ExoPlayer之SampleQueue

SampleQueue的作用

代码中关于SampleQueue的注释说明 - A queue of media samples.
SampleQueue对应于媒体文件数据采样的处理：
ProgressiveMediaPeriod在startLoading方法中会通过Loader对象在后台线程执行内部类ExtractingLoadable.load方法开始加载media data。 然后交由extractor.read()处理，最终由trackOutput(SampleQueue)真正开始从ExtractorInput中采样。采样的media data会保存到SampleDataQueue中，SampleDataQueue相当于是一个容器。

接口说明

com.google.android.exoplayer2.source.SampleQueue:

public class SampleQueue implements TrackOutput {
   
...
  /**
   * Creates a sample queue.
   *
   * @param allocator An {@link Allocator} from which allocations for sample data can be obtained.
   * @param playbackLooper The looper associated with the media playback thread.
   * @param drmSessionManager The {@link DrmSessionManager} to obtain {@link DrmSession DrmSessions}
   *     from. The created instance does not take ownership of this {@link DrmSessionManager}.
   */
  public SampleQueue(Allocator allocator, Looper playbackLooper, DrmSessionManager<?> drmSessionManager) {
   
    sampleDataQueue = new SampleDataQueue(allocator);
    this.playbackLooper = playbackLooper;
    this.drmSessionManager = drmSessionManager;
    extrasHolder = new SampleExtrasHolder();
    capacity = SAMPLE_CAPACITY_INCREMENT;
    sourceIds = new int[capacity];
    offsets = new long[capacity];
    timesUs = new long[capacity];
    flags = new int[capacity];
    sizes = new int[capacity];
    cryptoDatas = new CryptoData[capacity];
    formats = new Format[capacity];
    largestDiscardedTimestampUs = Long.MIN_VALUE;
    largestQueuedTimestampUs = Long.MIN_VALUE;
    upstreamFormatRequired = true;
    upstreamKeyframeRequired = true;
  }

  ...

  /**
   * Returns whether there is data available for reading.
   *
   * <p>Note: If the stream has ended then a buffer with the end of stream flag can always be read
   * from {@link #read}. Hence an ended stream is always ready.
   *
   * @param loadingFinished Whether no more samples will be written to the sample queue. When true,
   *     this method returns true if the sample queue is empty, because an empty sample queue means
   *     the end of stream has been reached. When false, this method returns false if the sample
   *     queue is empty.
   */
  @SuppressWarnings("ReferenceEquality") // See comments in setUpstreamFormat
  @CallSuper
  public synchronized boolean isReady(boolean loadingFinished) {
   
    if (!hasNextSample()) {
   
      return loadingFinished
          || isLastSampleQueued
          || (upstreamFormat != null && upstreamFormat != downstreamFormat);
    }
    int relativeReadIndex = getRelativeIndex(readPosition);
    if (formats[relativeReadIndex] != downstreamFormat) {
   
      // A format can be read.
      return true;
    }
    return mayReadSample(relativeReadIndex);
  }

  /**
   * Attempts to read from the queue.
   *
   * <p>{@link Format Formats} read from this method may be associated to a {@link DrmSession}
   * through {@link FormatHolder#drmSession}, which is populated in two scenarios:
   *
   * <ul>
   *   <li>The {@link Format} has a non-null {@link Format#drmInitData}.
   *   <li>The {@link DrmSessionManager} provides placeholder sessions for this queue's track type.
   *       See {@link DrmSessionManager#acquirePlaceholderSession(Looper, int)}.
   * </ul>
   *
   * @param formatHolder A {@link FormatHolder} to populate in the case of reading a format.
   * @param buffer A {@link DecoderInputBuffer} to populate in the case of reading a sample or the
   *     end of the stream. If the end of the stream has been reached, the {@link
   *     C#BUFFER_FLAG_END_OF_STREAM} flag will be set on the buffer. If a {@link
   *     DecoderInputBuffer#isFlagsOnly() flags-only} buffer is passed, only the buffer flags may be
   *     populated by this method and the read position of the queue will not change.
   * @param formatRequired Whether the caller requires that the format of the stream be read even if
   *     it's not changing. A sample will never be read if set to true, however it is still possible
   *     for the end of stream or nothing to be read.
   * @param loadingFinished True if an empty queue should be considered the end of the stream.
   * @param decodeOnlyUntilUs If a buffer is read, the {@link C#BUFFER_FLAG_DECODE_ONLY} flag will
   *     be set if the buffer's timestamp is less than this value.
   * @return The result, which can be {@link C#RESULT_NOTHING_READ}, {@link C#RESULT_FORMAT_READ} or
   *     {@link C#RESULT_BUFFER_READ}.
   */
  @CallSuper
  public int read(
      FormatHolder formatHolder,
      DecoderInputBuffer buffer,
      boolean formatRequired,
      boolean loadingFinished,
      long decodeOnlyUntilUs) {
   
    int result =
        readSampleMetadata(
            formatHolder, buffer, formatRequired, loadingFinished, decodeOnlyUntilUs, extrasHolder);
    if (result == C.RESULT_BUFFER_READ && !buffer.isEndOfStream() && !buffer.isFlagsOnly()) {
   
      sampleDataQueue.readToBuffer(buffer
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