ExoPlayer音画同步原理

ExoPlayer和大部分播放器一样，默认采用Audio Master的同步方式，其视频同步原因在MediaCodecVideoRender.java的processOutputBuffer()函数中，下面结合函数详细看下音画同步的原理。

protected boolean processOutputBuffer(
      long positionUs,
      long elapsedRealtimeUs,
      @Nullable MediaCodecAdapter codec,
      @Nullable ByteBuffer buffer,
      int bufferIndex,
      int bufferFlags,
      int sampleCount,
      long bufferPresentationTimeUs,
      boolean isDecodeOnlyBuffer,
      boolean isLastBuffer,
      Format format)
      throws ExoPlaybackException {
    //Video不支持Bypass
    Assertions.checkNotNull(codec); // Can not render video without codec

    if (initialPositionUs == C.TIME_UNSET) {
      initialPositionUs = positionUs;
    }

    if (bufferPresentationTimeUs != lastBufferPresentationTimeUs) {
      frameReleaseHelper.onNextFrame(bufferPresentationTimeUs);
      this.lastBufferPresentationTimeUs = bufferPresentationTimeUs;
    }

    //outputStreamOffsetUs通常为0
    long outputStreamOffsetUs = getOutputStreamOffsetUs();
    long presentationTimeUs = bufferPresentationTimeUs - outputStreamOffsetUs;

    //isDecodeOnlyBuffer直接跳过
    if (isDecodeOnlyBuffer && !isLastBuffer) {
      skipOutputBuffer(codec, bufferIndex, presentationTimeUs);
      return true;
    }

    // Note: Use of double rather