在之前的文章ExoPlayer中常见MediaSource子类的区别和使用场景中介绍了Exoplayer中各种子MediaSource的使用场景，这篇我们着重详细介绍下实现多路流混合播放的用法。常见的使用场景有：视频文件+电影字幕、正片视频+广告视频、背景视频+背景音乐等。
初始化Exoplayer就不多说了，随便查查文档就知道。
ExoPlayer mExoPlayer = new ExoPlayer.Builder(context, new DefaultMediaSourceFactory(context)).build();1.视频文件+电影字幕（MergingMediaSource）
Uri videoUri = Uri.parse("https://example.com/video.mp4");
Uri subtitleUri = Uri.parse("https://example.com/subtitles.srt");
MediaSource videoSource = new ExtractorMediaSource.Factory(dataSourceFactory)
.createMediaSource(videoUri);
MediaSource subtitleSource = new SingleSampleMediaSource.Factory(dataSourceFactory)
.createMediaSource(subtitleUri, Format.createTextSampleFormat(
"subs", MimeTypes.TEXT_SUBRIP, C.SELECTION_FLAG_DEFAULT, "en"));
MediaSource mergedSource = new MergingMediaSource(videoSource, subtitleSource);2.广告视频+正片视频（ConcatenatingMediaSource）
DefaultMediaSourceFactory mediaSourceFactory = new DefaultMediaSourceFactory(dataSourceFactory);
// 创建两个视频的 MediaSource
MediaSource video1Source = mediaSourceFactory.createMediaSource(MediaItem.fromUri("asset://android_asset/01.mp4"));
MediaSource video2Source = mediaSourceFactory.createMediaSource(MediaItem.fromUri("asset://android_asset/02.mp4"));
ConcatenatingMediaSource concatenatingMediaSource = new ConcatenatingMediaSource(video1Source,video2Source);
mExoPlayer.setMediaSource(concatenatingMediaSource);这样就能让两个视频按顺序播放且无缝衔接，若还想它两循环播放，可用LoopingMediaSource进一步封装。
//无限循环
LoopingMediaSource loopingMediaSource = new LoopingMediaSource(concatenatingMediaSource);
//循环5次
LoopingMediaSource loopingMediaSource = new LoopingMediaSource(concatenatingMediaSource,5);
mExoPlayer.setMediaSource(loopingMediaSource);3.视频+音频
DefaultMediaSourceFactory mediaSourceFactory = new DefaultMediaSourceFactory(dataSourceFactory);
MediaSource video2Source = mediaSourceFactory.createMediaSource(MediaItem.fromUri("asset://android_asset/02.mp4"));
MediaSource audio1Source = mediaSourceFactory.createMediaSource(MediaItem.fromUri("asset://android_asset/audio/ori.mp2"));
MergingMediaSource audioMerged = new MergingMediaSource(video2Source , audio1Source);
mExoPlayer.setMediaSource(audioMerged );
上面是1个视频+1个音频，当然也可以支持1个视频+多个音频，比如电影中有多个不同语言的音轨
DefaultMediaSourceFactory mediaSourceFactory = new DefaultMediaSourceFactory(dataSourceFactory);
MediaSource video1Source = mediaSourceFactory.createMediaSource(MediaItem.fromUri("asset://android_asset/01.mp4"));
// 合并两个音频源
MediaSource audio1Source = mediaSourceFactory.createMediaSource(MediaItem.fromUri("asset://android_asset/audio/ori.mp2"));
MediaSource audio2Source = mediaSourceFactory.createMediaSource(MediaItem.fromUri("asset://android_asset/audio/acc.mp2"));
MergingMediaSource audioMerged = new MergingMediaSource(audio1Source,audio2Source);
//合并视频和音频
MergingMediaSource finalSource = new MergingMediaSource(audioMerged,video1Source);
mExoPlayer.setMediaSource(finalSource);这样就可以实现一个视频混合多个音轨文件的播放了，那么如何动态切换不同音轨呢？TrackSelector
DefaultTrackSelector trackSelector = new DefaultTrackSelector(context);
mExoPlayer = new ExoPlayer.Builder(context, renderersFactory)
.setTrackSelector(trackSelector)
.build();
//查看所有音轨信息
private class PlayerEventListener implements Player.Listener {
@SuppressLint("UnsafeOptInUsageError")
@Override
public void onTracksChanged(Tracks tracks) {
audioList.clear();
Player.Listener.super.onTracksChanged(tracks);
ImmutableList<Tracks.Group> trackGroups = tracks.getGroups();
for (int index = 0; index < trackGroups.size(); index++) {
Tracks.Group group = trackGroups.get(index);
for (int jIndex = 0; jIndex < group.length; jIndex++) {
Format format = group.getTrackFormat(jIndex);
LOG.info("onTracksChanged format=" + Format.toLogString(format));
if (MimeTypes.isAudio(format.sampleMimeType)) {
audioList.add(format);
}
}
}
currentTrackGroups = mExoPlayer.getCurrentTrackGroups();
}
mExoPlayer.addListener(new PlayerEventListener());
// 用户选择第 index 个音轨
TrackGroup selectedGroup = null;
selectedGroup = currentTrackGroups.get(1); //根据需要选择第几个音轨
// 应用新音轨
mExoPlayer.setTrackSelectionParameters(mExoPlayer.getTrackSelectionParameters()
.buildUpon()
.setOverrideForType(new TrackSelectionOverride(selectedGroup, 0)) // 需要切换到的音轨索引
.build());
这里有个问题就是如果视频和音频时长不一致，特别是想混合多个音频和多个视频时就会出问题，无法播放，报错如下：
E/ExoPlayerImplInternal(11191): Playback error
E/ExoPlayerImplInternal(11191): com.google.android.exoplayer2.ExoPlaybackException: Source error
E/ExoPlayerImplInternal(11191): at com.google.android.exoplayer2.ExoPlayerImplInternal.handleIoException(ExoPlayerImplInternal.java:684)
E/ExoPlayerImplInternal(11191): at com.google.android.exoplayer2.ExoPlayerImplInternal.handleMessage(ExoPlayerImplInternal.java:660)
E/ExoPlayerImplInternal(11191): at android.os.Handler.dispatchMessage(Handler.java:98)
E/ExoPlayerImplInternal(11191): at android.os.Looper.loop(Looper.java:136)
E/ExoPlayerImplInternal(11191): at android.os.HandlerThread.run(HandlerThread.java:61)
E/ExoPlayerImplInternal(11191): Caused by: com.google.android.exoplayer2.source.MergingMediaSource$IllegalMergeException
E/ExoPlayerImplInternal(11191): at com.google.android.exoplayer2.source.MergingMediaSource.onChildSourceInfoRefreshed(MergingMediaSource.java:252)
E/ExoPlayerImplInternal(11191): at com.google.android.exoplayer2.source.MergingMediaSource.onChildSourceInfoRefreshed(MergingMediaSource.java:52)
E/ExoPlayerImplInternal(11191): at com.google.android.exoplayer2.source.CompositeMediaSource.lambda$prepareChildSource$0$com-google-android-exoplayer2-source-CompositeMediaSource(CompositeMediaSource.java:120)
E/ExoPlayerImplInternal(11191): at com.google.android.exoplayer2.source.CompositeMediaSource$$ExternalSyntheticLambda0.onSourceInfoRefreshed(D8$$SyntheticClass:0)
E/ExoPlayerImplInternal(11191): at com.google.android.exoplayer2.source.BaseMediaSource.refreshSourceInfo(BaseMediaSource.java:94)
E/ExoPlayerImplInternal(11191): at com.google.android.exoplayer2.source.ConcatenatingMediaSource.updateTimelineAndScheduleOnCompletionActions(ConcatenatingMediaSource.java:746)
E/ExoPlayerImplInternal(11191): at com.google.android.exoplayer2.source.ConcatenatingMediaSource.handleMessage(ConcatenatingMediaSource.java:716)
E/ExoPlayerImplInternal(11191): at com.google.android.exoplayer2.source.ConcatenatingMediaSource.$r8$lambda$xvlxaabNVihM68DRWdn_WPenrXk(ConcatenatingMediaSource.java)
E/ExoPlayerImplInternal(11191): at com.google.android.exoplayer2.source.ConcatenatingMediaSource$$ExternalSyntheticLambda0.handleMessage(D8$$SyntheticClass:0)
E/ExoPlayerImplInternal(11191): ... 3 more这个主要是播放时长不一致，无法同步时序导致，下一篇再讨论如何解决此类情况。
