作者：巴延兴
一、简介
媒体子系统为开发者提供一套接口，方便开发者使用系统的媒体资源，主要包含音视频开发、相机开发、流媒体开发等模块。每个模块都提供给上层应用对应的接口，本文会对音视频开发中的音视频播放框架做一个详细的介绍。
二、目录
foundation/multimedia/media_standard
├── frameworks #框架代码
│ ├── js
│ │ ├── player
│ ├── native
│ │ ├── player #native实现
│ └── videodisplaymanager #显示管理
│ ├── include
│ └── src
├── interfaces
│ ├── inner_api #内部接口
│ │ └── native
│ └── kits #外部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接口使用
OpenHarmony系统中，音视频播放通过N-API接口提供给上层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++直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();
}
首先根据RunCase可以大致了解一下播放音视频的主要流程，创建播放器，设置播放源，设置回调方法（包含播放过程中的多种状态的回调），设置播放显示的Surface，这些准备工作做好之后，需要调用播放器的PrepareASync方法，这个方法完成后，播放状态会变成Prepared状态，这时就可以调用播放器的play接口，进行音视频的播放了。 RegisterTable()方法中，将字符串和对应的方法映射到Map中，这样后续的DoNext会根据输入的命令，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方法中核心的代码是func()的调用，这个func就是之前注册进Map中字符串对应的方法，在RegisterTable方法中将空字符串""和"play"对绑定为Player::Play方法，默认不输入命令参数时，是播放操作。
五、调用流程
本段落主要针对媒体播放的框架层代码进行分析，所以在流程中涉及到了IPC调用相关的客户端和服务端，代码暂且分析到调用gstreamer引擎。 首先Sample通过PlayerFactory创建了一个播放器实例（PlayerImpl对象），创建过程中调用Init函数。
int32_t PlayerImpl::Init()
{
playerService_ = MediaServiceFactory::GetInstance().CreatePlayerService();
CHECK_AND_RETURN_RET_LOG(playerService_ != nullptr, MSERR_UNKNOWN, "failed to create player service");
return MSERR_OK;
}
MediaServiceFactory::GetInstance()返回的是MediaClient对象，所以CreateplayerService函数实际上是调用了MediaClient对应的方法。
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;
}
这个方法中主要通过PlayerClient::Create(playerProxy)方法创建了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的远程调用来实现的，具体地是通过IPC中的proxy端向远端服务发起远程调用请求。 我们以播放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端发送调用请求后，对应的Stub端会在PlayerServiceStub::OnRemoteRequest接收到请求，根据请求的参数进行对应的函数调用。播放操作对应的调用Stub的Play方法。
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()方式来获取得到。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;
}
最终我们的Play调用到了PlayerServer的Play()。在媒体播放的整个过程中会涉及到很多的状态，所以在Play中进行一些状态的判读后调用OnPlay方法。这个方法中发起了一个播放的任务。
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;
}
在播放任务中调用了PlayerServer::PreparedState::Play()
int32_t PlayerServer::PreparedState::Play()
{
return server_.HandlePlay();
}
在Play里面直接调用PlayerServer的HandlePlay方法，HandlePlay方法通过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
本文主要对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，然后对播放的主要步骤进行了详细地分析。 媒体播放主要分为以下几个层次： （1） 提供给应用调用的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，向远程服务发起调用请求。 （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功能。
更多原创内容请关注：深开鸿技术团队
入门到精通、技巧到案例，系统化分享OpenHarmony开发技术，欢迎投稿和订阅，让我们一起携手前行共建生态。