AndroidMediaPlayer Queue Playback Fix

Problem

When passing 3 tracks to playQueue(), only the first track was playing. After the first track completed, the queue wasn’t continuing to play tracks 2 and 3, and onQueueCompleted() was being called immediately.

Root Cause

The original implementation had several issues with track index tracking:

1. Initial index was 0: Starting with currentTrackIndex = 0 meant the first track would match immediately on STATE_READY, but this index wasn’t being properly managed for subsequent tracks.

2. onMediaItemTransition wasn’t calling onTrackStarted: When ExoPlayer transitioned to the next track, we were incrementing the index but not notifying the listener that a new track started. This meant tracks 2 and 3 never got their start callbacks.

3. Race condition in state tracking: The logic in onPlaybackStateChanged and onMediaItemTransition wasn’t coordinated, leading to missed callbacks.

Solution

The fix involves:

1. Start with index -1: This ensures we can detect the first track properly when it becomes ready.

2. Add hasNotifiedTrackStart flag: This prevents duplicate notifications and helps track whether we’ve notified about the current track.

3. Call onTrackStarted in both places:
   • In onPlaybackStateChanged when STATE_READY is reached (for the first track)
   • In onMediaItemTransition when transitioning between tracks (for subsequent tracks)

4. Use player.currentMediaItemIndex: Instead of manually incrementing, we rely on ExoPlayer’s own index tracking to know which track is currently playing.

5. Better logging: Added detailed logging to track state changes and transitions.

Key Changes

Before

private var currentTrackIndex = 0

override fun onMediaItemTransition(mediaItem: MediaItem?, reason: Int) {
    if (reason == Player.MEDIA_ITEM_TRANSITION_REASON_AUTO) {
        // Only notified completion, not start of new track
        if (currentTrackIndex > 0) {
            currentListener?.onTrackCompleted(currentTrackIndex - 1)
        }
        currentTrackIndex++  // Just increment, no start callback
    }
}

After

private var currentTrackIndex = -1
private var hasNotifiedTrackStart = false

override fun onMediaItemTransition(mediaItem: MediaItem?, reason: Int) {
    if (mediaItem != null && reason == Player.MEDIA_ITEM_TRANSITION_REASON_AUTO) {
        // Notify completion of previous track
        if (currentTrackIndex >= 0 && hasNotifiedTrackStart) {
            currentListener?.onTrackCompleted(currentTrackIndex)
        }
        
        // Start new track
        val newIndex = player.currentMediaItemIndex
        if (newIndex >= 0 && newIndex < trackInfoList.size) {
            currentTrackIndex = newIndex
            val trackInfo = trackInfoList[currentTrackIndex]
            currentListener?.onTrackStarted(currentTrackIndex, trackInfo)  // NOW CALLED!
            hasNotifiedTrackStart = true
        }
    }
}

Expected Behavior Now

With 3 tracks queued, you should see:

1. Track 0 starts → onTrackStarted(0, trackInfo0)
2. Track 0 completes → onTrackCompleted(0)
3. Track 1 starts → onTrackStarted(1, trackInfo1)
4. Track 1 completes → onTrackCompleted(1)
5. Track 2 starts → onTrackStarted(2, trackInfo2)
6. Track 2 completes → onTrackCompleted(2)
7. Queue completes → onQueueCompleted()

Testing

Build and run the app again. The logs should now show:

• “Playback state changed” messages with proper state tracking
• “Media item transition” messages when moving between tracks
• All onTrackStarted and onTrackCompleted callbacks for each of the 3 tracks
• onQueueCompleted only after all tracks have played