一、简介
媒体子系统为开发者提供一套接口，不便开发者应用零碎的媒体资源，次要蕴含音视频开发、相机开发、流媒体开发等模块。每个模块都提供给下层利用对应的接口，本文会对音视频开发中的音视频播放框架做一个具体的介绍。
二、目录
foundation/multimedia/media_standard
├── frameworks #框架代码│ ├── js│ │ ├── player│ ├── native│ │ ├── player #native实现│ └── videodisplaymanager #显示治理│ ├── include│ └── src├── interfaces│ ├── inner_api #外部接口│ │ └── native│ └── kits #内部JS接口├── sa_profile #服务配置文件└── services ├── engine #engine代码 │ └── gstreamer ├── etc #服务配置文件 ├── include #头文件 └── services ├── sa_media #media服务 │ ├── client #media客户端 │ ├── ipc #media ipc调用 │ └── server #media服务端 ├── factory #engine工厂 └── player #player服务 ├── client #player客户端 ├── ipc #player ipc调用 └── server #player服务端三、播放的总体流程
四、Native接口应用
OpenHarmony零碎中，音视频播放通过N-API接口提供给下层JS调用，N-API相当于是JS和Native之间的桥梁，在OpenHarmony源码中，提供了C++间接调用的音视频播放例子，在foundation/multimedia/player_framework/test/nativedemo/player目录中。
void PlayerDemo::RunCase(const string &path){ player_ = OHOS::Media::PlayerFactory::CreatePlayer(); if (player_ == nullptr) { cout << "player_ is null" << endl; return; } RegisterTable(); std::shared_ptr<PlayerCallbackDemo> cb = std::make_shared<PlayerCallbackDemo>(); cb->SetBufferingOut(SelectBufferingOut()); int32_t ret = player_->SetPlayerCallback(cb); if (ret != 0) { cout << "SetPlayerCallback fail" << endl; } if (SelectSource(path) != 0) { cout << "SetSource fail" << endl; return; } sptr<Surface> producerSurface = nullptr; producerSurface = GetVideoSurface(); if (producerSurface != nullptr) { ret = player_->SetVideoSurface(producerSurface); if (ret != 0) { cout << "SetVideoSurface fail" << endl; } } SetVideoScaleType(); if (SelectRendererMode() != 0) { cout << "set renderer info fail" << endl; } ret = player_->PrepareAsync(); if (ret != 0) { cout << "PrepareAsync fail" << endl; return; } cout << "Enter your step:" << endl; DoNext();}首先依据RunCase能够大抵理解一下播放音视频的次要流程，创立播放器，设置播放源，设置回调办法（蕴含播放过程中的多种状态的回调），设置播放显示的Surface，这些筹备工作做好之后，须要调用播放器的PrepareASync办法，这个办法实现后，播放状态会变成Prepared状态，这时就能够调用播放器的play接口，进行音视频的播放了。
RegisterTable()办法中，将字符串和对应的办法映射到Map中，这样后续的DoNext会依据输出的命令，来决定播放器具体的操作。
void PlayerDemo::DoNext(){ std::string cmd; while (std::getline(std::cin, cmd)) { auto iter = playerTable_.find(cmd); if (iter != playerTable_.end()) { auto func = iter->second; if (func() != 0) { cout << "Operation error" << endl; } if (cmd.find("stop") != std::string::npos && dataSrc_ != nullptr) { dataSrc_->Reset(); } continue; } else if (cmd.find("quit") != std::string::npos || cmd == "q") { break; } else { DoCmd(cmd); continue; } }}void PlayerDemo::RegisterTable(){ (void)playerTable_.emplace("prepare", std::bind(&Player::Prepare, player_)); (void)playerTable_.emplace("prepareasync", std::bind(&Player::PrepareAsync, player_)); (void)playerTable_.emplace("", std::bind(&Player::Play, player_)); // ENTER -> play (void)playerTable_.emplace("play", std::bind(&Player::Play, player_)); (void)playerTable_.emplace("pause", std::bind(&Player::Pause, player_)); (void)playerTable_.emplace("stop", std::bind(&Player::Stop, player_)); (void)playerTable_.emplace("reset", std::bind(&Player::Reset, player_)); (void)playerTable_.emplace("release", std::bind(&Player::Release, player_)); (void)playerTable_.emplace("isplaying", std::bind(&PlayerDemo::GetPlaying, this)); (void)playerTable_.emplace("isloop", std::bind(&PlayerDemo::GetLooping, this)); (void)playerTable_.emplace("speed", std::bind(&PlayerDemo::GetPlaybackSpeed, this));}以上的DoNext办法中外围的代码是func()的调用，这个func就是之前注册进Map中字符串对应的办法，在RegisterTable办法中将空字符串""和"play"对绑定为Player::Play办法，默认不输出命令参数时，是播放操作。
五、调用流程
本段落次要针对媒体播放的框架层代码进行剖析，所以在流程中波及到了IPC调用相干的客户端和服务端，代码暂且剖析到调用gstreamer引擎。首先Sample通过PlayerFactory创立了一个播放器实例（PlayerImpl对象），创立过程中调用Init函数。
void PlayerDemo::DoNext(){ std::string cmd; while (std::getline(std::cin, cmd)) { auto iter = playerTable_.find(cmd); if (iter != playerTable_.end()) { auto func = iter->second; if (func() != 0) { cout << "Operation error" << endl; } if (cmd.find("stop") != std::string::npos && dataSrc_ != nullptr) { dataSrc_->Reset(); } continue; } else if (cmd.find("quit") != std::string::npos || cmd == "q") { break; } else { DoCmd(cmd); continue; } }}void PlayerDemo::RegisterTable(){ (void)playerTable_.emplace("prepare", std::bind(&Player::Prepare, player_)); (void)playerTable_.emplace("prepareasync", std::bind(&Player::PrepareAsync, player_)); (void)playerTable_.emplace("", std::bind(&Player::Play, player_)); // ENTER -> play (void)playerTable_.emplace("play", std::bind(&Player::Play, player_)); (void)playerTable_.emplace("pause", std::bind(&Player::Pause, player_)); (void)playerTable_.emplace("stop", std::bind(&Player::Stop, player_)); (void)playerTable_.emplace("reset", std::bind(&Player::Reset, player_)); (void)playerTable_.emplace("release", std::bind(&Player::Release, player_)); (void)playerTable_.emplace("isplaying", std::bind(&PlayerDemo::GetPlaying, this)); (void)playerTable_.emplace("isloop", std::bind(&PlayerDemo::GetLooping, this)); (void)playerTable_.emplace("speed", std::bind(&PlayerDemo::GetPlaybackSpeed, this));}MediaServiceFactory::GetInstance()返回的是MediaClient对象，所以CreateplayerService函数实际上是调用了MediaClient对应的办法。
std::shared_ptr<IPlayerService> MediaClient::CreatePlayerService(){ std::lock_guard<std::mutex> lock(mutex_); if (!IsAlived()) { MEDIA_LOGE("media service does not exist."); return nullptr; } sptr<IRemoteObject> object = mediaProxy_->GetSubSystemAbility( IStandardMediaService::MediaSystemAbility::MEDIA_PLAYER, listenerStub_->AsObject()); CHECK_AND_RETURN_RET_LOG(object != nullptr, nullptr, "player proxy object is nullptr."); sptr<IStandardPlayerService> playerProxy = iface_cast<IStandardPlayerService>(object); CHECK_AND_RETURN_RET_LOG(playerProxy != nullptr, nullptr, "player proxy is nullptr."); std::shared_ptr<PlayerClient> player = PlayerClient::Create(playerProxy); CHECK_AND_RETURN_RET_LOG(player != nullptr, nullptr, "failed to create player client."); playerClientList_.push_back(player); return player;}这个办法中次要通过PlayerClient::Create(playerProxy)办法创立了PlayerClient实例，并且将该实例一层层向上传，最终传给了PlayerImpl的playerService_变量，后续对于播放器的操作，PlayerImpl都是通过调用PlayerClient实例实现的。
int32_t PlayerImpl::Play(){ CHECK_AND_RETURN_RET_LOG(playerService_ != nullptr, MSERR_INVALID_OPERATION, "player service does not exist.."); MEDIA_LOGW("KPI-TRACE: PlayerImpl Play in"); return playerService_->Play();}int32_t PlayerImpl::Prepare(){ CHECK_AND_RETURN_RET_LOG(playerService_ != nullptr, MSERR_INVALID_OPERATION, "player service does not exist.."); MEDIA_LOGW("KPI-TRACE: PlayerImpl Prepare in"); return playerService_->Prepare();}int32_t PlayerImpl::PrepareAsync(){ CHECK_AND_RETURN_RET_LOG(playerService_ != nullptr, MSERR_INVALID_OPERATION, "player service does not exist.."); MEDIA_LOGW("KPI-TRACE: PlayerImpl PrepareAsync in"); return playerService_->PrepareAsync();}对于PlayerImpl来说，playerService_指向的PlayerClient就是具体的实现，PlayerClient的实现是通过IPC的近程调用来实现的，具体地是通过IPC中的proxy端向远端服务发动近程调用申请。
咱们以播放Play为例：
int32_t PlayerClient::Play(){ std::lock_guard<std::mutex> lock(mutex_); CHECK_AND_RETURN_RET_LOG(playerProxy_ != nullptr, MSERR_NO_MEMORY, "player service does not exist.."); return playerProxy_->Play();}int32_t PlayerServiceProxy::Play(){ MessageParcel data; MessageParcel reply; MessageOption option; if (!data.WriteInterfaceToken(PlayerServiceProxy::GetDescriptor())) { MEDIA_LOGE("Failed to write descriptor"); return MSERR_UNKNOWN; } int error = Remote()->SendRequest(PLAY, data, reply, option); if (error != MSERR_OK) { MEDIA_LOGE("Play failed, error: %{public}d", error); return error; } return reply.ReadInt32();}proxy端发送调用申请后，对应的Stub端会在PlayerServiceStub::OnRemoteRequest接管到申请，依据申请的参数进行对应的函数调用。播放操作对应的调用Stub的Play办法。
int32_t PlayerServiceStub::Play(){ MediaTrace Trace("binder::Play"); CHECK_AND_RETURN_RET_LOG(playerServer_ != nullptr, MSERR_NO_MEMORY, "player server is nullptr"); return playerServer_->Play();}这里最终是通过playerServer_调用Play函数。playerServer_在Stub初始化的时候通过PlayerServer::Create()形式来获取失去。也就是PlayerServer。
std::shared_ptr<IPlayerService> PlayerServer::Create(){ std::shared_ptr<PlayerServer> server = std::make_shared<PlayerServer>(); CHECK_AND_RETURN_RET_LOG(server != nullptr, nullptr, "failed to new PlayerServer"); (void)server->Init(); return server;}在媒体播放的整个过程中会波及到很多的状态，所以在Play中进行一些状态的判读后调用OnPlay办法。这个办法中发动了一个播放的工作。
int32_t PlayerServer::Play(){ std::lock_guard<std::mutex> lock(mutex_); if (lastOpStatus_ == PLAYER_PREPARED || lastOpStatus_ == PLAYER_PLAYBACK_COMPLETE || lastOpStatus_ == PLAYER_PAUSED) { return OnPlay(); } else { MEDIA_LOGE("Can not Play, currentState is %{public}s", GetStatusDescription(lastOpStatus_).c_str()); return MSERR_INVALID_OPERATION; }}int32_t PlayerServer::OnPlay(){ auto playingTask = std::make_shared<TaskHandler<void>>([this]() { MediaTrace::TraceBegin("PlayerServer::Play", FAKE_POINTER(this)); auto currState = std::static_pointer_cast<BaseState>(GetCurrState()); (void)currState->Play(); }); int ret = taskMgr_.LaunchTask(playingTask, PlayerServerTaskType::STATE_CHANGE); CHECK_AND_RETURN_RET_LOG(ret == MSERR_OK, ret, "Play failed"); lastOpStatus_ = PLAYER_STARTED; return MSERR_OK;}在播放工作中调用了PlayerServer::PreparedState::Play()
int32_t PlayerServer::PreparedState::Play(){ return server_.HandlePlay();}在Play外面间接调用PlayerServer的HandlePlay办法，HandlePlay办法通过playerEngine_调用到了gstreamer引擎，gstreamer是最终播放的实现。
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;}六、总结
本文次要对OpenHarmony 3.2 Beta多媒体子系统的媒体播放进行介绍，首先梳理了整体的播放流程，而后对播放的次要步骤进行了具体地剖析。
媒体播放次要分为以下几个档次：
（1） 提供给利用调用的Native接口，这个实际上通过OHOS::Media::PlayerFactory::CreatePlayer()调用返回PlayerImpl实例。
（2） PlayerClient，这部分通过IPC的proxy调用，向近程服务发动调用申请。
（3） PlayerServer，这部分是播放服务的实现端，提供给Client端调用。
（4） Gstreamer，这部分是提供给PlayerServer调用，真正实现媒体播放的性能。