OpenHarmony 3.2 Beta多媒体系列：音视频播放gstreamer
本帖最后由 sunsili 于 2022-11-25 14:58 编辑OpenHarmony 3.2 Beta多媒体系列：音视频播放gstreamer
巴延兴深圳开鸿数字产业发展有限公司
资深OS框架开发工程师
一、 简介
多媒体播放框架主要的实现在PlayerServer服务中，这个服务提供了媒体播放框架所需要的实现环境，继续跟踪代码分析发现，PlayerServer主要通过gstreamer适配层，对gstreamer进行调用。gstreamer属于更加具体的实现，所以本篇文章主要是分析PlayerServer通过适配层调用到gstreamer的过程。
此前，我在《OpenHarmony 3.2 Beta多媒体系列-音视频播放框架》一文中，主要分析了多媒体播放的框架层代码，本地接口通过服务端的proxy代理类进行IPC调用，最终调用到PlayerServer服务端。本篇主要分析了多媒体gstreamer的调用，涉及到从PlayerServer到gstreamer的整体流程。
二、 目录gstreamer ├── BUILD.gn ├── common │ ├── BUILD.gn │ ├── playbin_adapter │ │ ├── i_playbin_ctrler.h │ │ ├── playbin2_ctrler.cpp │ │ ├── playbin2_ctrler.h │ │ ├── playbin_ctrler_base.cpp │ │ ├── playbin_ctrler_base.h │ │ ├── playbin_msg_define.h │ │ ├── playbin_sink_provider.h │ │ ├── playbin_state.cpp │ │ ├── playbin_state.h │ │ ├── playbin_task_mgr.cpp │ │ └── playbin_task_mgr.h │ ├── state_machine │ │ ├── state_machine.cpp │ │ └── state_machine.h ├── factory │ ├── BUILD.gn │ └── engine_factory.cpp └── player ├── BUILD.gn ├── player_codec_ctrl.cpp ├── player_codec_ctrl.h ├── player_engine_gst_impl.cpp ├── player_engine_gst_impl.h ├── player_sinkprovider.cpp ├── player_sinkprovider.h ├── player_track_parse.cpp └── player_track_parse.h
目录主要是多媒体子系统中的engine部分，涉及到了gstreamer的适配层，gstreamer具体的实现是在third_party/gstreamer目录中。
三 、Gstreamer介绍
1. 简介
Gstreamer是一个跨平台的多媒体框架，应用程序可以通过管道（Pipeline）的方式，将多媒体处理的各个步骤串联起来，达到预期的效果。每个步骤通过元素（Element）基于GObject对象系统通过插件（plugins）的方式实现，方便了各项功能的扩展。
2.Gstreamer几个重要的概念
ElementElement是Gstreamer中最重要的对象类型之一。一个element实现一个功能（读取文件，解码，输出等），程序需要创建多个element，并按顺序将其串联起来，构成一个完整的Pipeline。
PadPad是一个element的输入/输出接口，分为src pad（生产数据）和sink pad（消费数据）两种。两个element必须通过pad才能连接起来，pad拥有当前element能处理数据类型的能力（capabilities），会在连接时通过比较src pad和sink pad中所支持的能力，来选择最恰当的数据类型用于传输，如果element不支持，程序会直接退出。在element通过pad连接成功后，数据会从上一个element的src pad传到下一个element的sink pad然后进行处理。
Bin和PipelineBin是一个容器，用于管理多个element，改变bin的状态时，bin会自动去修改所包含的element的状态，也会转发所收到的消息。如果没有bin，我们需要依次操作我们所使用的element。通过bin降低了应用的复杂度。Pipeline继承自bin，为程序提供一个bus用于传输消息，并且对所有子element进行同步。当将Pipeline的状态设置为PLAYING时，Pipeline会在一个/多个新的线程中通过element处理数据。
四、调用流程
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五、源码分析
1. PrepareAsync分析
首先，在PlayerServer的PrepareAsync中会调用OnPrepare(false)，具体是在OnPrepare(false)中实现，参数传入false，表明调用的是异步方法。int32_t PlayerServer::PrepareAsync(){ std::lock_guard<std::mutex> lock(mutex_); MEDIA_LOGW("KPI-TRACE: PlayerServer PrepareAsync in");
if (lastOpStatus_ == PLAYER_INITIALIZED || lastOpStatus_ == PLAYER_STOPPED) { return OnPrepare(false); } else { MEDIA_LOGE("Can not Prepare, currentState is %{public}s", GetStatusDescription(lastOpStatus_).c_str()); return MSERR_INVALID_OPERATION; }}
OnPrepare方法中，先通过playerEngine_调用SerVideoSurface的方法，将surface_设置到PlayerEngineGstImpl中（producerSurface_），接着启动一个任务，调用目前状态的Prepare()方法。int32_t PlayerServer::OnPrepare(bool sync){ CHECK_AND_RETURN_RET_LOG(playerEngine_ != nullptr, MSERR_NO_MEMORY, "playerEngine_ is nullptr"); int32_t ret = MSERR_OK;
#ifdef SUPPORT_VIDEO if (surface_ != nullptr) { ret = playerEngine_->SetVideoSurface(surface_); CHECK_AND_RETURN_RET_LOG(ret == MSERR_OK, MSERR_INVALID_OPERATION, "Engine SetVideoSurface Failed!"); }#endif
lastOpStatus_ = PLAYER_PREPARED;
auto preparedTask = std::make_shared<TaskHandler<int32_t>>(() { MediaTrace::TraceBegin("PlayerServer::PrepareAsync", FAKE_POINTER(this)); auto currState = std::static_pointer_cast<BaseState>(GetCurrState()); return currState->Prepare(); });
ret = taskMgr_.LaunchTask(preparedTask, PlayerServerTaskType::STATE_CHANGE); CHECK_AND_RETURN_RET_LOG(ret == MSERR_OK, ret, "Prepare launch task failed");
if (sync) { (void)preparedTask->GetResult(); // wait HandlePrpare } return MSERR_OK;}
进入Preparing状态后，会触发PlayerServer的HandlePrepare()方法被调用，在这个方法里会通过playerEngine_调用PrepareAsync方法，这个方法调用的是PlayerEngineGstImpl对应的PrepareAsync方法。int32_t PlayerServer::HandlePrepare(){ int32_t ret = playerEngine_->PrepareAsync(); CHECK_AND_RETURN_RET_LOG(ret == MSERR_OK, MSERR_INVALID_OPERATION, "Server Prepare Failed!"); if (config_.leftVolume <= 1.0f || config_.rightVolume <= 1.0f) { ret = playerEngine_->SetVolume(config_.leftVolume, config_.rightVolume); MEDIA_LOGD("Prepared SetVolume leftVolume:%{public}f rightVolume:%{public}f, ret:%{public}d", \ config_.leftVolume, config_.rightVolume, ret); } (void)playerEngine_->SetLooping(config_.looping);
{ auto rateTask = std::make_shared<TaskHandler<void>>(() { auto currState = std::static_pointer_cast<BaseState>(GetCurrState()); (void)currState->SetPlaybackSpeed(config_.speedMode); });
(void)taskMgr_.LaunchTask(rateTask, PlayerServerTaskType::RATE_CHANGE); } return MSERR_OK;}首先初始化playBinCtrler_，后续的操作都是通过PlayBinCtrlerBase对象来操作的，所以PlayBinCtrlerInit()方法会创建PlayBinCtrlerBase对象（playBinCtrler_），创建好以后通过playBinCtrler_进行SetSource和SetXXXListener的设置。int32_t PlayerEngineGstImpl::PrepareAsync(){ std::unique_lock<std::mutex> lock(mutex_); MEDIA_LOGD("Prepare in");
int32_t ret = PlayBinCtrlerInit(); CHECK_AND_RETURN_RET_LOG(ret == MSERR_OK, MSERR_INVALID_VAL, "PlayBinCtrlerInit failed");
CHECK_AND_RETURN_RET_LOG(playBinCtrler_ != nullptr, MSERR_INVALID_VAL, "playBinCtrler_ is nullptr"); ret = playBinCtrler_->PrepareAsync(); CHECK_AND_RETURN_RET_LOG(ret == MSERR_OK, ret, "PrepareAsync failed");
// The duration of some resources without header information cannot be obtained. MEDIA_LOGD("Prepared ok out"); return MSERR_OK;}初始化完成以后，接下来进行playBinCtrler_的PrepareAsync的调用，PlayBinCtrlerBase中的PrepareAsync的方法间接地调用了PrepareAsyncInternal。int32_t PlayBinCtrlerBase::PrepareAsync(){ MEDIA_LOGD("enter");
std::unique_lock<std::mutex> lock(mutex_); return PrepareAsyncInternal();}
PrepareAsyncInternal首先判断当前的状态，如果是preparingState或preparedState，那么就直接返回成功，否则继续向下调用。接下来会调用EnterInitializedState(),这个方法中会创建playbin，设置signal的回调以及gstreamer参数的设置。最后调用目前状态的Prepare方法，此时的状态是InitializedState。int32_t PlayBinCtrlerBase::PrepareAsyncInternal(){ if ((GetCurrState() == preparingState_) || (GetCurrState() == preparedState_)) { MEDIA_LOGI("already at preparing state, skip"); return MSERR_OK; }
CHECK_AND_RETURN_RET_LOG((!uri_.empty() || appsrcWrap_), MSERR_INVALID_OPERATION, "Set uri firsty!");
int32_t ret = EnterInitializedState(); CHECK_AND_RETURN_RET(ret == MSERR_OK, ret);
auto currState = std::static_pointer_cast<BaseState>(GetCurrState()); ret = currState->Prepare(); CHECK_AND_RETURN_RET_LOG(ret == MSERR_OK, ret, "PrepareAsyncInternal failed");
return MSERR_OK;}
InitializedState的Prepare方法又通过ctrler_调回到PlayBinCtrlerBase的ChangeState方法，这个方法是在PlayBinCtrlerBase的父类StateMachine中，它是一个状态机，管理着各种状态的切换。int32_t PlayBinCtrlerBase::InitializedState::Prepare(){ ctrler_.ChangeState(ctrler_.preparingState_); return MSERR_OK;}
很多表示状态的类在PlayBinCtrlerBase中进行声明，这些子类的具体实现功能在playbin_state.cpp中。private: class BaseState; class IdleState; class InitializedState; class PreparingState; class PreparedState; class PlayingState; class PausedState; class StoppedState; class StoppingState; class PlaybackCompletedState;
接下来看一下状态机的ChangeState方法，可以看出切换状态的时候，先调用切换前状态的StateExit()方法，再调用切换后状态的StateEnter()。如果需要一些操作，我们可以在状态的StateEnter和StateExit中进行。void StateMachine::ChangeState(const std::shared_ptr<State> &state){ ...... if (currState_ != nullptr && currState_->GetStateName() == "stopping_state" && state->GetStateName() != "stopped_state") { return; } if (currState_) { currState_->StateExit(); } currState_ = state; state->StateEnter();}
因为上面切换状态调用的是ctrler_.ChangeState(ctrler_.preparingState_)，所以接下来看一下PreparingState状态的StateEnter方法。这个方法中首先是调用了ctrler_.ReportMessage(msg)，字面上看是用来上报msg信息的。void PlayBinCtrlerBase::PreparingState::StateEnter(){ PlayBinMessage msg = { PLAYBIN_MSG_SUBTYPE, PLAYBIN_SUB_MSG_BUFFERING_START, 0, {} }; ctrler_.ReportMessage(msg);
GstStateChangeReturn ret; (void)ChangePlayBinState(GST_STATE_PAUSED, ret);
MEDIA_LOGD("PreparingState::StateEnter finished");}
ctrler_是PlayBinCtrlerBase类型的变量，直接看PlayBinCtrlerBase的ReportMessage方法，这个方法的核心，是创建一个任务后，将任务放入消息队列中，等待消息被处理，这里我们最想知道的是这个消息会在什么地方被处理。msgReportHandler创建了TaskHandler，这个里面会调用notifier_(msg)，这里的notifier_比较重要，我们可以顺着这个变量向上分析。void PlayBinCtrlerBase::ReportMessage(const PlayBinMessage &msg){ ...... auto msgReportHandler = std::make_shared<TaskHandler<void>>(() { notifier_(msg); }); int32_t ret = msgQueue_->EnqueueTask(msgReportHandler); if (ret != MSERR_OK) { MEDIA_LOGE("async report msg failed, type: %{public}d, subType: %{public}d, code: %{public}d", msg.type, msg.subType, msg.code); };
if (msg.type == PlayBinMsgType::PLAYBIN_MSG_EOS) { ProcessEndOfStream(); }}notifier_是在PlayBinCtrlerBase被创建的时候赋值的。PlayBinCtrlerBase::PlayBinCtrlerBase(const PlayBinCreateParam &createParam) : renderMode_(createParam.renderMode), notifier_(createParam.notifier), sinkProvider_(createParam.sinkProvider){ MEDIA_LOGD("enter ctor, instance: 0x%{public}06" PRIXPTR "", FAKE_POINTER(this));}
在源码分析的前期PlayerEngineGstImpl初始化PlayBinCtrlerBase的时候进行了创建notifier = std::bind(&PlayerEngineGstImpl::OnNotifyMessage, this, std::placeholders::_1)notifier相当于是调用了PlayerEngineGstImpl::OnNotifyMessage方法。所以上述中的处理函数就是PlayerEngineGstImpl::OnNotifyMessage。int32_t PlayerEngineGstImpl::PlayBinCtrlerPrepare(){ uint8_t renderMode = IPlayBinCtrler::PlayBinRenderMode::DEFAULT_RENDER; auto notifier = std::bind(&PlayerEngineGstImpl::OnNotifyMessage, this, std::placeholders::_1);
{ std::unique_lock<std::mutex> lk(trackParseMutex_); sinkProvider_ = std::make_shared<PlayerSinkProvider>(producerSurface_); sinkProvider_->SetAppInfo(appuid_, apppid_); }
IPlayBinCtrler::PlayBinCreateParam createParam = { static_cast<IPlayBinCtrler::PlayBinRenderMode>(renderMode), notifier, sinkProvider_ }; playBinCtrler_ = IPlayBinCtrler::Create(IPlayBinCtrler::PlayBinKind::PLAYBIN2, createParam); ...... return MSERR_OK;}在OnNotifyMessage中指定了各种消息类型对应的执行函数，上述代码中创建的Message类型是PLAYBIN_MSG_SUBTYPE，子类型为PLAYBIN_SUB_MSG_BUFFERING_START。void PlayerEngineGstImpl::OnNotifyMessage(const PlayBinMessage &msg){ const std::unordered_map<int32_t, MsgNotifyFunc> MSG_NOTIFY_FUNC_TABLE = { { PLAYBIN_MSG_ERROR, std::bind(&PlayerEngineGstImpl::HandleErrorMessage, this, std::placeholders::_1) }, { PLAYBIN_MSG_SEEKDONE, std::bind(&PlayerEngineGstImpl::HandleSeekDoneMessage, this, std::placeholders::_1) }, { PLAYBIN_MSG_SPEEDDONE, std::bind(&PlayerEngineGstImpl::HandleInfoMessage, this, std::placeholders::_1) }, { PLAYBIN_MSG_BITRATEDONE, std::bind(&PlayerEngineGstImpl::HandleInfoMessage, this, std::placeholders::_1)}, { PLAYBIN_MSG_EOS, std::bind(&PlayerEngineGstImpl::HandleInfoMessage, this, std::placeholders::_1) }, { PLAYBIN_MSG_STATE_CHANGE, std::bind(&PlayerEngineGstImpl::HandleInfoMessage, this, std::placeholders::_1) }, { PLAYBIN_MSG_SUBTYPE, std::bind(&PlayerEngineGstImpl::HandleSubTypeMessage, this, std::placeholders::_1) }, { PLAYBIN_MSG_AUDIO_SINK, std::bind(&PlayerEngineGstImpl::HandleAudioMessage, this, std::placeholders::_1) }, { PLAYBIN_MSG_POSITION_UPDATE, std::bind(&PlayerEngineGstImpl::HandlePositionUpdateMessage, this, std::placeholders::_1) }, }; if (MSG_NOTIFY_FUNC_TABLE.count(msg.type) != 0) { MSG_NOTIFY_FUNC_TABLE.at(msg.type)(msg); }}
最终的流程走到了PlayerEngineGstImpl::HandleBufferingStart()，在这个方法中，主要通过obs_将format传给IPlayerEngineObs的OnInfo方法。void PlayerEngineGstImpl::HandleBufferingStart(){ percent_ = 0; Format format;(void)format.PutIntValue(std::string(PlayerKeys::PLAYER_BUFFERING_START), 0); std::shared_ptr<IPlayerEngineObs> notifyObs = obs_.lock(); if (notifyObs != nullptr) { notifyObs->OnInfo(INFO_TYPE_BUFFERING_UPDATE, 0, format); }}
我们重点看一下obs_是哪里设置的，在PlayerServer的初始化InitPlayEngine。shared_from_this()相当于是把PlayerServer自身赋值给obs，PlayerServer也是实现了IPlayerEngineObs对应的接口。int32_t PlayerServer::InitPlayEngine(const std::string &url){ ...... int32_t ret = taskMgr_.Init(); auto engineFactory = EngineFactoryRepo::Instance().GetEngineFactory(IEngineFactory::Scene::SCENE_PLAYBACK, url);
playerEngine_ = engineFactory->CreatePlayerEngine(appUid_, appPid_);
if (dataSrc_ == nullptr) { ret = playerEngine_->SetSource(url); } else { ret = playerEngine_->SetSource(dataSrc_); }
std::shared_ptr<IPlayerEngineObs> obs = shared_from_this(); ret = playerEngine_->SetObs(obs);
lastOpStatus_ = PLAYER_INITIALIZED; ChangeState(initializedState_);
return MSERR_OK;}
这样我们就跟踪到了PlayerServer的OnInfo()方法。void PlayerServer::OnInfo(PlayerOnInfoType type, int32_t extra, const Format &infoBody){ std::lock_guard<std::mutex> lockCb(mutexCb_);
int32_t ret = HandleMessage(type, extra, infoBody); if (playerCb_ != nullptr && ret == MSERR_OK) { playerCb_->OnInfo(type, extra, infoBody); }}
2. Play分析
从PlayerServer开始跟踪，调用到PlayerServer的OnPlay()方法。int32_t PlayerServer::Play(){ ...... if (lastOpStatus_ == PLAYER_PREPARED || lastOpStatus_ == PLAYER_PLAYBACK_COMPLETE || lastOpStatus_ == PLAYER_PAUSED) { return OnPlay(); } else { return MSERR_INVALID_OPERATION; }}
在OnPlay中会启动一个任务，在任务中获取当前的状态，然后调用当前状态的Play()方法。int32_t PlayerServer::OnPlay(){ ...... auto playingTask = std::make_shared<TaskHandler<void>>(() { auto currState = std::static_pointer_cast<BaseState>(GetCurrState()); (void)currState->Play(); });
int ret = taskMgr_.LaunchTask(playingTask, PlayerServerTaskType::STATE_CHANGE);
lastOpStatus_ = PLAYER_STARTED; return MSERR_OK;}
前面调用了PrepareAsync，所以当前的状态是Prepared，调用到了PreparedState的Play()方法，这个方法还是按照之前Prepare的方式，调回到PlayerServer的HandlePlay()。int32_t PlayerServer::PreparedState::Play(){ return server_.HandlePlay();}在PlayServer中通过播放引擎继续向下调用。int32_t PlayerServer::HandlePlay(){ int32_t ret = playerEngine_->Play(); CHECK_AND_RETURN_RET_LOG(ret == MSERR_OK, MSERR_INVALID_OPERATION, "Engine Play Failed!");
return MSERR_OK;}
在PlayerEngineGstImpl的Play()方法会继续调用playBinCtrler_的Play()方法。int32_t PlayerEngineGstImpl::Play(){ ...... playBinCtrler_->Play(); return MSERR_OK;}
PlayBinCtrlerBase的Play()方法根据当前的State，调用currSate->Play()。int32_t PlayBinCtrlerBase::Play(){ ...... auto currState = std::static_pointer_cast<BaseState>(GetCurrState()); int32_t ret = currState->Play();
return MSERR_OK;}
在PreparedState的Play()方法中改变了PlayBin的状态为playing。 int32_t PlayBinCtrlerBase::PreparedState::Play(){ GstStateChangeReturn ret; return ChangePlayBinState(GST_STATE_PLAYING, ret);}
ChangePlayBinState主要是调用了gst_element_set_state(GST_ELEMENT_CAST(ctrler_.playbin_),GST_STATE_PLAYING),这个直接调用了gstreamer三方库的实现，调用完这个方法以后，gstreamer就开始进行播放了。<div align="left">int32_t PlayBinCtrlerBase::BaseState::ChangePlayBinState(GstState targetState, GstStateChangeReturn &ret)</div><div align="left">{</div><div align="left"> ......</div><div align="left"> ret = gst_element_set_state(GST_ELEMENT_CAST(ctrler_.playbin_), targetState);</div><div align="left"> if (ret == GST_STATE_CHANGE_FAILURE) {</div><div align="left"> MEDIA_LOGE("Failed to change playbin's state to %{public}s", gst_element_state_get_name(targetState));</div><div align="left"> return MSERR_INVALID_OPERATION;</div><div align="left"> }</div><div align="left">
</div><div align="left"> return MSERR_OK;</div><div align="left">}</div>
六、总结
本篇文章主要从PlayerServer播放服务开始分析音视频播放的流程，涉及到gstreamer引擎的调用，相对于多媒体播放框架来说，更加底层，便于熟悉从框架到gstreamer的整体流程。
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