OpenHarmony 3.2 Beta多媒体系列——音视频播放gstreamer

OpenHarmony开发者

发布于 2022-11-24 10:47

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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。

OpenHarmony 3.2 Beta多媒体系列——音视频播放gstreamer-鸿蒙开发者社区

2.Gstreamer几个重要的概念

Element

Element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。

Pad

Pad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Pipeline

Bin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

OpenHarmony 3.2 Beta多媒体系列——音视频播放gstreamer-鸿蒙开发者社区

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>>([this]() {
        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>>([this]() {
            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>>([this, msg]() { 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>>([this]() {
        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。

int32_t PlayBinCtrlerBase::BaseState::ChangePlayBinState(GstState targetState, GstStateChangeReturn &ret)
{
    ......
    ret = gst_element_set_state(GST_ELEMENT_CAST(ctrler_.playbin_), targetState);
    if (ret == GST_STATE_CHANGE_FAILURE) {
        MEDIA_LOGE("Failed to change playbin's state to %{public}s", gst_element_state_get_name(targetState));
        return MSERR_INVALID_OPERATION;
    }

    return MSERR_OK;
}