Stagefright框架解读（—）音视频Playback流程

转载请注明出处：http://blog.csdn.net/itachi85/article/details/7216639

从Android 2.0，Google引进了Stagefright，并在android2.3时用Stagefright在Android中是以shared library的形式存在(libstagefright.so)，其中AwesomePlayer可用來播放video/audio。AwesomePlayer提供許多API，可以让上层的应用用程式(Java/JNI)來呼叫，我在这里简单说明一下video playback的流程（采用的是android2.2的源码）。

在Java中，若要播放一个影片，我們通常会这样写：

MediaPlayer mp = new MediaPlayer();mp.setDataSource(PATH_TO_FILE);mp.prepare()；mp.start();

在Stagefright中，会看到如下的处理：

1.将影片文件的绝对路径指定给uri:

status_t AwesomePlayer::setDataSource(        const char *uri, const KeyedVector<String8, String8> *headers) {    Mutex::Autolock autoLock(mLock);    return setDataSource_l(uri, headers);}status_t AwesomePlayer::setDataSource_l(        const char *uri, const KeyedVector<String8, String8> *headers) {    reset_l();    mUri = uri;    if (headers) {        mUriHeaders = *headers;    }    // The actual work will be done during preparation in the call to    // ::finishSetDataSource_l to avoid blocking the calling thread in    // setDataSource for any significant time.    return OK;}

2.启动mQueue:

status_t AwesomePlayer::prepare() {    Mutex::Autolock autoLock(mLock);    return prepare_l();}status_t AwesomePlayer::prepare_l() {    if (mFlags & PREPARED) {        return OK;    }    if (mFlags & PREPARING) {        return UNKNOWN_ERROR;    }    mIsAsyncPrepare = false;    status_t err = prepareAsync_l();    if (err != OK) {        return err;    }    while (mFlags & PREPARING) {        mPreparedCondition.wait(mLock);    }    return mPrepareResult;}status_t AwesomePlayer::prepareAsync() {    Mutex::Autolock autoLock(mLock);    if (mFlags & PREPARING) {        return UNKNOWN_ERROR;  // async prepare already pending    }    mIsAsyncPrepare = true;    return prepareAsync_l();}status_t AwesomePlayer::prepareAsync_l() {    if (mFlags & PREPARING) {        return UNKNOWN_ERROR;  // async prepare already pending    }    if (!mQueueStarted) {        mQueue.start();        mQueueStarted = true;    }    mFlags |= PREPARING;    mAsyncPrepareEvent = new AwesomeEvent(            this, &AwesomePlayer::onPrepareAsyncEvent);    mQueue.postEvent(mAsyncPrepareEvent);    return OK;}

3.onprepareAsyncEvent被触发，根据传来文件的header来创建相应的解析器，并初始化音视频解码器：

void AwesomePlayer::onPrepareAsyncEvent() {    sp<Prefetcher> prefetcher;    {        Mutex::Autolock autoLock(mLock);        if (mFlags & PREPARE_CANCELLED) {            LOGI("prepare was cancelled before doing anything");            abortPrepare(UNKNOWN_ERROR);            return;        }        if (mUri.size() > 0) {           //在这个方法中创建解析器          <strong>status_t err = finishSetDataSource_l();</strong>            if (err != OK) {                abortPrepare(err);                return;            }        }        if (mVideoTrack != NULL && mVideoSource == NULL) {            //初始化视频解码器           <strong>status_t err = initVideoDecoder();</strong>            if (err != OK) {                abortPrepare(err);                return;            }        }        if (mAudioTrack != NULL && mAudioSource == NULL) {            //初始化音频解码器           <strong>status_t err = initAudioDecoder();</strong>            if (err != OK) {                abortPrepare(err);                return;            }        }        prefetcher = mPrefetcher;    } if (prefetcher != NULL) {        {            Mutex::Autolock autoLock(mLock);            if (mFlags & PREPARE_CANCELLED) {                LOGI("prepare was cancelled before preparing the prefetcher");                prefetcher.clear();                abortPrepare(UNKNOWN_ERROR);                return;            }        }        LOGI("calling prefetcher->prepare()");        status_t result =            prefetcher->prepare(&AwesomePlayer::ContinuePreparation, this);        prefetcher.clear();        if (result == OK) {            LOGI("prefetcher is done preparing");        } else {            Mutex::Autolock autoLock(mLock);            CHECK_EQ(result, -EINTR);            LOGI("prefetcher->prepare() was cancelled early.");            abortPrepare(UNKNOWN_ERROR);            return;        }    } Mutex::Autolock autoLock(mLock);    if (mIsAsyncPrepare) {        if (mVideoWidth < 0 || mVideoHeight < 0) {            notifyListener_l(MEDIA_SET_VIDEO_SIZE, 0, 0);        } else {            notifyListener_l(MEDIA_SET_VIDEO_SIZE, mVideoWidth, mVideoHeight);        }        notifyListener_l(MEDIA_PREPARED);    }    mPrepareResult = OK;    mFlags &= ~(PREPARING|PREPARE_CANCELLED);    mFlags |= PREPARED;    mAsyncPrepareEvent = NULL;    mPreparedCondition.broadcast();    postBufferingEvent_l();}

status_t AwesomePlayer::finishSetDataSource_l() {    sp<DataSource> dataSource;    if (!strncasecmp("http://", mUri.string(), 7)) {        mConnectingDataSource = new HTTPDataSource(mUri, &mUriHeaders);        mLock.unlock();        status_t err = mConnectingDataSource->connect();        mLock.lock();        if (err != OK) {            mConnectingDataSource.clear();            LOGI("mConnectingDataSource->connect() returned %d", err);            return err;        }        dataSource = new CachingDataSource(                mConnectingDataSource, 64 * 1024, 10);        mConnectingDataSource.clear();    } else {        dataSource = DataSource::CreateFromURI(mUri.string(), &mUriHeaders);    }    if (dataSource == NULL) {        return UNKNOWN_ERROR;    }    <strong>sp<MediaExtractor> extractor = MediaExtractor::Create(dataSource);</strong>    if (extractor == NULL) {        return UNKNOWN_ERROR;    }    dataSource->getDrmInfo(&mDecryptHandle, &mDrmManagerClient);    if (mDecryptHandle != NULL            && RightsStatus::RIGHTS_VALID != mDecryptHandle->status) {        notifyListener_l(MEDIA_ERROR, MEDIA_ERROR_UNKNOWN, ERROR_NO_LICENSE);    }    if (dataSource->flags() & DataSource::kWantsPrefetching) {        mPrefetcher = new Prefetcher;    }   <strong> return setDataSource_l(extractor)</strong>;}

4.使用extractor对文件进行A/V分离：

status_t AwesomePlayer::setDataSource_l(const sp<MediaExtractor> &extractor) {    bool haveAudio = false;    bool haveVideo = false;    for (size_t i = 0; i < extractor->countTracks(); ++i) {        sp<MetaData> meta = extractor->getTrackMetaData(i);        const char *mime;        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;        }        if (haveAudio && haveVideo) {            break;        }    }    if (!haveAudio && !haveVideo) {        return UNKNOWN_ERROR;    }    mExtractorFlags = extractor->flags();    return OK;}

5.将解析后的音视频数据分别交给VideoTrack和AudioTrack:

void AwesomePlayer::setVideoSource(sp<MediaSource> source) {    CHECK(source != NULL);    if (mPrefetcher != NULL) {        source = mPrefetcher->addSource(source);    }    mVideoTrack = source;}void AwesomePlayer::setAudioSource(sp<MediaSource> source) { CHECK(source != NULL); if (mPrefetcher != NULL) { source = mPrefetcher->addSource(source); } mAudioTrack = source;}

6.根据mVideoTrck中的编码类型来选择 video decoder 同理根据mAudioTrack中的编码类型来选择 audio decoder:

status_t AwesomePlayer::initVideoDecoder() {    mVideoSource = OMXCodec::Create(            mClient.interface(), mVideoTrack->getFormat(),            false, // createEncoder            mVideoTrack);    if (mVideoSource != NULL) {        int64_t durationUs;        if (mVideoTrack->getFormat()->findInt64(kKeyDuration, &durationUs)) {            Mutex::Autolock autoLock(mMiscStateLock);            if (mDurationUs < 0 || durationUs > mDurationUs) {                mDurationUs = durationUs;            }        }        CHECK(mVideoTrack->getFormat()->findInt32(kKeyWidth, &mVideoWidth));        CHECK(mVideoTrack->getFormat()->findInt32(kKeyHeight, &mVideoHeight));        status_t err = mVideoSource->start();        if (err != OK) {            mVideoSource.clear();            return err;        }    }    return mVideoSource != NULL ? OK : UNKNOWN_ERROR;}

7.将mVideoEvent放入mQueue中，开始解码播放，并交由mvideoRenderer来画出 audio的数据则交由audioplayer来管理，它最终将解码的数据交给audioTrack并由audioTrack和audioFlinger进行交互，最终将数据交给audio hal层，这个我们以后会做讲解：

status_t AwesomePlayer::play() {    Mutex::Autolock autoLock(mLock);    return play_l();}status_t AwesomePlayer::play_l() {    if (mFlags & PLAYING) {        return OK;    }    if (!(mFlags & PREPARED)) {        status_t err = prepare_l();        if (err != OK) {            return err;        }    }    mFlags |= PLAYING;    mFlags |= FIRST_FRAME;    bool deferredAudioSeek = false;    if (mAudioSource != NULL) {        if (mAudioPlayer == NULL) {            if (mAudioSink != NULL) {                //音频数据由audioplayer进行管理               mAudioPlayer = new AudioPlayer(mAudioSink);                mAudioPlayer->setSource(mAudioSource);                // We've already started the MediaSource in order to enable                // the prefetcher to read its data.                  //调用audioPlayer的start方法则是调用audioSource对数据进行解码                   //并将解码似得数据最终交给audioTrack,并调用audioTrack的start方法与audioFlinger进行交互                  status_t err = mAudioPlayer->start(                        true /* sourceAlreadyStarted */);                if (err != OK) {                    delete mAudioPlayer;                    mAudioPlayer = NULL;                    mFlags &= ~(PLAYING | FIRST_FRAME);                    return err;                }                delete mTimeSource;                mTimeSource = mAudioPlayer;                deferredAudioSeek = true;                mWatchForAudioSeekComplete = false;                mWatchForAudioEOS = true;            }        } else {            mAudioPlayer->resume();        }        postCheckAudioStatusEvent_l();    }    if (mTimeSource == NULL && mAudioPlayer == NULL) {        mTimeSource = new SystemTimeSource;    }    if (mVideoSource != NULL) {        // Kick off video playback        //将mVideoEvent放入queue中         postVideoEvent_l();    }    if (deferredAudioSeek) {        // If there was a seek request while we were paused        // and we're just starting up again, honor the request now.        seekAudioIfNecessary_l();    }    if (mFlags & AT_EOS) {        // Legacy behaviour, if a stream finishes playing and then        // is started again, we play from the start...        seekTo_l(0);    }    if (mDecryptHandle != NULL) {        int64_t position;        getPosition(&position);        mDrmManagerClient->setPlaybackStatus(mDecryptHandle,                Playback::START, position / 1000);    }    return OK;}void AwesomePlayer::postVideoEvent_l(int64_t delayUs) { if (mVideoEventPending) { return; } mVideoEventPending = true; mQueue.postEventWithDelay(mVideoEvent, delayUs < 0 ? 10000 : delayUs);}

void AwesomePlayer::onVideoEvent(){  mVideoSource->read(&mVideoBuffer, &options);  mVideoRenderer->render(mVideoBuffer);  postVideoEvent_l();}

更多相关文章

随机推荐