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。

二、基础知识点

前言的内容对于初学ExoPlayer开发者而言还是有些抽象，下面我们梳理一下ExoPlayer的关键类，方便理解本篇内容。

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。

三、自适应流切换分析

3.1 原理图

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。

通过原理图我们可以了解到以下信息：

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。

3.2 核心逻辑

核心逻辑主要为：

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 自适应流清单文件解析

ExoPlayer中支持DASH、HLS、Smoothing-Stream协议，我们这里以HLS和DASH协议进行流程分析，毕竟目前使用Smoothing-Stream也就微软自己为主。接下来先看看HLS和DASH的清单文件，方便我们后续测试。

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=YES#EXT-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.m3u8#EXT-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.m3u8#EXT-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.m3u8#EXT-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.m3u8#EXT-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.m3u8#EXT-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

<?xml version="1.0" encoding="UTF-8"?><!--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。

解析的流程主要如下

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。

3.2.2 Renderer与TrackGroup、Selection选择

在ExoPlayer中，DefaultTrackSelector主要负责这项工作，核心逻辑如下

  @Override  protected final Pair<@NullableType RendererConfiguration[], @NullableType ExoTrackSelection[]>      selectTracks(          MappedTrackInfo mappedTrackInfo,          @Capabilities int[][][] rendererFormatSupports,          @AdaptiveSupport 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.@NullableType 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++) {      @C.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    @NullableType    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    @NullableType    RendererConfiguration[] rendererConfigurations = new RendererConfiguration[rendererCount];    for (int i = 0; i < rendererCount; i++) {      @C.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);  }

这部分逻辑相对很复杂，但是我们com.google.android.exoplayer2.trackselection.AdaptiveTrackSelection.Factory#createTrackSelections要做必要的分析。

在ExoPlayer中默认使用改工厂适配Selection，具体逻辑如下

   @Override    public final @NullableType ExoTrackSelection[] createTrackSelections(        @NullableType 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++) {        @Nullable 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AdaptiveTrackSeletion        selections[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;    @Nullable 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);  }

3.2.4 网速检测和AdaptiveTrackSelection分片选择

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

 @Override  public 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 selection    if (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format    int 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Track         newSelectedIndex = 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Format        if (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0.7f    long 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

由于每种采样队列的分片Format有一些差别，可能需要解码器检测到格式变化。这个时候解码器可能需要重启，当然重启是多路流切换的最基本的做法，但是这种往往会出现卡顿或者短暂的黑屏，体验反而比较差。ExoPlayer对于无论是MergingMediaSource方式的多路流切换还是自适应流的切换导致onInputFormatChanged被调用做了相当多的优化，从而实现解码器的重复利用。

核心逻辑如下：

<?xml version="1.0" encoding="UTF-8"?><!--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>0

3.2.6 完成切换

通过上述必要的逻辑，就能实现分片的切换，当然，每个部分代码量实在太多包括，资源加载部分也是一个核心环节，这里就不在继续分析了。

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。

<?xml version="1.0" encoding="UTF-8"?><!--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>1

四、实验

4.1 实验目的：

实现手动切换分片

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

<?xml version="1.0" encoding="UTF-8"?><!--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>2

4.2.2 设置到Builder中

<?xml version="1.0" encoding="UTF-8"?><!--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>3

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

<?xml version="1.0" encoding="UTF-8"?><!--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>4

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

<?xml version="1.0" encoding="UTF-8"?><!--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>5

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

<?xml version="1.0" encoding="UTF-8"?><!--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>1