ExoPlayer架构详解与源码分析（14）——ProgressiveMediaPeriod

系列文章目录

ExoPlayer架构详解与源码分析（1）——前言
ExoPlayer架构详解与源码分析（2）——Player
ExoPlayer架构详解与源码分析（3）——Timeline
ExoPlayer架构详解与源码分析（4）——整体架构
ExoPlayer架构详解与源码分析（5）——MediaSource
ExoPlayer架构详解与源码分析（6）——MediaPeriod
ExoPlayer架构详解与源码分析（7）——SampleQueue
ExoPlayer架构详解与源码分析（8）——Loader
ExoPlayer架构详解与源码分析（9）——TsExtractor
ExoPlayer架构详解与源码分析（10）——H264Reader
ExoPlayer架构详解与源码分析（11）——DataSource
ExoPlayer架构详解与源码分析（12）——Cache
ExoPlayer架构详解与源码分析（13）——TeeDataSource和CacheDataSource
ExoPlayer架构详解与源码分析（14）——ProgressiveMediaPeriod

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前言

中途间隔了一段时间，之前写了那么多铺垫，终于看到ProgressiveMediaPeriod实现部分了

ProgressiveMediaPeriod

有了之前的那些铺垫，这里直接看源码了

  @Overridepublic void prepare(Callback callback, long positionUs) {this.callback = callback;loadCondition.open();//保证继续加载的开关打开，Loader不阻塞startLoading();}private void startLoading() {ExtractingLoadable loadable =//创建loadable 共Loader加载new ExtractingLoadable(//extractorOutput监听，也就是Loader加载过程中会回调ProgressiveMediaPeriod的track，endTracksuri, dataSource, progressiveMediaExtractor, /* extractorOutput= */ this, loadCondition);if (prepared) {//如果已经准备完成Assertions.checkState(isPendingReset());if (durationUs != C.TIME_UNSET && pendingResetPositionUs > durationUs) {//当前定位位置已经超过总时长，直接加载结束loadingFinished = true;pendingResetPositionUs = C.TIME_UNSET;return;}//通过seekMap查找出当前时间对于的数据位置loadable.setLoadPosition(checkNotNull(seekMap).getSeekPoints(pendingResetPositionUs).first.position,pendingResetPositionUs);for (SampleQueue sampleQueue : sampleQueues) {//将所有的轨道开始时间同步sampleQueue.setStartTimeUs(pendingResetPositionUs);}pendingResetPositionUs = C.TIME_UNSET;}//获取开始加载时所有轨道已经提前的数据块总数，后面通过当前的数据块总数和开始的数量对比，可以判断出是否加载了新的数据extractedSamplesCountAtStartOfLoad = getExtractedSamplesCount();long elapsedRealtimeMs =loader.startLoading(//Loader开始加载，具体过程参照Loader部分的文章，此时加载状态的回调this，也就是加载完成后会调用ProgressiveMediaPeriod  onLoadCompleted，seekMaploadable, this, loadErrorHandlingPolicy.getMinimumLoadableRetryCount(dataType));DataSpec dataSpec = loadable.dataSpec;//触发监听mediaSourceEventDispatcher.loadStarted(new LoadEventInfo(loadable.loadTaskId, dataSpec, elapsedRealtimeMs),C.DATA_TYPE_MEDIA,C.TRACK_TYPE_UNKNOWN,/* trackFormat= */ null,C.SELECTION_REASON_UNKNOWN,/* trackSelectionData= */ null,/* mediaStartTimeUs= */ loadable.seekTimeUs,durationUs);}@Override//Loader加载时如果解析器需要输出Sample数据会先回调track，获取TrackOutputpublic TrackOutput track(int id, int type) {return prepareTrackOutput(new TrackId(id, /* isIcyTrack= */ false));}//构建TrackOutputprivate TrackOutput prepareTrackOutput(TrackId id) {int trackCount = sampleQueues.length;for (int i = 0; i < trackCount; i++) {//查询当前的sampleQueue是否已创建直接返回if (id.equals(sampleQueueTrackIds[i])) {return sampleQueues[i];}}SampleQueue trackOutput =//创建新的SampleQueue对应一个新的轨道，这里传入了缓存分配器allocatorSampleQueue.createWithDrm(allocator, drmSessionManager, drmEventDispatcher);trackOutput.setUpstreamFormatChangeListener(this);//设置Format改变的监听@NullableTypeTrackId[] sampleQueueTrackIds = Arrays.copyOf(this.sampleQueueTrackIds, trackCount + 1);sampleQueueTrackIds[trackCount] = id;this.sampleQueueTrackIds = Util.castNonNullTypeArray(sampleQueueTrackIds);//更新全局的SampleQueue数组@NullableType SampleQueue[] sampleQueues = Arrays.copyOf(this.sampleQueues, trackCount + 1);sampleQueues[trackCount] = trackOutput;this.sampleQueues = Util.castNonNullTypeArray(sampleQueues);return trackOutput;}@Override//当解析器已经将所有轨道解析出来时会调用此方法，参照H264Reader部分public void endTracks() {sampleQueuesBuilt = true;//当前方法在解析的子线程中回调，需要将方法方法当前ProgressiveMediaPeriod的线程中执行maybeFinishPreparehandler.post(maybeFinishPrepareRunnable);}private void maybeFinishPrepare() {if (released || prepared || !sampleQueuesBuilt || seekMap == null) {return;}//确保所有轨道均已解析for (SampleQueue sampleQueue : sampleQueues) {if (sampleQueue.getUpstreamFormat() == null) {return;}}//阻塞住loader的解析，防止继续更新sampleQueuesloadCondition.close();//创建TrackGroup，trackState 供后续的selectTracks使用int trackCount = sampleQueues.length;TrackGroup[] trackArray = new TrackGroup[trackCount];boolean[] trackIsAudioVideoFlags = new boolean[trackCount];for (int i = 0; i < trackCount; i++) {Format trackFormat = checkNotNull(sampleQueues[i].getUpstreamFormat());@Nullable String mimeType = trackFormat.sampleMimeType;boolean isAudio = MimeTypes.isAudio(mimeType);boolean isAudioVideo = isAudio || MimeTypes.isVideo(mimeType);trackIsAudioVideoFlags[i] = isAudioVideo;haveAudioVideoTracks |= isAudioVideo;...trackFormat = trackFormat.copyWithCryptoType(drmSessionManager.getCryptoType(trackFormat));trackArray[i] = new TrackGroup(/* id= */ Integer.toString(i), trackFormat);}trackState = new TrackState(new TrackGroupArray(trackArray), trackIsAudioVideoFlags);prepared = true;//标记准备完成checkNotNull(callback).onPrepared(this);//通知上层prepared ，上层接下就会去调用ProgressiveMediaPeriod.selectTracks获取轨道信息}@Override//获取轨道public long selectTracks(@NullableType ExoTrackSelection[] selections,//TrackSelector 部分会说到boolean[] mayRetainStreamFlags,@NullableType SampleStream[] streams,//Renderer部分会说的boolean[] streamResetFlags,long positionUs) {assertPrepared();//确保已经准备完成TrackGroupArray tracks = trackState.tracks;boolean[] trackEnabledStates = trackState.trackEnabledStates;int oldEnabledTrackCount = enabledTrackCount;// 去除原来mayRetainStreamFlags标记的无需保留的轨道for (int i = 0; i < selections.length; i++) {if (streams[i] != null && (selections[i] == null || !mayRetainStreamFlags[i])) {int track = ((SampleStreamImpl) streams[i]).track;Assertions.checkState(trackEnabledStates[track]);enabledTrackCount--;trackEnabledStates[track] = false;streams[i] = null;}}//如果是第一次selectTracks，而且positionUs 位置又不为0，或者之前一次selectTracks禁用了所有的轨道，这2种情况就需要Seekboolean seekRequired = seenFirstTrackSelection ? oldEnabledTrackCount == 0 : positionUs != 0;// 开始通过selections选择新的轨道for (int i = 0; i < selections.length; i++) {if (streams[i] == null && selections[i] != null) {ExoTrackSelection selection = selections[i];Assertions.checkState(selection.length() == 1);Assertions.checkState(selection.getIndexInTrackGroup(0) == 0);int track = tracks.indexOf(selection.getTrackGroup());Assertions.checkState(!trackEnabledStates[track]);enabledTrackCount++;trackEnabledStates[track] = true;streams[i] = new SampleStreamImpl(track);//向入参中赋值streamResetFlags[i] = true;if (!seekRequired) {SampleQueue sampleQueue = sampleQueues[track];seekRequired =!sampleQueue.seekTo(positionUs, /* allowTimeBeyondBuffer= */ true)&& sampleQueue.getReadIndex() != 0;}}}if (enabledTrackCount == 0) {pendingDeferredRetry = false;notifyDiscontinuity = false;if (loader.isLoading()) {// Discard as much as we can synchronously.for (SampleQueue sampleQueue : sampleQueues) {sampleQueue.discardToEnd();}loader.cancelLoading();} else {for (SampleQueue sampleQueue : sampleQueues) {sampleQueue.reset();}}} else if (seekRequired) {positionUs = seekToUs(positionUs);// We'll need to reset renderers consuming from all streams due to the seek.for (int i = 0; i < streams.length; i++) {if (streams[i] != null) {streamResetFlags[i] = true;}}}seenFirstTrackSelection = true;return positionUs;}@Override//解析器解析出SeekMap时回调public void seekMap(SeekMap seekMap) {handler.post(() -> setSeekMap(seekMap));}@Overridepublic boolean continueLoading(long playbackPositionUs) {if (loadingFinished//加载完成，出错等情况返回false|| loader.hasFatalError()|| pendingDeferredRetry|| (prepared && enabledTrackCount == 0)) {return false;}boolean continuedLoading = loadCondition.open();if (!loader.isLoading()) {//当前没有正在加载startLoading();//再次startLoadingcontinuedLoading = true;}return continuedLoading;}@Override//Loader加载完成后回调public void onLoadCompleted(ExtractingLoadable loadable, long elapsedRealtimeMs, long loadDurationMs) {if (durationUs == C.TIME_UNSET && seekMap != null) {//此时durationUs 未知，seekMap 已经有了boolean isSeekable = seekMap.isSeekable();long largestQueuedTimestampUs =//查询所有轨道中时间戳最大值getLargestQueuedTimestampUs(/* includeDisabledTracks= */ true);durationUs =//将最大值+10毫秒作为当前媒体的时长largestQueuedTimestampUs == Long.MIN_VALUE? 0: largestQueuedTimestampUs + DEFAULT_LAST_SAMPLE_DURATION_US;//此时durationUs 等信息已知可以通过MediaSource更新一把TimeLinelistener.onSourceInfoRefreshed(durationUs, isSeekable, isLive);}StatsDataSource dataSource = loadable.dataSource;//StatsDataSource可以缓存Uri和ResponseHeader，获取这些值构建LoadEventInfo LoadEventInfo loadEventInfo =new LoadEventInfo(loadable.loadTaskId,loadable.dataSpec,dataSource.getLastOpenedUri(),dataSource.getLastResponseHeaders(),elapsedRealtimeMs,loadDurationMs,dataSource.getBytesRead());loadErrorHandlingPolicy.onLoadTaskConcluded(loadable.loadTaskId);//触发监听mediaSourceEventDispatcher.loadCompleted(loadEventInfo,C.DATA_TYPE_MEDIA,C.TRACK_TYPE_UNKNOWN,/* trackFormat= */ null,C.SELECTION_REASON_UNKNOWN,/* trackSelectionData= */ null,/* mediaStartTimeUs= */ loadable.seekTimeUs,durationUs);loadingFinished = true;//此时的loadingFinished 已为true，没设置为false前是无法continueLoading的//请求上层继续加载，是否继续加载由上层决定，如果上层同意继续加载会调用ProgressiveMediaPeriod的continueLoadingcheckNotNull(callback).onContinueLoadingRequested(this);}

这里再总结下ProgressiveMediaPeriod执行过程：

1. 首先调用prepare方法启动Loader去获取资源的轨道信息，此处参考ExoPlayer架构详解与源码分析（8）——Loader
2. Loader中的解析器获取到信息后回回调endTrack通知ProgressiveMediaPeriod prepare完成
3. ProgressiveMediaPeriod 会进一步通知上层此时MeidaSource已经准备完成，上层再Renderer（四大组件之一）绘制前会通过ExoPlayer的另一大组件TrackSelector（后面会说到），调用ProgressiveMediaPeriod 的selectTracks选择轨道数据，也就将SampleQueue中的数据提供出去，此处参考ExoPlayer架构详解与源码分析（7）——SampleQueue
4. 同时Loader从打开到当前加载数据量超过1M（默认值）时就会阻塞当前的Loader，然后询问上层是否继续加载
5. 上层主要是通过后面要将LoadControl判断，如果上层决定继续加载更多数据，就会调用ProgressiveMediaPeriod continueLoading解开阻塞的锁继续加载。因为我们知道数据是加载到内存中的，如果无限制的加载肯定是不行的，需要有节制的加载和释放数据。而LoadControl就负责这块工作后面会讲到。
6. 而上面过程当需要释放已经播放的内存时就会调用discardBuffer方法释放Sample中的内存
7. 当所有的数据加载完成的时候会调用onLoadCompleted将loadingFinished 标记为true

总结

到这里Exoplayer中最复杂的组件MediaSource就全部分析完毕了，之前说了MediaSource在整个运载火箭中的角色就类似于燃料系统，确保火箭顺利升空，在运行过程中持续稳定的为火箭提供燃料。这些燃料提供给谁了呢，或者说是被谁消耗了呢。这就是下面要讲的火箭的核心发动机Renderers，也是Exoplayer四大组件中另一个重要的角色。

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