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
@Override
public 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);
}
@Override
protected 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 need
DefaultAudioSink.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 renderList
MultiTrackAudioRender 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);
}
@Override
public 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 renderList
if(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.AdaptiveSupport
int[] 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.Capabilities
int[] 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。