Track selection

When a media item contains multiple tracks, track selection is the process that determines which of them are chosen for playback. The track selection process is configured by TrackSelectionParameters, which allows many different constraints and overrides influencing track selection to be specified.

Querying the available tracks

You can listen to Player.Listener.onTracksChanged to be notified about changes to tracks, including:

  • The available tracks becoming known when preparation of the media item being played completes. Note that the player needs to prepare a media item to know what tracks it contains.
  • The available tracks changing due to playback transitioning from one media item to another.
  • Changes to the selected tracks.
player.addListener(new Player.Listener() {
  @Override
  public void onTracksChanged(Tracks tracks) {
    // Update UI using current tracks.
  }
});

You can also query the current tracks by calling player.getCurrentTracks(). The returned Tracks contains a list of Track.Groups, where tracks within a single Group present the same content but in different formats.

As an example of how tracks can be grouped, consider an adaptive playback where a main video feed is provided in five bitrates, and an alternative video feed (e.g., a different camera angle in a sports match) is provided in two bitrates. In this case there will be two video track groups, one corresponding to the main video feed containing five tracks, and a second for the alternative video feed containing two tracks.

Audio tracks whose languages differ are not grouped, because content in different languages is not considered to be the same. Conversely, audio tracks in the same language that only differ in properties such as bitrate, sampling rate, channel count and so on can be grouped. This also applies to text tracks.

Each Group can be queried to determine which tracks are supported for playback, which are currently selected, and what Format each track uses:

for (Tracks.Group trackGroup : tracks.getGroups()) {
  // Group level information.
  @C.TrackType int trackType = trackGroup.getTrackType();
  boolean trackInGroupIsSelected = trackGroup.isSelected();
  boolean trackInGroupIsSupported = trackGroup.isSupported();
  for (int i = 0; i < trackGroup.length; i++) {
    // Individual track information.
    boolean isSupported = trackGroup.isTrackSupported(i);
    boolean isSelected = trackGroup.isTrackSelected(i);
    Format trackFormat = trackGroup.getTrackFormat(i);
  }
}
  • A track is ‘supported’ if the Player is able to decode and render its samples. Note that even if multiple track groups of the same type (for example multiple audio track groups) are supported, it only means that they are supported individually and the player is not necessarily able to play them at the same time.
  • A track is ‘selected’ if it has been chosen for playback given the current TrackSelectionParameters. If multiple tracks within one track group are selected, the player uses these tracks for adaptive playback (for example, multiple video tracks with different bitrates). Note that only one of these tracks will be played at any one time.

Modifying track selection parameters

The track selection process can be configured using Player.setTrackSelectionParameters. This can be done both before and during playback. The example below demonstrates how to obtain the current TrackSelectionParameters from the player, modify them, and update the Player with the modified result:

player.setTrackSelectionParameters(
    player.getTrackSelectionParameters()
        .buildUpon()
        .setMaxVideoSizeSd()
        .setPreferredAudioLanguage("hu")
        .build());

Constraint based track selection

Most options in TrackSelectionParameters allow you to specify constraints, which are independent of the tracks that are actually available. Available constraints include:

  • Maximum and minimum video width, height, frame rate, and bitrate.
  • Maximum audio channel count and bitrate.
  • Preferred MIME types for video and audio.
  • Preferred audio languages and role flags.
  • Preferred text languages and role flags.

ExoPlayer uses sensible defaults for these constraints, for example restricting video resolution to the display size and preferring the audio language that matches the user’s system Locale setting.

There are several benefits to using constraint based track selection rather than selecting specific tracks from those that are available:

  • You can specify constraints before knowing what tracks a media item provides. This means that constraints can be specified before the player has prepared a media item, whereas selecting specific tracks requires application code to wait until the available tracks become known.
  • Constraints are applied for all media items in a playlist, even when those items have different available tracks. For example, a preferred audio language constraint will be automatically applied for all media items, even if the Format of the track in that language varies from one media item to the next. This is not the case when selecting specific tracks, as described below.

Selecting specific tracks

It’s possible to select specific tracks using TrackSelectionParameters. First, the player’s currently available tracks should be queried using Player.getCurrentTracks. Second, having identified which tracks to select, they can be set on TrackSelectionParameters using a TrackSelectionOverride. For example, to select the first track from a specific audioTrackGroup:

player.setTrackSelectionParameters(
    player.getTrackSelectionParameters()
        .buildUpon()
        .setOverrideForType(
            new TrackSelectionOverride(
                audioTrackGroup.getMediaTrackGroup(),
                /* trackIndex= */ 0))
        .build());

A TrackSelectionOverride will only apply to media items that contain a TrackGroup exactly matching the one specified in the override. Hence an override may not apply to a subsequent media item if that item contains different tracks.

Disabling track types or groups

Track types like video, audio or text, can be disabled completely using TrackSelectionParameters.Builder.setTrackTypeDisabled. A disabled track type will be disabled for all media items:

player.setTrackSelectionParameters(
    player.getTrackSelectionParameters()
        .buildUpon()
        .setTrackTypeDisabled(C.TRACK_TYPE_VIDEO, /* disabled= */ true)
        .build());

Alternatively, it’s possible to prevent the selection of tracks from a specific TrackGroup by specifying an empty override for that group:

player.setTrackSelectionParameters(
    player.getTrackSelectionParameters()
        .buildUpon()
        .addOverride(
            new TrackSelectionOverride(
                disabledTrackGroup.getMediaTrackGroup(),
                /* trackIndices= */ ImmutableList.of()))
        .build());

Customizing the track selector

Track selection is the responsibility of a TrackSelector, an instance of which can be provided whenever an ExoPlayer is built and later obtained with ExoPlayer.getTrackSelector().

DefaultTrackSelector trackSelector = new DefaultTrackSelector(context);
ExoPlayer player =
    new ExoPlayer.Builder(context)
        .setTrackSelector(trackSelector)
        .build();

DefaultTrackSelector is a flexible TrackSelector suitable for most use cases. It uses the TrackSelectionParameters set in the Player, but also provides some advanced customization options that can be specified in the DefaultTrackSelector.ParametersBuilder:

trackSelector.setParameters(
    trackSelector
        .buildUponParameters()
        .setAllowVideoMixedMimeTypeAdaptiveness(true));

Tunneling

Tunneled playback can be enabled in cases where the combination of renderers and selected tracks supports it. This can be done by using DefaultTrackSelector.ParametersBuilder.setTunnelingEnabled(true).