Android12(S)MultiMediaLearning（七）NuPlayerGenericSource

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 extractor;
// ......
// 创建MediaExtractor
extractor = MediaExtractorFactory::Create(dataSource, NULL);
// 获取文件的metadata
sp 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 i) {
// 获取MediaSource
sp track = extractor->getTrack(i);
if (track == NULL) {
continue;
}
sp 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 mediaBuffers;
status_t err = NO_ERROR;
sp 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 id) {
mediaBuffers[id]->release();
}
break;
}
for (; id 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 buffer = mediaBufferToABuffer(mbuf, trackType);
if (numBuffers == 0 && actualTimeUs != nullptr) {
*actualTimeUs = timeUs;
}
if (seeking && buffer != nullptr) {
sp meta = buffer->meta();
if (meta != nullptr && mode == MediaPlayerSeekMode::SEEK_CLOSEST
&& seekTimeUs > timeUs) {
sp 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 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 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 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 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(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 *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 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 msg = new AMessage(kWhatFetchSubtitleData, this);
msg->setInt64("timeUs", timeUs);
msg->setInt32("generation", mFetchSubtitleDataGeneration);
msg->post();
}
if (mTimedTextTrack.mSource != NULL
&& !mTimedTextTrack.mPackets->hasBufferAvailable(&eosResult)) {
sp 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。