Android 12(S) MultiMedia Learning（七）NuPlayer GenericSource

本节来看一下NuPlayer Source中的GenericSource，GenericSource主要是用来播放本地视频的，接下来着重来看以下5个方法：

prepare，start，pause，seek，dequeueAccessUnit

相关代码位置：

http://aospxref.com/android-12.0.0_r3/xref/frameworks/av/media/libmediaplayerservice/nuplayer/NuPlayer.cpp

http://aospxref.com/android-12.0.0_r3/xref/frameworks/av/media/libmediaplayerservice/nuplayer/GenericSource.cpp

http://aospxref.com/android-12.0.0_r3/xref/frameworks/av/media/libdatasource/DataSourceFactory.cpp

a. prepare

prepare的过程中做了以下几件事情（这边的代码比较简单，顺着看就行所以就不贴代码了）：

1. 根据setDataSource过程中传进来的uri来创建DataSource，由于GenericSource一般用来播放本地视频，所以会创建一个FileSource（这里的dataSource实现了最基本的读写文件的接口）

2. 利用创建的DataSource来读取文件，使用media.extractor服务来选择并创建一个合适的MediaExtractor（media.extractor服务后面可能会来记录一下它的工作原理）

3. 利用MediaExtractor来获取文件的metadata，以及各个track的metadata（后面用于创建以及初始化decoder），调用getTrack方法从MediaExtractor中获取IMediaSource，audio和video track均拥有自己的IMediaSource，IMediaSource实现了demux功能

4. 为音频和视频分别创建一个AnotherPacketSource作为数据容器，与IMediaSource一起封装成为Track对象，之后的函数调用就是操作音频和视频的Track

status_t NuPlayer::GenericSource::initFromDataSource() {
    sp<IMediaExtractor> extractor;
    // ......
    // 创建MediaExtractor
    extractor = MediaExtractorFactory::Create(dataSource, NULL);

    // 获取文件的metadata
    sp<MetaData> fileMeta = extractor->getMetaData();

    // 获取track数量
    size_t numtracks = extractor->countTracks();

    // ......
    // 获取文件的时长
    if (mFileMeta != NULL) {
        int64_t duration;
        if (mFileMeta->findInt64(kKeyDuration, &duration)) {
            mDurationUs = duration;
        }
    }

    for (size_t i = 0; i < numtracks; ++i) {
        // 获取MediaSource
        sp<IMediaSource> track = extractor->getTrack(i);
        if (track == NULL) {
            continue;
        }

        sp<MetaData> meta = extractor->getTrackMetaData(i);
        if (meta == NULL) {
            ALOGE("no metadata for track %zu", i);
            return UNKNOWN_ERROR;
        }

        const char *mime;
        CHECK(meta->findCString(kKeyMIMEType, &mime));

        // 构建Track
        if (!strncasecmp(mime, "audio/", 6)) {
            if (mAudioTrack.mSource == NULL) {
                mAudioTrack.mIndex = i;
                mAudioTrack.mSource = track;
                // 为track构建数据容器AnotherPacketSource
                mAudioTrack.mPackets =
                    new AnotherPacketSource(mAudioTrack.mSource->getFormat());

                if (!strcasecmp(mime, MEDIA_MIMETYPE_AUDIO_VORBIS)) {
                    mAudioIsVorbis = true;
                } else {
                    mAudioIsVorbis = false;
                }

                mMimes.add(String8(mime));
            }
        } else if (!strncasecmp(mime, "video/", 6)) {
            if (mVideoTrack.mSource == NULL) {
                mVideoTrack.mIndex = i;
                mVideoTrack.mSource = track;
                mVideoTrack.mPackets =
                    new AnotherPacketSource(mVideoTrack.mSource->getFormat());

                // video always at the beginning
                mMimes.insertAt(String8(mime), 0);
            }
        }

        mSources.push(track);
        return UNKNOWN_ERROR;
    }

    // 获取加密视频的信息
    (void)checkDrmInfo();

    // 这里会算视频的biterate，先忽略
    mBitrate = totalBitrate;

    return OK;
}

b. start

NuPlayer的start方法会同步调用Source的start方法，这时候就开始读取数据了。

调用postReadBuffer发送两个消息，最后会调用到readBuffer方法当中做数据的读取

void NuPlayer::GenericSource::start() {
    // ......
    if (mAudioTrack.mSource != NULL) {
        postReadBuffer(MEDIA_TRACK_TYPE_AUDIO);
    }

    if (mVideoTrack.mSource != NULL) {
        postReadBuffer(MEDIA_TRACK_TYPE_VIDEO);
    }

    mStarted = true;
}

readBuffer看起来比较长，但是并不是很复杂：

1、根据trackType获取对应的Track

2、根据actualTimeUs判断是否需要seek，如需要则构建ReadOptions

3、调用IMediaSource的read或者readMultiple方法读取数据

4、将读到的数据加入到AnotherPacketSource

void NuPlayer::GenericSource::readBuffer(
        media_track_type trackType, int64_t seekTimeUs, MediaPlayerSeekMode mode,
        int64_t *actualTimeUs, bool formatChange) {
    Track *track;
    size_t maxBuffers = 1;
    // 根据tracktype获取Track
    switch (trackType) {
        case MEDIA_TRACK_TYPE_VIDEO:
            track = &mVideoTrack;
            maxBuffers = 8;  // too large of a number may influence seeks
            break;
        case MEDIA_TRACK_TYPE_AUDIO:
            track = &mAudioTrack;
            maxBuffers = 64;
            break;
        case MEDIA_TRACK_TYPE_SUBTITLE:
            track = &mSubtitleTrack;
            break;
        case MEDIA_TRACK_TYPE_TIMEDTEXT:
            track = &mTimedTextTrack;
            break;
        default:
            TRESPASS();
    }

    if (track->mSource == NULL) {
        return;
    }
    // 如果seekTimeUs >= 0，说明发生了seek，封装ReadOptions在read时作为参数传下去
    if (actualTimeUs) {
        *actualTimeUs = seekTimeUs;
    }

    MediaSource::ReadOptions options;

    bool seeking = false;
    if (seekTimeUs >= 0) {
        options.setSeekTo(seekTimeUs, mode);
        seeking = true;
    }
    // 每次读取都会读取maxBuffer数量的buffer（audio 64，video 8），这时候就有两种读取方式，每次调用IMediaSource的read方法读一个buffer，或者调用readMultiple一次性读取多个buffer上来。无论哪种方法都会读满maxBuffers
    const bool couldReadMultiple = (track->mSource->supportReadMultiple());

    if (couldReadMultiple) {
        options.setNonBlocking();
    }

    int32_t generation = getDataGeneration(trackType);
    for (size_t numBuffers = 0; numBuffers < maxBuffers; ) {
        Vector<MediaBufferBase *> mediaBuffers;
        status_t err = NO_ERROR;

        sp<IMediaSource> source = track->mSource;
        mLock.unlock();
        if (couldReadMultiple) {
            err = source->readMultiple(
                    &mediaBuffers, maxBuffers - numBuffers, &options);
        } else {
            MediaBufferBase *mbuf = NULL;
            err = source->read(&mbuf, &options);
            if (err == OK && mbuf != NULL) {
                mediaBuffers.push_back(mbuf);
            }
        }
        mLock.lock();

        options.clearNonPersistent();

        size_t id = 0;
        size_t count = mediaBuffers.size();

        // in case track has been changed since we don't have lock for some time.
        if (generation != getDataGeneration(trackType)) {
            for (; id < count; ++id) {
                mediaBuffers[id]->release();
            }
            break;
        }

        for (; id < count; ++id) {
            int64_t timeUs;
            MediaBufferBase *mbuf = mediaBuffers[id];
            // 记录读到的audio/video的媒体位置
            if (!mbuf->meta_data().findInt64(kKeyTime, &timeUs)) {
                mbuf->meta_data().dumpToLog();
                track->mPackets->signalEOS(ERROR_MALFORMED);
                break;
            }
            if (trackType == MEDIA_TRACK_TYPE_AUDIO) {
                mAudioTimeUs = timeUs;
            } else if (trackType == MEDIA_TRACK_TYPE_VIDEO) {
                mVideoTimeUs = timeUs;
            }
            // 如果seek了，会清除AnotherpacketSource中的数据，并添加seek标志
            queueDiscontinuityIfNeeded(seeking, formatChange, trackType, track);

            sp<ABuffer> buffer = mediaBufferToABuffer(mbuf, trackType);
            if (numBuffers == 0 && actualTimeUs != nullptr) {
                *actualTimeUs = timeUs;
            }
            if (seeking && buffer != nullptr) {
                sp<AMessage> meta = buffer->meta();
                if (meta != nullptr && mode == MediaPlayerSeekMode::SEEK_CLOSEST
                        && seekTimeUs > timeUs) {
                    sp<AMessage> extra = new AMessage;
                    extra->setInt64("resume-at-mediaTimeUs", seekTimeUs);
                    meta->setMessage("extra", extra);
                }
            }
            // 将数据加入到AnotherPacketSource当中
            track->mPackets->queueAccessUnit(buffer);
            formatChange = false;
            seeking = false;
            ++numBuffers;
        }
        if (id < count) {
            // Error, some mediaBuffer doesn't have kKeyTime.
            for (; id < count; ++id) {
                // 清除暂存容器的数据用于再次的数据读取
                mediaBuffers[id]->release();
            }
            break;
        }

        if (err == WOULD_BLOCK) {
            break;
        } else if (err == INFO_FORMAT_CHANGED) {
#if 0
            track->mPackets->queueDiscontinuity(
                    ATSParser::DISCONTINUITY_FORMATCHANGE,
                    NULL,
                    false /* discard */);
#endif
        } else if (err != OK) {
            // 如果读取错误，则说明eos
            queueDiscontinuityIfNeeded(seeking, formatChange, trackType, track);
            track->mPackets->signalEOS(err);
            break;
        }
    }

    // 这个应该是播放网络资源时，不断下载缓存
    if (mIsStreaming
        && (trackType == MEDIA_TRACK_TYPE_VIDEO || trackType == MEDIA_TRACK_TYPE_AUDIO)) {
        status_t finalResult;
        int64_t durationUs = track->mPackets->getBufferedDurationUs(&finalResult);

        // TODO: maxRebufferingMarkMs could be larger than
        // mBufferingSettings.mResumePlaybackMarkMs
        int64_t markUs = (mPreparing ? mBufferingSettings.mInitialMarkMs
            : mBufferingSettings.mResumePlaybackMarkMs) * 1000LL;
        if (finalResult == ERROR_END_OF_STREAM || durationUs >= markUs) {
            if (mPreparing || mSentPauseOnBuffering) {
                Track *counterTrack =
                    (trackType == MEDIA_TRACK_TYPE_VIDEO ? &mAudioTrack : &mVideoTrack);
                if (counterTrack->mSource != NULL) {
                    durationUs = counterTrack->mPackets->getBufferedDurationUs(&finalResult);
                }
                if (finalResult == ERROR_END_OF_STREAM || durationUs >= markUs) {
                    if (mPreparing) {
                        notifyPrepared();
                        mPreparing = false;
                    } else {
                        sendCacheStats();
                        mSentPauseOnBuffering = false;
                        sp<AMessage> notify = dupNotify();
                        notify->setInt32("what", kWhatResumeOnBufferingEnd);
                        notify->post();
                    }
                }
            }
            return;
        }
　　　// 自己调用自己，循环读取
        postReadBuffer(trackType);
    }
}

接下来看看queueDiscontinuityIfNeeded，这个方法很简单，其实就是调用了AnotherPacketSource的queueDiscontinuity方法。这个在后面的博文中会简单介绍工作原理

void NuPlayer::GenericSource::queueDiscontinuityIfNeeded(
        bool seeking, bool formatChange, media_track_type trackType, Track *track) {
    // formatChange && seeking: track whose source is changed during selection
    // formatChange && !seeking: track whose source is not changed during selection
    // !formatChange: normal seek
    if ((seeking || formatChange)
            && (trackType == MEDIA_TRACK_TYPE_AUDIO
            || trackType == MEDIA_TRACK_TYPE_VIDEO)) {
        ATSParser::DiscontinuityType type = (formatChange && seeking)
                ? ATSParser::DISCONTINUITY_FORMATCHANGE
                : ATSParser::DISCONTINUITY_NONE;
        track->mPackets->queueDiscontinuity(type, NULL /* extra */, true /* discard */);
    }
}

c. seek

有了前面的底子，seek方法就很简单了，NuPlayer调用seekTo方法之后，会调用到readBuffer方法做数据读取

status_t NuPlayer::GenericSource::seekTo(int64_t seekTimeUs, MediaPlayerSeekMode mode) {
    ALOGV("seekTo: %lld, %d", (long long)seekTimeUs, mode);
    sp<AMessage> msg = new AMessage(kWhatSeek, this);
    msg->setInt64("seekTimeUs", seekTimeUs);
    msg->setInt32("mode", mode);

    // Need to call readBuffer on |mLooper| to ensure the calls to
    // IMediaSource::read* are serialized. Note that IMediaSource::read*
    // is called without |mLock| acquired and MediaSource is not thread safe.
    sp<AMessage> response;
    status_t err = msg->postAndAwaitResponse(&response);
    if (err == OK && response != NULL) {
        CHECK(response->findInt32("err", &err));
    }

    return err;
}

status_t NuPlayer::GenericSource::doSeek(int64_t seekTimeUs, MediaPlayerSeekMode mode) {
    if (mVideoTrack.mSource != NULL) {
        ++mVideoDataGeneration;

        int64_t actualTimeUs;
        readBuffer(MEDIA_TRACK_TYPE_VIDEO, seekTimeUs, mode, &actualTimeUs);

        if (mode != MediaPlayerSeekMode::SEEK_CLOSEST) {
            seekTimeUs = std::max<int64_t>(0, actualTimeUs);
        }
        mVideoLastDequeueTimeUs = actualTimeUs;
    }

    if (mAudioTrack.mSource != NULL) {
        ++mAudioDataGeneration;
        readBuffer(MEDIA_TRACK_TYPE_AUDIO, seekTimeUs, MediaPlayerSeekMode::SEEK_CLOSEST);
        mAudioLastDequeueTimeUs = seekTimeUs;
    }

    if (mSubtitleTrack.mSource != NULL) {
        mSubtitleTrack.mPackets->clear();
        mFetchSubtitleDataGeneration++;
    }

    if (mTimedTextTrack.mSource != NULL) {
        mTimedTextTrack.mPackets->clear();
        mFetchTimedTextDataGeneration++;
    }

    ++mPollBufferingGeneration;
    schedulePollBuffering();
    return OK;
}

d. pause

上层调用pause之后，NuPlayer相应的也会调用GenericSource的pause方法，这个方法很简单，直接置mStarted为false。

void NuPlayer::GenericSource::pause() {
    Mutex::Autolock _l(mLock);
    mStarted = false;
}

e. dequeueAccessUnit 

NuPlayerDecoder会调用这个方法来从Source中获取读到的数据，这是个比较重要的方法。

1、读取时会先去判断当前播放器的状态，如果是pause或者是stop，mStarted为false，则会停止本次数据的读取。

2、接着判断数据池中的数据是否足够，如果不够则读取数据

3、从数据池中出队列一个数据

4、再次判断数据池中的数据是否足够，如果不够则读取数据

status_t NuPlayer::GenericSource::dequeueAccessUnit(
        bool audio, sp<ABuffer> *accessUnit) {
    Mutex::Autolock _l(mLock);
    // If has gone through stop/releaseDrm sequence, we no longer send down any buffer b/c
    // the codec's crypto object has gone away (b/37960096).
    // Note: This will be unnecessary when stop() changes behavior and releases codec (b/35248283).
    if (!mStarted && mIsDrmReleased) {
        return -EWOULDBLOCK;
    }

    Track *track = audio ? &mAudioTrack : &mVideoTrack;

    if (track->mSource == NULL) {
        return -EWOULDBLOCK;
    }

    status_t finalResult;
    // 先判断AnotherPacketSource中的数据是否足够，如果不足够就调用postReadBuffer方法读取数据
    if (!track->mPackets->hasBufferAvailable(&finalResult)) {
        if (finalResult == OK) {
            postReadBuffer(
                    audio ? MEDIA_TRACK_TYPE_AUDIO : MEDIA_TRACK_TYPE_VIDEO);
            return -EWOULDBLOCK;
        }
        return finalResult;
    }
    // 从AnotherPacketSource中出队列一个buffer
    status_t result = track->mPackets->dequeueAccessUnit(accessUnit);

    // start pulling in more buffers if cache is running low
    // so that decoder has less chance of being starved
    // 再判断数据池中的数据是否足够，如不够就去读取（本地播放）
    if (!mIsStreaming) {
        if (track->mPackets->getAvailableBufferCount(&finalResult) < 2) {
            postReadBuffer(audio? MEDIA_TRACK_TYPE_AUDIO : MEDIA_TRACK_TYPE_VIDEO);
        }
    } else {
        int64_t durationUs = track->mPackets->getBufferedDurationUs(&finalResult);
        // TODO: maxRebufferingMarkMs could be larger than
        // mBufferingSettings.mResumePlaybackMarkMs
        int64_t restartBufferingMarkUs =
             mBufferingSettings.mResumePlaybackMarkMs * 1000LL / 2;
        if (finalResult == OK) {
            if (durationUs < restartBufferingMarkUs) {
                postReadBuffer(audio? MEDIA_TRACK_TYPE_AUDIO : MEDIA_TRACK_TYPE_VIDEO);
            }
            if (track->mPackets->getAvailableBufferCount(&finalResult) < 2
                && !mSentPauseOnBuffering && !mPreparing) {
                mCachedSource->resumeFetchingIfNecessary();
                sendCacheStats();
                mSentPauseOnBuffering = true;
                sp<AMessage> notify = dupNotify();
                notify->setInt32("what", kWhatPauseOnBufferingStart);
                notify->post();
            }
        }
    }

    if (result != OK) {
        if (mSubtitleTrack.mSource != NULL) {
            mSubtitleTrack.mPackets->clear();
            mFetchSubtitleDataGeneration++;
        }
        if (mTimedTextTrack.mSource != NULL) {
            mTimedTextTrack.mPackets->clear();
            mFetchTimedTextDataGeneration++;
        }
        return result;
    }

    int64_t timeUs;
    status_t eosResult; // ignored
    CHECK((*accessUnit)->meta()->findInt64("timeUs", &timeUs));
    if (audio) {
        mAudioLastDequeueTimeUs = timeUs;
    } else {
        mVideoLastDequeueTimeUs = timeUs;
    }

    if (mSubtitleTrack.mSource != NULL
            && !mSubtitleTrack.mPackets->hasBufferAvailable(&eosResult)) {
        sp<AMessage> msg = new AMessage(kWhatFetchSubtitleData, this);
        msg->setInt64("timeUs", timeUs);
        msg->setInt32("generation", mFetchSubtitleDataGeneration);
        msg->post();
    }

    if (mTimedTextTrack.mSource != NULL
            && !mTimedTextTrack.mPackets->hasBufferAvailable(&eosResult)) {
        sp<AMessage> msg = new AMessage(kWhatFetchTimedTextData, this);
        msg->setInt64("timeUs", timeUs);
        msg->setInt32("generation", mFetchTimedTextDataGeneration);
        msg->post();
    }

    return result;
}

到这里GenericSource的主要工作原理就学习完成了。