.sessionToken }
         val next = playing ?: kept ?: controllers.firstOrNull()
         if (next?.sessionToken != controller?.sessionToken) {
             controller?.unregisterCallback(callback)
             controller = next
             last = null
             next?.registerCallback(callback, handler)
         }
         update()
     }
 
     /** True while the player that is followed plays. */
     val playing: Boolean
         get() = controller?.playbackState?.state == PlaybackState.STATE_PLAYING
 
     /** Pauses the player. True when it played. */
     fun pause(): Boolean {
         if (!playing) return false
         controller?.transportControls?.pause()
         return true
     }
 
     fun play() {
         controller?.transportControls?.play()
     }
 
     fun stop() {
         handler.removeCallbacks(wake)
         controller?.unregisterCallback(callback)
         controller = null
     }
 
     /** Makes the line [steps] lines from the line that is on the current line, by a change of [offsetMs]. */
     fun shiftLines(steps: Int) {
         val clock = clock() ?: return
         if (index.size == 0) return
         val position = clock.positionAt(now())
         val target = (index.indexAt(position + offsetMs) + steps).coerceIn(0, index.size - 1)
         offsetMs = index.cues[target].startMs - position
     }
 
     private fun clock(): PlayClock? {
         val state = controller?.playbackState ?: return null
         val playing = state.state == PlaybackState.STATE_PLAYING
         val clock = if (state.position >= 0) {
             // Some players leave the time of the report at zero. Then the report is from now.
             val reportedAt = state.lastPositionUpdateTime.takeIf { it > 0 } ?: now()
             val speed = state.playbackSpeed.takeIf { it > 0f } ?: 1f
             PlayClock(state.position, reportedAt, speed, playing)
         } else {
             // No position in this report: hold the position of the report before.
             PlayClock(last?.positionAt(now()) ?: 0, now(), 1f, playing)
         }
         return clock.also { last = it }
     }
 
     private fun update() {
         handler.removeCallbacks(wake)
         val clock = clock()
         if (clock == null) {
             onLine(-1, false, false)
             return
         }
         val position = clock.positionAt(now()) + offsetMs
         val at = index.indexAt(position)
         onLine(at, true, clock.playing)
         val next = index.nextChangeAfter(position) ?: return
         clock.waitUntil(next - offsetMs, now())?.let { handler.postDelayed(wake, it) }
     }
 }