ExoPlayer 自适应流切换分析
一、前言
自适应流切换属于多路流切换的方式中的一种，ExoPlayer作为MediaCodec使用的集大成者，不仅具备通过MergingMediaSource实现不同流的组合切换，同样也具备基于MGEG-DASH、HLS、smoothing-stream 协议的的自适应流切换。当然，在项目中每种方案的选型都要充分考虑团队条件。
主要区别如下：
MergingMediaSource 方式更适合团队人力有限，后台服务支持有限的情况，不需要在资源传输和编码上做更多的考虑，普通的CDN部署就可以，相比更加节省成本。而自适应流相对要求比较专业，对服务器的部署、资源分片、资源编码也是有一定要求的。 MergingMediaSource 方式可实现不同编码的流合并，而自适应流方面部分协议如HLS有较严格的要求，主要要求是ts分片的编码尽可能保持一致，这样做的目的是为尽可能实现MediaCodec的重复利用。当然，MergingMediaSource方式如果每路流的Format差别不大，视频解码器完全可以通过PPS、SPS或flush buffer 的方式实现MediaCodec利用，音频解码器也是可以通过输入特定字节特征实现MediaCodec复用。 在ExoPlayer中，MergingMediaSource 中的同一类型（视频类型、音频类型、字幕类型等）的数据，由于缺乏必要的码率参数， 无法将相似Format的Track数据合并为一组，因此使用的FixedTrackSelection对同一类型的资源，自然而然也不支持多路流的自动切换。而自适应流完成可以实现Format分组，最终创建AdaptiveTrackSelection 动态管理各路流。
Renderer渲染器：负责解码器的Format支持能力检测、解码器的注册、解码器的销毁、解码器的复用、采样数据读取、数据渲染或输出、丢帧、跳帧以及音画同步等工作。ExoPlayer支持Renderer的拆解、组合、关闭和启用，也支持自定义的解码器接入，比如通过SimpleDecoder实现FFMPEG对视频和音频的解码渲染。
MediaClock媒体时钟：负责音画同步、播放进度管理等工作。在ExoPlayer中国存在两种时钟，一种是独立时钟StandaloneMediaClock，另一种是通过音频Renderer实现的Audio Master模式的时钟。
Extractor 解封装器：负责将媒体资源中的每一路流的Moov信息、采样表、Format、采样数据（如SPS、PPS、各种帧数据）拆解出来，同时会对一些数据，便于Track和Format的选择以及码流切换。ExoPlayer内置了大量的解封装器，同样也支持自定义的Extractor来实现特定目的。当然，自适应流Format的解析一般是通过MediaSource去解析的，只有视频容器需要通过Extractor去解析。
Decoder解码器：负责解码采样数据，其中MediaCodec具备使用硬解和软解的能力的同时，而且支持渲染。
TrackSelector：多路流切换的核心类，负责通过Format检测实现TrackSelection的分组、以及TrackGroup和Renderer、TrackGroup和TrackSelection的配对工作。
DataSource 数据源：负责提供数据。
MediaSource 媒体源：在ExoPlayer中，得益于对从DataSource中抽象出了MediaSource，使得ExoPlayer在多路流管理方面更加灵活方便。
SeekPoint：在ExoPlayer中，SeekPoint 往往是IDR帧即将开始的位置。
FixedTrackSelection：固定流选择器，播放过程中TrackSelection保持固定，普通协议默认的TrackSelection，但也因为这个原因，使得其适用于MergingMediaSource方式的多路流切换。
AdptiveTrackSelection: 自适应流选择器，可以根据Bandwidth实现动态选择分片。当然，可以通过一些策略，实现用户自行的切换，类似bilibili的码流切换。
TrackGroup : 同一类型资源的Track Format 分组。
MapTrackInfo : 这个类实际上是一个Renderer和TrackGroup相关信息的集合类，主要保存Renderer能力信息和TrackGroup信息，某种程度上可以看到数据格式和Renderer全局信息。
selection分组原理：TrackSelector会通过SELECTION_ELIGIBILITY_FIXED、SELECTION_ELIGIBILITY_ADAPTIVE对资源进行分组，当然前提是各个Format之间能够相互兼容，具体兼容逻辑参考VideoTrackInfo类和AudioTrackInfo类。
Bandwidth：ExoPlayer中对网速检测的重要工具，检测结果用于AdaptiveTrackSelection进行分片选择。
DefaultMediaSourceFactory 用于实现DataSource转换为MediaSource的路由工厂，通过mineType和uri后缀识别出创建那种MediaSource。
在不同网速时自动切换为兼容当前bitrate的媒体流，匹配条件一般在自适应流的清单文件中就已经提前设定了，保证当前网络的bitrate大于清单协议中媒体流的最低bandWidth，就可以切换到指定的媒体流Track。
默认情况下，自适应流的切换不需要查找SeekPoint，而是通过选择下一个分片实现。 默认情况下，自适应流通过网速检测实现了分片切换。 从图上可知，每个分片的的播放时间和I帧的开始位置也需要做到严格对齐。
注意：之所以强调默认情况，一个重要的原因是ExopPlayer具备高度可扩展性，我们可以通过修改部分代码实现其他行为。
清单文件解析
建立Renderer与TrackGroup、Selection之间的映射关系
开始分片加载
网速带宽检测 与 AdaptiveTrackSelection 选择合适的分片
解码器复用或重启
完成切换
3.2.1.1 hls 协议清单文件
EXTM3U#EXT-X-MEDIA:TYPE=AUDIO,GROUP-ID="bipbop_audio",LANGUAGE="eng",NAME="BipBop Audio 1",AUTOSELECT=YES,DEFAULT=YESEXT-X-MEDIA:TYPE=AUDIO,GROUP-ID="bipbop_audio",LANGUAGE="eng",NAME="BipBop Audio 2",AUTOSELECT=NO,DEFAULT=NO,URI="alternate_audio_aac/prog_index.m3u8"#EXT-X-MEDIA:TYPE=SUBTITLES,GROUP-ID="subs",NAME="English",DEFAULT=YES,AUTOSELECT=YES,FORCED=NO,LANGUAGE="en",CHARACTERISTICS="public.accessibility.transcribes-spoken-dialog, public.accessibility.describes-music-and-sound",URI="subtitles/eng/prog_index.m3u8"EXT-X-MEDIA:TYPE=SUBTITLES,GROUP-ID="subs",NAME="English (Forced)",DEFAULT=NO,AUTOSELECT=NO,FORCED=YES,LANGUAGE="en",URI="subtitles/eng_forced/prog_index.m3u8"EXT-X-MEDIA:TYPE=SUBTITLES,GROUP-ID="subs",NAME="Français",DEFAULT=NO,AUTOSELECT=YES,FORCED=NO,LANGUAGE="fr",CHARACTERISTICS="public.accessibility.transcribes-spoken-dialog, public.accessibility.describes-music-and-sound",URI="subtitles/fra/prog_index.m3u8"EXT-X-MEDIA:TYPE=SUBTITLES,GROUP-ID="subs",NAME="Français (Forced)",DEFAULT=NO,AUTOSELECT=NO,FORCED=YES,LANGUAGE="fr",URI="subtitles/fra_forced/prog_index.m3u8"EXT-X-MEDIA:TYPE=SUBTITLES,GROUP-ID="subs",NAME="Español",DEFAULT=NO,AUTOSELECT=YES,FORCED=NO,LANGUAGE="es",CHARACTERISTICS="public.accessibility.transcribes-spoken-dialog, public.accessibility.describes-music-and-sound",URI="subtitles/spa/prog_index.m3u8"EXT-X-MEDIA:TYPE=SUBTITLES,GROUP-ID="subs",NAME="Español (Forced)",DEFAULT=NO,AUTOSELECT=NO,FORCED=YES,LANGUAGE="es",URI="subtitles/spa_forced/prog_index.m3u8"EXT-X-MEDIA:TYPE=SUBTITLES,GROUP-ID="subs",NAME="日本語",DEFAULT=NO,AUTOSELECT=YES,FORCED=NO,LANGUAGE="ja",CHARACTERISTICS="public.accessibility.transcribes-spoken-dialog, public.accessibility.describes-music-and-sound",URI="subtitles/jpn/prog_index.m3u8"EXT-X-MEDIA:TYPE=SUBTITLES,GROUP-ID="subs",NAME="日本語 (Forced)",DEFAULT=NO,AUTOSELECT=NO,FORCED=YES,LANGUAGE="ja",URI="subtitles/jpn_forced/prog_index.m3u8"#EXT-X-STREAM-INF:BANDWIDTH=263851,CODECS="mp4a.40.2, avc1.4d400d",RESOLUTION=416x234,AUDIO="bipbop_audio",SUBTITLES="subs"gear1/prog_index.m3u8EXT-X-I-FRAME-STREAM-INF:BANDWIDTH=28451,CODECS="avc1.4d400d",URI="gear1/iframe_index.m3u8"#EXT-X-STREAM-INF:BANDWIDTH=577610,CODECS="mp4a.40.2, avc1.4d401e",RESOLUTION=640x360,AUDIO="bipbop_audio",SUBTITLES="subs"gear2/prog_index.m3u8EXT-X-I-FRAME-STREAM-INF:BANDWIDTH=181534,CODECS="avc1.4d401e",URI="gear2/iframe_index.m3u8"#EXT-X-STREAM-INF:BANDWIDTH=915905,CODECS="mp4a.40.2, avc1.4d401f",RESOLUTION=960x540,AUDIO="bipbop_audio",SUBTITLES="subs"gear3/prog_index.m3u8EXT-X-I-FRAME-STREAM-INF:BANDWIDTH=297056,CODECS="avc1.4d401f",URI="gear3/iframe_index.m3u8"#EXT-X-STREAM-INF:BANDWIDTH=1030138,CODECS="mp4a.40.2, avc1.4d401f",RESOLUTION=1280x720,AUDIO="bipbop_audio",SUBTITLES="subs"gear4/prog_index.m3u8EXT-X-I-FRAME-STREAM-INF:BANDWIDTH=339492,CODECS="avc1.4d401f",URI="gear4/iframe_index.m3u8"#EXT-X-STREAM-INF:BANDWIDTH=1924009,CODECS="mp4a.40.2, avc1.4d401f",RESOLUTION=1920x1080,AUDIO="bipbop_audio",SUBTITLES="subs"gear5/prog_index.m3u8EXT-X-I-FRAME-STREAM-INF:BANDWIDTH=669554,CODECS="avc1.4d401f",URI="gear5/iframe_index.m3u8"#EXT-X-STREAM-INF:BANDWIDTH=41457,CODECS="mp4a.40.2",AUDIO="bipbop_audio",SUBTITLES="subs"gear0/prog_index.m3u8
3.2.1.2 DASH协议清单文件
<!--Generated with https://github.com/google/shaka-packager version 97fc982-release--><MPD xmlns="urn:mpeg:dash:schema:mpd:2011" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xlink="http://www.w3.org/1999/xlink" xsi:schemaLocation="urn:mpeg:dash:schema:mpd:2011 DASH-MPD.xsd" xmlns:cenc="urn:mpeg:cenc:2013" minBufferTime="PT2S" type="static" profiles="urn:mpeg:dash:profile:isoff-on-demand:2011" mediaPresentationDuration="PT734S"><Period id="0"><AdaptationSet id="0" contentType="audio" lang="en"><Representation id="0" bandwidth="131596" codecs="mp4a.40.2" mimeType="audio/mp4" audioSamplingRate="44100"><AudioChannelConfiguration schemeIdUri="urn:mpeg:dash:23003:3:audio_channel_configuration:2011" value="2"/><BaseURL>tears_audio_eng.mp4</BaseURL><SegmentBase indexRange="745-1664" timescale="44100"><Initialization range="0-744"/></SegmentBase></Representation></AdaptationSet><AdaptationSet id="1" contentType="video" maxWidth="1920" maxHeight="856" frameRate="12288/512" par="38:17"><Representation id="1" bandwidth="769255" codecs="avc1.42c01e" mimeType="video/mp4" sar="852:857" width="320" height="142"><BaseURL>tears_h264_baseline_240p_800.mp4</BaseURL><SegmentBase indexRange="827-1602" timescale="12288"><Initialization range="0-826"/></SegmentBase></Representation><Representation id="2" bandwidth="1774254" codecs="avc1.4d401f" mimeType="video/mp4" sar="2242:2249" width="854" height="380"><BaseURL>tears_h264_main_480p_2000.mp4</BaseURL><SegmentBase indexRange="829-1604" timescale="12288"><Initialization range="0-828"/></SegmentBase></Representation><Representation id="3" bandwidth="7203938" codecs="avc1.4d4028" mimeType="video/mp4" sar="855:857" width="1280" height="570"><BaseURL>tears_h264_main_720p_8000.mp4</BaseURL><SegmentBase indexRange="830-1605" timescale="12288"><Initialization range="0-829"/></SegmentBase></Representation><Representation id="4" bandwidth="18316946" codecs="avc1.64002a" mimeType="video/mp4" sar="856:857" width="1920" height="856"><BaseURL>tears_h264_high_1080p_20000.mp4</BaseURL><SegmentBase indexRange="832-1607" timescale="12288"><Initialization range="0-831"/></SegmentBase></Representation></AdaptationSet></Period></MPD>
两种协议的共同点中我们可以明确的发现又很多共同点：
bandWidth: 网络带宽，也就是下载的速率，但是在清单文件中一般表示的是支持该数据流的最低下载速率。
mimeType : 资源类型
codecs: 资源编码类型 width: 视频宽度 height: 视频高度
和其他协议的资源不同的是，由于使用清单文件的原因，基本可以实现在解封装之前就能获取到必要的Format信息。
解析时清单文件时，如果使用的是HLS协议，ExoPlayer内部利用HlsPlaylistParser类作为清单文件解析工具，如果是DASH则使用DashManifestParser解析清单，依次类推，smoothing-stream 使用SsmanifestParser进行将进行清单文件解析。
使用DefaultMediaSourceFactory创建对应的自适应流MediaSource，如HlsMediaSource、DashMediaSource、SsMediaSource。 自适应流MediaSource创建Parser解析工具，当然HlsMediaSource比较特殊，不直接持有Parser，而是通过DefaultHlsPlaylistTracker时线对Parser的调度。 创建Loader和ParsingLoadable，ParsingLoadable类似于Runnable，属于加载任务，在ParsingLoadable中可以实现边加载边解析。 对每组Track Format进行分组，使用TrackGroup保存分组信息。
protected final Pair< RendererConfiguration[], ExoTrackSelection[]>selectTracks(MappedTrackInfo mappedTrackInfo,int[][][] rendererFormatSupports,int[] rendererMixedMimeTypeAdaptationSupport,MediaPeriodId mediaPeriodId,Timeline timeline)throws ExoPlaybackException {Parameters parameters;synchronized (lock) {parameters = this.parameters;if (parameters.constrainAudioChannelCountToDeviceCapabilities&& Util.SDK_INT >= 32&& spatializer != null) {// Initialize the spatializer now so we can get a reference to the playback looper with// Looper.myLooper().spatializer.ensureInitialized(this, checkStateNotNull(Looper.myLooper()));}}int rendererCount = mappedTrackInfo.getRendererCount();// 建立Renderers 与 TrackGroup 映射，注意，我们前面提到的MappedTrackInfo，保存有Renderer和TrackGroup的相关信息// 这里也会尝试对不同TrackGroup Format进行依据兼容性进行合并分组到definition中ExoTrackSelection. Definition[] definitions =selectAllTracks(mappedTrackInfo,rendererFormatSupports,rendererMixedMimeTypeAdaptationSupport,parameters);//对筛选出来的defintions 二次过滤applyTrackSelectionOverrides(mappedTrackInfo, parameters, definitions);applyLegacyRendererOverrides(mappedTrackInfo, parameters, definitions);//三次过滤，对Renderer不能使用解除映关系// Disable renderers if needed.for (int i = 0; i < rendererCount; i++) {.TrackType int rendererType = mappedTrackInfo.getRendererType(i);if (parameters.getRendererDisabled(i)|| parameters.disabledTrackTypes.contains(rendererType)) {definitions[i] = null;}}//建立TrackGroup与Selection之间的映射关系，并且传入BandwidthMeter用于获取网速策略的数据ExoTrackSelection[] rendererTrackSelections =trackSelectionFactory.createTrackSelections(definitions, getBandwidthMeter(), mediaPeriodId, timeline);// Initialize the renderer configurations to the default configuration for all renderers with// selections, and null otherwise.//创建Renderer初始化配置RendererConfiguration[] rendererConfigurations = new RendererConfiguration[rendererCount];for (int i = 0; i < rendererCount; i++) {.TrackType int rendererType = mappedTrackInfo.getRendererType(i);boolean forceRendererDisabled =parameters.getRendererDisabled(i) || parameters.disabledTrackTypes.contains(rendererType);boolean rendererEnabled =!forceRendererDisabled&& (mappedTrackInfo.getRendererType(i) == C.TRACK_TYPE_NONE|| rendererTrackSelections[i] != null);rendererConfigurations[i] = rendererEnabled ? RendererConfiguration.DEFAULT : null;}// Configure audio and video renderers to use tunneling if appropriate.if (parameters.tunnelingEnabled) {//这种属于隧道渲染，具体流程好像是MediaCodec不负责解码，可以将未解码的数据输出到驱动层，由驱动层处理，如ac3音频数据直接输出到AudioTrack中，这方面资料太少，后续在研究。maybeConfigureRenderersForTunneling(mappedTrackInfo, rendererFormatSupports, rendererConfigurations, rendererTrackSelections);}return Pair.create(rendererConfigurations, rendererTrackSelections);}
在ExoPlayer中默认使用改工厂适配Selection，具体逻辑如下
public final ExoTrackSelection[] createTrackSelections(Definition[] definitions,BandwidthMeter bandwidthMeter,MediaPeriodId mediaPeriodId,Timeline timeline) {ImmutableList<ImmutableList<AdaptationCheckpoint>> adaptationCheckpoints =getAdaptationCheckpoints(definitions);ExoTrackSelection[] selections = new ExoTrackSelection[definitions.length];for (int i = 0; i < definitions.length; i++) {Definition definition = definitions[i];if (definition == null || definition.tracks.length == 0) {continue;}//遍历所有的definition，对分组中tracks 数量为1的创建FixedTrackSelection，对存在多个的创建AdaptiveTrackSeletionselections[i] =definition.tracks.length == 1? new FixedTrackSelection(definition.group,/* track= */ definition.tracks[0],/* type= */ definition.type): createAdaptiveTrackSelection(definition.group,definition.tracks,definition.type,bandwidthMeter,adaptationCheckpoints.get(i));}return selections;}
至此，Renderer 、TrackGroup、selection的映射关系建立完成。
那么这里有个疑问，如果利用MergingMediaSource合并多路流并修改参数，能否也实现AdaptiveTrackSelection，进入试下自适应能力？答案是否定的，因为MergingMediaSource合并的是完整的资源，在使用过程中并不会调用TrackSelection相关方法，当然ExoPlayer也没有实现资源的动态分片。
3.2.3 分片加载
DASH、HLS、Smoothing-Stream 加载分片的时候，单个分片都是用各自的实现的ChunkSource类，但是对于存在多个分片情况，ExoPlayer利用ChunkSampleStream和HlsSampleStreamWrapper将分片队列管理起来，核心方法是continueLoading中的实现，大致相同，这里以HlsSampleStreamWrapper的为参考。
public boolean continueLoading(long positionUs) {if (loadingFinished || loader.isLoading() || loader.hasFatalError()) {return false;}boolean pendingReset = isPendingReset();//获取资源队列List<BaseMediaChunk> chunkQueue;//获取上次加载资源时间位置long loadPositionUs;if (pendingReset) {chunkQueue = Collections.emptyList();loadPositionUs = pendingResetPositionUs;} else {chunkQueue = readOnlyMediaChunks;loadPositionUs = getLastMediaChunk().endTimeUs;}//获取下一个分片chunkSource.getNextChunk(positionUs, loadPositionUs, chunkQueue, nextChunkHolder);boolean endOfStream = nextChunkHolder.endOfStream;Chunk loadable = nextChunkHolder.chunk;nextChunkHolder.clear();if (endOfStream) {// 如果没有资源可以加载了，标记加载结束pendingResetPositionUs = C.TIME_UNSET;loadingFinished = true;return true;}if (loadable == null) {return false;}//下面逻辑是加载状态直接的判断loadingChunk = loadable;if (isMediaChunk(loadable)) {BaseMediaChunk mediaChunk = (BaseMediaChunk) loadable;if (pendingReset) {// Only set the queue start times if we're not seeking to a chunk boundary. If we are// seeking to a chunk boundary then we want the queue to pass through all of the samples in// the chunk. Doing this ensures we'll always output the keyframe at the start of the chunk,// even if its timestamp is slightly earlier than the advertised chunk start time.if (mediaChunk.startTimeUs != pendingResetPositionUs) {primarySampleQueue.setStartTimeUs(pendingResetPositionUs);for (SampleQueue embeddedSampleQueue : embeddedSampleQueues) {embeddedSampleQueue.setStartTimeUs(pendingResetPositionUs);}}pendingResetPositionUs = C.TIME_UNSET;}mediaChunk.init(chunkOutput);mediaChunks.add(mediaChunk);} else if (loadable instanceof InitializationChunk) {((InitializationChunk) loadable).init(chunkOutput);}long elapsedRealtimeMs =loader.startLoading(loadable, this, loadErrorHandlingPolicy.getMinimumLoadableRetryCount(loadable.type));mediaSourceEventDispatcher.loadStarted(new LoadEventInfo(loadable.loadTaskId, loadable.dataSpec, elapsedRealtimeMs),loadable.type,primaryTrackType,loadable.trackFormat,loadable.trackSelectionReason,loadable.trackSelectionData,loadable.startTimeUs,loadable.endTimeUs);return true;}
getNextChunk是continueLoading中的核心方法，继续看代码。
public final void getNextChunk(long playbackPositionUs,long loadPositionUs,List<? extends MediaChunk> queue,ChunkHolder out) {if (fatalError != null) {return;}StreamElement streamElement = manifest.streamElements[streamElementIndex];if (streamElement.chunkCount == 0) {// There aren't any chunks for us to load.//没有分片可以加载了out.endOfStream = !manifest.isLive;return;}int chunkIndex; //获取最后一次加载片段的索引if (queue.isEmpty()) {chunkIndex = streamElement.getChunkIndex(loadPositionUs);} else {chunkIndex =(int) (queue.get(queue.size() - 1).getNextChunkIndex() - currentManifestChunkOffset);if (chunkIndex < 0) {//这个片段不存在，说明加载片段不完整// This is before the first chunk in the current manifest.fatalError = new BehindLiveWindowException();return;}}if (chunkIndex >= streamElement.chunkCount) {// This is beyond the last chunk in the current manifest.//这个索引不合法，直接停止加载out.endOfStream = !manifest.isLive;return;}long bufferedDurationUs = loadPositionUs - playbackPositionUs;long timeToLiveEdgeUs = resolveTimeToLiveEdgeUs(playbackPositionUs);MediaChunkIterator[] chunkIterators = new MediaChunkIterator[trackSelection.length()];for (int i = 0; i < chunkIterators.length; i++) {int trackIndex = trackSelection.getIndexInTrackGroup(i);chunkIterators[i] = new StreamElementIterator(streamElement, trackIndex, chunkIndex);}//最关键的地方，这里会根据网速，筛选出下一个分片trackSelection.updateSelectedTrack(playbackPositionUs, bufferedDurationUs, timeToLiveEdgeUs, queue, chunkIterators);long chunkStartTimeUs = streamElement.getStartTimeUs(chunkIndex);long chunkEndTimeUs = chunkStartTimeUs + streamElement.getChunkDurationUs(chunkIndex);long chunkSeekTimeUs = queue.isEmpty() ? loadPositionUs : C.TIME_UNSET;int currentAbsoluteChunkIndex = chunkIndex + currentManifestChunkOffset;//这里是重点，获取选下一个需要加载的分片int trackSelectionIndex = trackSelection.getSelectedIndex();//获取分片解封装器ChunkExtractor chunkExtractor = chunkExtractors[trackSelectionIndex];int manifestTrackIndex = trackSelection.getIndexInTrackGroup(trackSelectionIndex);Uri uri = streamElement.buildRequestUri(manifestTrackIndex, chunkIndex);//绑定要加载的片段信息out.chunk =newMediaChunk(trackSelection.getSelectedFormat(),dataSource,uri,currentAbsoluteChunkIndex,chunkStartTimeUs,chunkEndTimeUs,chunkSeekTimeUs,trackSelection.getSelectionReason(),trackSelection.getSelectionData(),chunkExtractor);}
网速检测使用的默认的DefaultBandWidthMeter进行测试，具体原理是监控数据的某一段时间的下载流量，计算出平均网速。AdaptiveTrack Selection#updateSelectedTrack中会利用选择测量数据，重新计算使用哪个队列的数据。
int newSelectedIndex = determineIdealSelectedIndex(nowMs, chunkDurationUs);核心逻辑就是通过bitrate去做比较
@Overridepublic void updateSelectedTrack(long playbackPositionUs,long bufferedDurationUs,long availableDurationUs,List<? extends MediaChunk> queue,MediaChunkIterator[] mediaChunkIterators) {long nowMs = clock.elapsedRealtime();long chunkDurationUs = getNextChunkDurationUs(mediaChunkIterators, queue);// Make initial selectionif (reason == C.SELECTION_REASON_UNKNOWN) {reason = C.SELECTION_REASON_INITIAL;//初始化或者还没有选择过则直接一次性选择，不走兼容流程selectedIndex = determineIdealSelectedIndex(nowMs, chunkDurationUs);return;}int previousSelectedIndex = selectedIndex;@C.SelectionReason int previousReason = reason;int formatIndexOfPreviousChunk =queue.isEmpty() ? C.INDEX_UNSET : indexOf(Iterables.getLast(queue).trackFormat);if (formatIndexOfPreviousChunk != C.INDEX_UNSET) {previousSelectedIndex = formatIndexOfPreviousChunk;previousReason = Iterables.getLast(queue).trackSelectionReason;}//选择匹配bitrate的formatint newSelectedIndex = determineIdealSelectedIndex(nowMs, chunkDurationUs);//判断如果该format所在的Track不在黑名单中，则走兼容逻辑if (!isBlacklisted(previousSelectedIndex, nowMs)) {// Revert back to the previous selection if conditions are not suitable for switching.Format currentFormat = getFormat(previousSelectedIndex);Format selectedFormat = getFormat(newSelectedIndex);//注意：进入这个地方就是一个坑点，如果切码流失败，原因是这里通过一些条件又给重置回去了long minDurationForQualityIncreaseUs =minDurationForQualityIncreaseUs(availableDurationUs, chunkDurationUs);if (selectedFormat.bitrate > currentFormat.bitrate&& bufferedDurationUs < minDurationForQualityIncreaseUs) {// The selected track is a higher quality, but we have insufficient buffer to safely switch// up. Defer switching up for now.//如果选择的码流大于当前的，但是buffering的数据不够去安全的切换，因此还是选择当前TracknewSelectedIndex = previousSelectedIndex;} else if (selectedFormat.bitrate < currentFormat.bitrate&& bufferedDurationUs >= maxDurationForQualityDecreaseUs) {// The selected track is a lower quality, but we have sufficient buffer to defer switching// down for now.//选择的码流小于当前的，但是buffer数据是足够，不至于去切换newSelectedIndex = previousSelectedIndex;}}// If we adapted, update the trigger.//触发选择条件reason =newSelectedIndex == previousSelectedIndex ? previousReason : C.SELECTION_REASON_ADAPTIVE;selectedIndex = newSelectedIndex;}
上面提到2个坑点，bufferedDurationUs 大小影响切换，因此在项目中有必要规避此问题。
核心点determineIdealSelectedIndex，这里的逻辑就是获取当前带宽，然后匹配Selection中的采样队列
private int determineIdealSelectedIndex(long nowMs, long chunkDurationUs) {//获取带宽数据long effectiveBitrate = getAllocatedBandwidth(chunkDurationUs);int lowestBitrateAllowedIndex = 0;for (int i = 0; i < length; i++) {if (nowMs == Long.MIN_VALUE || !isBlacklisted(i, nowMs)) {Format format = getFormat(i);//获取小于当前网速的Formatif (canSelectFormat(format, format.bitrate, effectiveBitrate)) {return i;} else {lowestBitrateAllowedIndex = i;}}}return lowestBitrateAllowedIndex;}
带宽数据获取
private long getTotalAllocatableBandwidth(long chunkDurationUs) {//注意这里将带宽x0.7f，实际上比实际值偏小了，因此设置网速时一定要除以0.7flong cautiousBandwidthEstimate =(long) (bandwidthMeter.getBitrateEstimate() * bandwidthFraction);long timeToFirstByteEstimateUs = bandwidthMeter.getTimeToFirstByteEstimateUs();if (timeToFirstByteEstimateUs == C.TIME_UNSET || chunkDurationUs == C.TIME_UNSET) {//默认情况下回到这里，带宽除以当前播放的速度 （倍速）return (long) (cautiousBandwidthEstimate / playbackSpeed);}float availableTimeToLoadUs =max(chunkDurationUs / playbackSpeed - timeToFirstByteEstimateUs, 0);return (long) (cautiousBandwidthEstimate * availableTimeToLoadUs / chunkDurationUs);}
3.2.5 解码器的复用和重启
核心逻辑如下：
protected DecoderReuseEvaluation onInputFormatChanged(FormatHolder formatHolder)throws ExoPlaybackException {waitingForFirstSampleInFormat = true;Format newFormat = checkNotNull(formatHolder.format);if (newFormat.sampleMimeType == null) {// If the new format is invalid, it is either a media bug or it is not intended to be played.// See also https://github.com/google/ExoPlayer/issues/8283.throw createRendererException(new IllegalArgumentException(),newFormat,PlaybackException.ERROR_CODE_DECODING_FORMAT_UNSUPPORTED);}setSourceDrmSession(formatHolder.drmSession);inputFormat = newFormat;if (bypassEnabled) {//等到队列数据被清空后初始化bypassDrainAndReinitialize = true;return null; // Need to drain batch buffer first.}//如果Codec已经被释放或者还没有创建的情况下，重新初始化if (codec == null) {availableCodecInfos = null;maybeInitCodecOrBypass();return null;}// We have an existing codec that we may need to reconfigure, re-initialize, or release to// switch to bypass. If the existing codec instance is kept then its operating rate and DRM// session may need to be updated.// Copy the current codec and codecInfo to local variables so they remain accessible if the// member variables are updated during the logic below.MediaCodecAdapter codec = this.codec;MediaCodecInfo codecInfo = this.codecInfo;Format oldFormat = codecInputFormat;if (drmNeedsCodecReinitialization(codecInfo, newFormat, codecDrmSession, sourceDrmSession)) {drainAndReinitializeCodec();return new DecoderReuseEvaluation(codecInfo.name,oldFormat,newFormat,REUSE_RESULT_NO,DISCARD_REASON_DRM_SESSION_CHANGED);}boolean drainAndUpdateCodecDrmSession = sourceDrmSession != codecDrmSession;Assertions.checkState(!drainAndUpdateCodecDrmSession || Util.SDK_INT >= 23);//对比新旧解码器的重用的条件，这里代码太多，不在深入讨论。DecoderReuseEvaluation evaluation = canReuseCodec(codecInfo, oldFormat, newFormat);int overridingDiscardReasons = 0;switch (evaluation.result) {case REUSE_RESULT_NO://不重用，直接重启drainAndReinitializeCodec();break;case REUSE_RESULT_YES_WITH_FLUSH://可以重复使用，但需要清除队列，这种情况下需要调用MediaCodec.flushif (!updateCodecOperatingRate(newFormat)) {overridingDiscardReasons |= DISCARD_REASON_OPERATING_RATE_CHANGED;} else {codecInputFormat = newFormat;if (drainAndUpdateCodecDrmSession) {if (!drainAndUpdateCodecDrmSessionV23()) {overridingDiscardReasons |= DISCARD_REASON_WORKAROUND;}} else if (!drainAndFlushCodec()) {overridingDiscardReasons |= DISCARD_REASON_WORKAROUND;}}break;case REUSE_RESULT_YES_WITH_RECONFIGURATION://可以重用，但是需要重新注册信息，如果是视频，需要重新注册SPS、PPS信息，具体信息在//com.google.android.exoplayer2.Format.Builder#initializationData 中，如果音频一般是资源类型if (!updateCodecOperatingRate(newFormat)) {overridingDiscardReasons |= DISCARD_REASON_OPERATING_RATE_CHANGED;} else {codecReconfigured = true;codecReconfigurationState = RECONFIGURATION_STATE_WRITE_PENDING;codecNeedsAdaptationWorkaroundBuffer =codecAdaptationWorkaroundMode == ADAPTATION_WORKAROUND_MODE_ALWAYS|| (codecAdaptationWorkaroundMode == ADAPTATION_WORKAROUND_MODE_SAME_RESOLUTION&& newFormat.width == oldFormat.width&& newFormat.height == oldFormat.height);codecInputFormat = newFormat;if (drainAndUpdateCodecDrmSession && !drainAndUpdateCodecDrmSessionV23()) {overridingDiscardReasons |= DISCARD_REASON_WORKAROUND;}}break;case REUSE_RESULT_YES_WITHOUT_RECONFIGURATION://直接重用解码器，无需注册任何信息if (!updateCodecOperatingRate(newFormat)) {overridingDiscardReasons |= DISCARD_REASON_OPERATING_RATE_CHANGED;} else {codecInputFormat = newFormat;if (drainAndUpdateCodecDrmSession && !drainAndUpdateCodecDrmSessionV23()) {overridingDiscardReasons |= DISCARD_REASON_WORKAROUND;}}break;default:throw new IllegalStateException(); // Never happens.}if (evaluation.result != REUSE_RESULT_NO&& (this.codec != codec || codecDrainAction == DRAIN_ACTION_REINITIALIZE)) {// Initial evaluation indicated reuse was possible, but codec re-initialization was triggered.// The reasons are indicated by overridingDiscardReasons.return new DecoderReuseEvaluation(codecInfo.name, oldFormat, newFormat, REUSE_RESULT_NO, overridingDiscardReasons);}return evaluation;}
但是如何验证切换完成了，实际上是有回调的，参考下面接口实现。
com.google.android.exoplayer2.audio.AudioRendererEventListener#onAudioInputFormatChanged(com.google.android.exoplayer2.Format)com.google.android.exoplayer2.video.VideoRendererEventListener#onVideoInputFormatChanged(com.google.android.exoplayer2.Format)
四、实验
4.2 实验方法：
自动以AdaptiveTrackSelection#Factory或者自定义BandwidthMeter，这里我们选择后者，因为改动较小。
4.2.1实现QmBandwidthMeter
private long bitrateEstimate;private long specificBitrate = C.TIME_UNSET;public synchronized long getBitrateEstimate() {//如果用户没有指定比特率，则使用默认的，如果指定了，则使用用户的设备if(specificBitrate == C.RATE_UNSET_INT) {return bitrateEstimate;}return specificBitrate;}
4.2.2 设置到Builder中
//创建带宽测量类bandwidthMeter = new QmBandwidthMeter.Builder(getApplicationContext()).build();//缓冲控制类DefaultLoadControl loadControl = new DefaultLoadControl.Builder().setAllocator(new DefaultAllocator(true,C.DEFAULT_BUFFER_SEGMENT_SIZE / 2)).setBufferDurationsMs(10_000, //低于20秒的缓冲就开始加载数据，不一定会buffer20_000, //一次加载最多加载20秒的数据DefaultLoadControl.DEFAULT_BUFFER_FOR_PLAYBACK_MS,DefaultLoadControl.DEFAULT_BUFFER_FOR_PLAYBACK_AFTER_REBUFFER_MS).build();ExoPlayer.Builder playerBuilder =new ExoPlayer.Builder(/* context= */ this).setLoadControl(loadControl).setBandwidthMeter(bandwidthMeter).setMediaSourceFactory(createMediaSourceFactory());setRenderersFactory(playerBuilder, intent.getBooleanExtra(IntentUtil.PREFER_EXTENSION_DECODERS_EXTRA, false));player = playerBuilder.build();
注意：这里设置缓冲控制类的前文已经说过， 分片选择存在坑点，由于bufferedDurationUs值过大，可能造成降码流失效， AdaptiveTrackSelection可能会重置会原来的selectionIndex为上一个分片Track，导致降码流失败，有必要做以下兼容。
4.2.3 以切换为下面的Hls分片为例子，实现从1920x1080 -> 640x360的切换
链接地址：https://devstreaming-cdn.apple.com/videos/streaming/examples/bipbop_4x3/bipbop_4x3_variant.m3u8
EXT-X-STREAM-INF:BANDWIDTH=1924009,CODECS="mp4a.40.2, avc1.4d401f",RESOLUTION=1920x1080,AUDIO="bipbop_audio",SUBTITLES="subs"gear5/prog_index.m3u8EXT-X-STREAM-INF:BANDWIDTH=577610,CODECS="mp4a.40.2, avc1.4d401e",RESOLUTION=640x360,AUDIO="bipbop_audio",SUBTITLES="subs"gear2/prog_index.m3u8
在起播后5s后设置带宽
【1】起播时设置带宽1924009/0.7f
【2】起播10s后设置带宽577610/0.7f
//起播时bandwidthMeter.setSpecificBitrate((long) Math.ceil(1924009/0.7f));//播放一段时间后bandwidthMeter.setSpecificBitrate((long) Math.ceil(577610/0.7f));
注意: 这里除以0.7f上面也说了，是因为带宽冗余机制造成的。
验证方法，实现下面的回调
com.google.android.exoplayer2.audio.AudioRendererEventListener#onAudioInputFormatChanged(com.google.android.exoplayer2.Format)com.google.android.exoplayer2.video.VideoRendererEventListener#onVideoInputFormatChanged(com.google.android.exoplayer2.Format)
符合预期，成功实现了降码流
五、总结
ExoPlayer不仅支持多路流合并方式切换，也支持自适应流切换，具备高度可定制化的能力，因此，对于体验要求较高的场景，可完全通过修改自适应流相关接口实现更加顺滑的多路流切换。
ExoPlayer自适应流切换如果要改造的为用户所能选择的方式，需要修改BandwidthMeter和AdaptiveTrackSelection的一些参数。还有就是分片长度、分片IDR帧对齐、编码格式一致（尽可能解码器复用）是需要考虑的因素，另外分片可能切换等待时间较长，也是需要注意的问题。