扩展ExoPlayer实现多音轨同时播放

如果要同时播放2个音频甚至播放多个音频怎么办呢？比如同时播放伴奏声音和原唱声音，而且要做到多音轨同步。这里涉及一个核心问题，多音轨播放如何同步，因为每个音轨有对应时钟。Android平台的ExoPlayer扩展性非常好，虽然本身不支持多音轨播放，但是我们可以扩展。通过扩展TrackSelector、AudioRender、RenderFactory来支持多音轨。

1、自定义AudioRender

继承于MediaCodecAudioRenderer，在构造方法传入index记录音轨索引，重写getMediaClock方法，默认使用第一个音轨作为同步时钟。也就是解决上面讨论的核心问题。

public class MultiTrackAudioRender extends MediaCodecAudioRenderer {private final int index;public MultiTrackAudioRender(int index,Context context,MediaCodecSelector mediaCodecSelector,@Nullable DrmSessionManager<FrameworkMediaCrypto> drmSessionManager,boolean playClearSamplesWithoutKeys,boolean enableDecoderFallback,@Nullable Handler eventHandler,@Nullable AudioRendererEventListener eventListener,AudioSink audioSink) {super(context, mediaCodecSelector, drmSessionManager, playClearSamplesWithoutKeys,enableDecoderFallback, eventHandler, eventListener, audioSink);this.index = index;}@Nullable@Overridepublic MediaClock getMediaClock() {if (index == 0) {return super.getMediaClock();}return null;}}

2、自定义RenderFactory

继承于DefaultRenderersFactory，把自定义的AudioRender添加到renderList里。

public class MultiTrackRenderFactory extends DefaultRenderersFactory {private final static int count = 2;public MultiTrackRenderFactory(Context context) {super(context);}@Overrideprotected void buildAudioRenderers(Context context, int extensionRendererMode, MediaCodecSelector mediaCodecSelector,@Nullable DrmSessionManager<FrameworkMediaCrypto> drmSessionManager,boolean playClearSamplesWithoutKeys, boolean enableDecoderFallback,AudioProcessor[] audioProcessors, Handler eventHandler,AudioRendererEventListener eventListener, ArrayList<Renderer> out) {// Create ProcessorChain, and add any AudioProcess you needDefaultAudioSink.DefaultAudioProcessorChain processorChain = new DefaultAudioSink.DefaultAudioProcessorChain(new SilenceSkippingAudioProcessor(),new SonicAudioProcessor());for (int i=0; i<count; i++) {AudioSink audioSink = new DefaultAudioSink(AudioCapabilities.getCapabilities(context),processorChain, true);// Using MultiTrackAudioRender and add into renderListMultiTrackAudioRender audioRender = new MultiTrackAudioRender(i, context, MediaCodecSelector.DEFAULT,drmSessionManager, playClearSamplesWithoutKeys, enableDecoderFallback,eventHandler, eventListener, audioSink);out.add(audioRender);}}
}

3、自定义TrackSelector

继承于DefaultTrackSelector，判断到媒体类型是音频，都添加renderList里。这样就实现支持多音轨播放。

public class MultiTrackSelector extends DefaultTrackSelector {public MultiTrackSelector(Context context) {super(context);}@Overridepublic TrackSelectorResult selectTracks(RendererCapabilities[] rendererCapabilities, TrackGroupArray trackGroups,MediaSource.MediaPeriodId periodId, Timeline timeline) throws ExoPlaybackException {if (trackGroups == null || rendererCapabilities == null) {return null;}LinkedList<Integer> renderList = new LinkedList<>();TrackSelection[] selections = new TrackSelection[rendererCapabilities.length];RendererConfiguration[] configs = new RendererConfiguration[rendererCapabilities.length];for (int i = 0; i < rendererCapabilities.length; i++) {// Add all audio tracks into renderListif(rendererCapabilities[i].getTrackType() == C.TRACK_TYPE_AUDIO) {renderList.add(i);configs[i] = RendererConfiguration.DEFAULT;}}for (int i = 0; i < trackGroups.length; i++) {if (trackGroups.get(i).getFormat(0) != null&& MimeTypes.isAudio(trackGroups.get(i).getFormat(0).sampleMimeType)) {Integer index = renderList.poll();if (index != null) {selections[index] = new FixedTrackSelection(trackGroups.get(i), 0);}}}int[] rendererTrackGroupCounts = new int[rendererCapabilities.length + 1];TrackGroup[][] rendererTrackGroups = new TrackGroup[rendererCapabilities.length + 1][];@RendererCapabilities.Capabilities int[][][] rendererFormatSupports = new int[rendererCapabilities.length + 1][][];for (int i = 0; i < rendererTrackGroups.length; i++) {rendererTrackGroups[i] = new TrackGroup[trackGroups.length];rendererFormatSupports[i] = new int[trackGroups.length][];}// Determine the extent to which each renderer supports mixed mimeType adaptation.@RendererCapabilities.AdaptiveSupportint[] rendererMixedMimeTypeAdaptationSupports =getMixedMimeTypeAdaptationSupports(rendererCapabilities);for (int groupIndex = 0; groupIndex < trackGroups.length; groupIndex++) {TrackGroup group = trackGroups.get(groupIndex);// Associate the group to a preferred renderer.int rendererIndex = findRenderer(rendererCapabilities, group);// Evaluate the support that the renderer provides for each track in the group.@RendererCapabilities.Capabilitiesint[] rendererFormatSupport =rendererIndex == rendererCapabilities.length? new int[group.length]: getFormatSupport(rendererCapabilities[rendererIndex], group);// Stash the results.int rendererTrackGroupCount = rendererTrackGroupCounts[rendererIndex];rendererTrackGroups[rendererIndex][rendererTrackGroupCount] = group;rendererFormatSupports[rendererIndex][rendererTrackGroupCount] = rendererFormatSupport;rendererTrackGroupCounts[rendererIndex]++;}// Create a track group array for each renderer, and trim each rendererFormatSupports entry.TrackGroupArray[] rendererTrackGroupArrays = new TrackGroupArray[rendererCapabilities.length];int[] rendererTrackTypes = new int[rendererCapabilities.length];for (int i = 0; i < rendererCapabilities.length; i++) {int rendererTrackGroupCount = rendererTrackGroupCounts[i];rendererTrackGroupArrays[i] =new TrackGroupArray(Util.nullSafeArrayCopy(rendererTrackGroups[i], rendererTrackGroupCount));rendererFormatSupports[i] =Util.nullSafeArrayCopy(rendererFormatSupports[i], rendererTrackGroupCount);rendererTrackTypes[i] = rendererCapabilities[i].getTrackType();}// Create a track group array for track groups not mapped to a renderer.int unmappedTrackGroupCount = rendererTrackGroupCounts[rendererCapabilities.length];TrackGroupArray unmappedTrackGroupArray =new TrackGroupArray(Util.nullSafeArrayCopy(rendererTrackGroups[rendererCapabilities.length], unmappedTrackGroupCount));// Package up the track information and selections.MappedTrackInfo mappedTrackInfo =new MappedTrackInfo(rendererTrackTypes,rendererTrackGroupArrays,rendererMixedMimeTypeAdaptationSupports,rendererFormatSupports,unmappedTrackGroupArray);return new TrackSelectorResult(configs, selections, mappedTrackInfo);}}

4、接入SimpleExoPlayer

创建MultiTrackSelector、MultiRenderFactory，作为参数传入到SimpleExoPlayer.Builder，来构造出SimpleExoPlayer。

DefaultTrackSelector trackSelector = new MultiTrackSelector(context);
RenderersFactory  renderersFactory = new MultiTrackRenderFactory(context);
SimpleExoPlayer.Builder builder    = new SimpleExoPlayer.Builder(context, renderersFactory, trackSelector, new DefaultLoadControlX(),DefaultBandwidthMeter.getSingletonInstance(context), handler.getLooper(), new AnalyticsCollector(Clock.DEFAULT), true, Clock.DEFAULT);
SimpleExoPlayer simpleExoPlayer = builder.build();

至此，实现多音轨同时播放。ExoPlayer架构设计合理，易于扩展，这是值得我们学习的地方。

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