Great question! Let's walk through how to handle adding or replacing subtitles for a video that's already playing in ExoPlayer—since the pre-player setup with MergingMediaSource doesn't cover this dynamic use case. First, let's clear up one key point: DynamicConcatenatingMediaSource isn't the right tool here. It's designed for stitching together a sequence of media items (like a playlist), not modifying the subtitle tracks of a single, currently playing video. Instead, we have two main approaches depending on your scenario:

1. Switching Built-In Subtitles (No MediaSource Rebuild Needed) #

If your video already has embedded subtitle tracks, you don't need to touch the media source at all. ExoPlayer lets you switch between available tracks dynamically using TrackSelectionParameters:

// Get current track selection parameters
TrackSelectionParameters currentParams = player.getTrackSelectionParameters();

// Fetch all available subtitle track groups
TrackGroups subtitleTrackGroups = player.getCurrentTracks().getGroups(C.TRACK_TYPE_TEXT);

// Example: Select the second subtitle track (index 1) from the first group
TrackSelectionOverride subtitleOverride = TrackSelectionOverride.create(subtitleTrackGroups.get(0), 1);

// Build updated parameters
TrackSelectionParameters newParams = currentParams.buildUpon()
    .setTrackTypeDisabled(C.TRACK_TYPE_TEXT, false) // Ensure subtitles aren't disabled
    .setOverrideForType(subtitleOverride)
    .build();

// Apply the new selection (seamless switch during playback)
player.setTrackSelectionParameters(newParams);

This will instantly switch to the selected subtitle track without interrupting playback.

2. Adding/Replacing Custom User-Provided Subtitles #

For adding a new subtitle file (from internal storage) or replacing existing ones, you'll need to rebuild your media source and update the player dynamically. Here's how to do it seamlessly:

Step 1: Create the Custom Subtitle MediaSource #

First, build a SingleSampleMediaSource for the user's subtitle file:

// Assume you have a DataSourceFactory set up (e.g., DefaultDataSourceFactory)
SingleSampleMediaSource customSubtitleSource = new SingleSampleMediaSource.Factory(dataSourceFactory)
    .createMediaSource(
        MediaItem.fromUri(Uri.fromFile(userSubtitleFile)), // Path to user's subtitle file
        Format.createTextSampleFormat(
            null, // Let ExoPlayer auto-generate an ID
            MimeTypes.TEXT_VTT, // Adjust based on subtitle type: MimeTypes.APPLICATION_SUBRIP for SRT
            Format.NO_VALUE,
            null
        )
    );

Step 2: Rebuild the MergingMediaSource #

Take your original video media source and merge it with the new subtitle source:

// Assume originalVideoSource is the MediaSource for your playing video
MergingMediaSource updatedMediaSource = new MergingMediaSource(originalVideoSource, customSubtitleSource);

Step 3: Update the Player Seamlessly #

Tell the player to switch to the new media source while keeping playback state and position:

// Parameters: newMediaSource, keepPlaying, resetPosition
player.prepare(updatedMediaSource, true, false);

• keepPlaying = true: The player resumes playback immediately after updating.
• resetPosition = false: The player stays at the current playback position.

Alternative: Using MediaItem with Subtitle Configurations #

If you're using MediaItem to load your video (instead of raw MediaSource), you can update the MediaItem directly with new subtitle configurations:

// Create a subtitle configuration for the user's file
MediaItem.SubtitleConfiguration subtitleConfig = new MediaItem.SubtitleConfiguration.Builder(Uri.fromFile(userSubtitleFile))
    .setMimeType(MimeTypes.TEXT_VTT)
    .setLanguage("en") // Set appropriate language code
    .setSelectionFlags(C.SELECTION_FLAG_DEFAULT) // Mark as default if needed
    .build();

// Update the existing MediaItem with the new subtitle
MediaItem updatedMediaItem = originalMediaItem.buildUpon()
    .addSubtitleConfiguration(subtitleConfig)
    // Use .clearSubtitleConfigurations() first if you want to replace all existing subtitles
    .build();

// Apply the updated MediaItem without resetting position
player.setMediaItem(updatedMediaItem, false);
player.prepare(false);

Key Notes #

• If you want to replace all existing subtitles (including built-in ones), use .clearSubtitleConfigurations() when updating the MediaItem, or exclude built-in tracks by only merging your custom subtitle source with the video.
• Always ensure your DataSourceFactory has the necessary permissions to access the user's internal storage file.

内容的提问来源于stack exchange，提问作者Siju