Stagefright (1) - Video Playback的流程

在Android上，預設的多媒體框架 (multimedia framework)是OpenCORE。OpenCORE的優點是兼顧了跨平台的移植性，而且已經過多方驗證，所以相當穩定；但是其缺點是過於龐大複 雜，需要耗費相當多的時間去維護。從Android 2.0開始，Google引進了架構稍為簡潔的Stagefright，並且有逐漸取代OpenCORE的趨勢。以下我們就先來看看 Stagefright是如何播放一個影片檔。Stagefright在Android中是以shared library的形式存在(libstagefright.so)，其中的module -- AwesomePlayer可用來播放video/audio。AwesomePlayer提供許多API，可以讓上層的應用程式(Java/JNI)來 呼叫，我們以一個簡單的程式來說明video playback的流程。在Java中，若要播放一個影片檔，我們會這樣寫：MediaPlayer mp = new MediaPlayer();mp.setDataSource(PATH_TO_FILE); ...... (1)mp.prepare(); ........................ (2)、(3)mp.start(); .......................... (4)在Stagefright中，則會看到相對應的處理；(1) 將檔案的絕對路徑指定給mUristatus_t AwesomePlayer::setDataSource(const char* uri, ...){  return setDataSource_l(uri, ...);}status_t AwesomePlayer::setDataSource_l(const char* uri, ...){  mUri = uri;}(2) 啟 動mQueue，作為event handler

status_t AwesomePlayer::prepare(){  return prepare_l();}status_t AwesomePlayer::prepare_l(){  prepareAsync_l();  while (mFlags & PREPARING)  {    mPreparedCondition.wait(mLock);  }}status_t AwesomePlayer::prepareAsync_l(){  mQueue.start();  mFlags |= PREPARING;  mAsyncPrepareEvent = new AwesomeEvent(                             this                             &AwesomePlayer::onPrepareAsyncEvent);  mQueue.postEvent(mAsyncPrepareEvent);}(3) onPrepareAsyncEvent被觸發void AwesomePlayer::onPrepareAsyncEvent(){  finishSetDataSource_l();  initVideoDecoder(); ...... (3.3)  initAudioDecoder();}status_t AwesomePlayer::finishSetDataSource_l(){  dataSource = DataSource::CreateFromURI(mUri.string(), ...);  sp<MediaExtractor> extractor =                     MediaExtractor::Create(dataSource); ..... (3.1)  return setDataSource_l(extractor); ......................... (3.2)}(3.1) 解 析mUri所指定的檔案，並且根據其header來選擇對應的extractorsp<MediaExtractor> MediaExtractor::Create(const sp<DataSource> &source, ...){  source->sniff(&tmp, ...);  mime = tmp.string();  if (!strcasecmp(mime, MEDIA_MIMETYPE_CONTAINER_MPEG4)  {    return new MPEG4Extractor(source);  }  else if (!strcasecmp(mime, MEDIA_MIMETYPE_AUDIO_MPEG))  {    return new MP3Extractor(source);  }  else if (!strcasecmp(mime, MEDIA_MIMETYPE_AUDIO_AMR_NB)  {    return new AMRExtractor(source);  }}(3.2) 使用extractor對檔案做A/V的分離 (mVideoTrack/mAudioTrack)status_t AwesomePlayer::setDataSource_l(const sp<MediaExtractor> &extractor){  for (size_t i = 0; i < extractor->countTracks(); ++i)  {    sp<MetaData> meta = extractor->getTrackMetaData(i);    CHECK(meta->findCString(kKeyMIMEType, &mime));    if (!haveVideo && !strncasecmp(mime, "video/", 6))    {      setVideoSource(extractor->getTrack(i));      haveVideo = true;    }    else if (!haveAudio && !strncasecmp(mime, "audio/", 6))    {      setAudioSource(extractor->getTrack(i));      haveAudio = true;    }  }}void AwesomePlayer::setVideoSource(sp<MediaSource> source){  mVideoTrack = source;}(3.3) 根 據mVideoTrack中的編碼類型來選擇video decoder (mVideoSource)status_t AwesomePlayer::initVideoDecoder(){  mVideoSource = OMXCodec::Create(mClient.interface(),                                  mVideoTrack->getFormat(),                                  false,                                  mVideoTrack);}(4) 將mVideoEvent放入mQueue中，開始解碼播放，並交由mVideoRenderer來畫出status_t AwesomePlayer::play(){  return play_l();}status_t AwesomePlayer::play_l(){  postVideoEvent_l();}void AwesomePlayer::postVideoEvent_l(int64_t delayUs){  mQueue.postEventWithDelay(mVideoEvent, delayUs);}void AwesomePlayer::onVideoEvent(){  mVideoSource->read(&mVideoBuffer, &options);  [Check Timestamp]  mVideoRenderer->render(mVideoBuffer);  postVideoEvent_l();}