Audio播放流程(四)---MediaPlayerService流程之AudioTrack的创建

首先在每次MediaPlayer调用create创建的时候，会调用一次prepare，这个时候会走入NuPlayerDriver::prepareAsync中的STATE_UNPREPARED，之后会先调用stop一下，从而将NuPlayer播放器的状态切换为STATE_STOPPED态，之后再次调用prepare进入NuPlayerDriver::prepareAsync的STATE_STOPPED流程

第二次在prepareAsync调用中

status_t NuPlayerDriver::prepareAsync() {ALOGV("prepareAsync(%p)", this);Mutex::Autolock autoLock(mLock);switch (mState) {case STATE_UNPREPARED:mState = STATE_PREPARING;mIsAsyncPrepare = true;mPlayer->prepareAsync();return OK;case STATE_STOPPED:// this is really just paused. handle as seek to startmAtEOS = false;mState = STATE_STOPPED_AND_PREPARING;mIsAsyncPrepare = true;mPlayer->seekToAsync(0, true /* needNotify */);return OK;default:return INVALID_OPERATION;};
}

这里会将播放器的seek调整到0，同时播放器的状态切换为STATE_STOPPED_AND_PREPARING

void NuPlayer::seekToAsync(int64_t seekTimeUs, bool needNotify) {android::CallStack cs("JON");sp<AMessage> msg = new AMessage(kWhatSeek, this);msg->setInt64("seekTimeUs", seekTimeUs);msg->setInt32("needNotify", needNotify);msg->post();
}

消息发往NuPlayerDriver在构造的时候开启的线程"NuPlayerDriver Looper"

void NuPlayer::onMessageReceived(const sp<AMessage> &msg) {...case kWhatSeek:onStart(seekTimeUs);...
}
void NuPlayer::onStart(int64_t startPositionUs) {...postScanSources();...
}
...
void NuPlayer::postScanSources() {if (mScanSourcesPending) {return;}sp<AMessage> msg = new AMessage(kWhatScanSources, this);msg->setInt32("generation", mScanSourcesGeneration);msg->post();mScanSourcesPending = true;
}

消息继续发往子线程处理

void NuPlayer::onMessageReceived(const sp<AMessage> &msg) {case kWhatScanSources:{if (mAudioSink != NULL && mAudioDecoder == NULL) {if (instantiateDecoder(true, &mAudioDecoder) == -EWOULDBLOCK) {rescan = true;}}...}
}
...
status_t NuPlayer::instantiateDecoder(bool audio, sp<DecoderBase> *decoder, bool checkAudioModeChange) {if (audio) {...这里是关键if (checkAudioModeChange) {determineAudioModeChange(format);}if (mOffloadAudio) {const bool hasVideo = (mSource->getFormat(false /*audio */) != NULL);format->setInt32("has-video", hasVideo);*decoder = AVNuFactory::get()->createPassThruDecoder(notify, mSource, mRenderer);} else {AVNuUtils::get()->setCodecOutputFormat(format);mSource->setOffloadAudio(false /* offload */);*decoder = AVNuFactory::get()->createDecoder(notify, mSource, mPID, mRenderer);}        ...
}

如上会完成2件重要的事情，一个是调用determineAudioModeChange完成AudioTrack的创建，另外就是创建音频的decoder，我们当前只关注AudioTrack的创建

void NuPlayer::determineAudioModeChange(const sp<AMessage> &audioFormat) {...走offload模式if (canOffload) {tryOpenAudioSinkForOffload(audioFormat, audioMeta, hasVideo);}....
}
void NuPlayer::tryOpenAudioSinkForOffload(const sp<AMessage> &format, const sp<MetaData> &audioMeta, bool hasVideo) {// Note: This is called early in NuPlayer to determine whether offloading// is possible; otherwise the decoders call the renderer openAudioSink directly.status_t err = mRenderer->openAudioSink(format, true /* offloadOnly */, hasVideo, AUDIO_OUTPUT_FLAG_NONE, &mOffloadAudio, mSource->isStreaming());if (err != OK) {// Any failure we turn off mOffloadAudio.mOffloadAudio = false;} else if (mOffloadAudio) {sendMetaDataToHal(mAudioSink, audioMeta);}
}

frameworks\av\media\libmediaplayerservice\nuplayer\NuPlayerRenderer.cpp

status_t NuPlayer::Renderer::openAudioSink(const sp<AMessage> &format,bool offloadOnly,bool hasVideo,uint32_t flags,bool *isOffloaded,bool isStreaming) {sp<AMessage> msg = new AMessage(kWhatOpenAudioSink, this);msg->setMessage("format", format);msg->setInt32("offload-only", offloadOnly);msg->setInt32("has-video", hasVideo);msg->setInt32("flags", flags);msg->setInt32("isStreaming", isStreaming);sp<AMessage> response;status_t postStatus = msg->postAndAwaitResponse(&response);...
}

如上消息发往Renderer的子线程NuPlayerRenderer处理，之后调用onOpenAudioSink

status_t NuPlayer::Renderer::onOpenAudioSink(const sp<AMessage> &format,bool offloadOnly,bool hasVideo,uint32_t flags,bool isStreaming) {...err = mAudioSink->open(sampleRate,numChannels,(audio_channel_mask_t)channelMask,audioFormat,0 /* bufferCount - unused */,&NuPlayer::Renderer::AudioSinkCallback,this,(audio_output_flags_t)offloadFlags,&offloadInfo);...err = mAudioSink->start();             ...
}

这里的mAudioSink在Audio播放流程(二)—NuPlayer流程之setDataSource中的setDataSource_pre有完成设置，指向的是MediaPlayerService中的AudioOutput

frameworks\av\media\libmediaplayerservice\MediaPlayerService.cpp

status_t MediaPlayerService::AudioOutput::open(uint32_t sampleRate, int channelCount, audio_channel_mask_t channelMask,audio_format_t format, int bufferCount,AudioCallback cb, void *cookie,audio_output_flags_t flags,const audio_offload_info_t *offloadInfo,bool doNotReconnect,uint32_t suggestedFrameCount)
{...t = new AudioTrack(mStreamType,sampleRate,format,channelMask,frameCount,flags,NULL, // callbackNULL, // user data0, // notification framesmSessionId,AudioTrack::TRANSFER_DEFAULT,NULL, // offload infomUid,mPid,mAttributes,doNotReconnect,targetSpeed);t->setVolume(mLeftVolume, mRightVolume);mTrack = t;...
}

如上完成了AudioTrack的创建

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