webrtc 的TaskQueue() 任务队列

TaskQueue 定义

见文件：rtc_base\task_queue.h

具体实现

class RTC_LOCKABLE RTC_EXPORT TaskQueue {public:// TaskQueue priority levels. On some platforms these will map to thread// priorities, on others such as Mac and iOS, GCD queue priorities.using Priority = ::webrtc::TaskQueueFactory::Priority;// 注意这个构造函数，以TaskQueueBase智能指针作为参数// TaskQueue 的真正实现，其实是这个TaskQueueBase explicit TaskQueue(std::unique_ptr<webrtc::TaskQueueBase,webrtc::TaskQueueDeleter> task_queue);~TaskQueue();}

创建一个 TaskQueue

  //首先声明相应对象：一个任务队列工厂、一个任务队列对象std::shared_ptr<webrtc::TaskQueueFactory> video_encoder_task_queue_factory_;rtc::TaskQueue video_encoder_queue_;//在指定类构造函数列表中创建任务队列//假如我们的调用类是VideoHandler//具体的实现如下：VideoHandbler::VideoHandbler():  video_encoder_task_queue_factory_(webrtc::CreateDefaultTaskQueueFactory()),video_encoder_queue_(video_encoder_task_queue_factory_->CreateTaskQueue("VideoEncoderQueue",TaskQueueFactory::Priority::NORMAL)){}
//具体过程：
//首先创建一个默认的任务队列工厂
//然后基于任务队列工厂创建一个任务队列TaskQueueBase，
//这个新创建的任务队列工厂TaskQueueBase，作为构造函数参数传给了 TaskQueue()，从而创建了我们需要的video_encoder_queue_//TaskQueue的构造函数，以TaskQueueBase的智能指针作为参数
//通过成员变量impl_ 接收该指针
//后面会发现，任务最终都传给了TaskQueueBase()TaskQueue::TaskQueue(std::unique_ptr<webrtc::TaskQueueBase, webrtc::TaskQueueDeleter> task_queue): impl_(task_queue.release()) {}//向TaskQueue投递任务时，最终还是通过 imple_抛给了TaskQueueBase()
void TaskQueue::PostTask(std::unique_ptr<webrtc::QueuedTask> task) {return impl_->PostTask(std::move(task));
}

TaskQueueLibevent

//每个平台都有各自的实现
//我们主要看TaskQueueLibevent()的实现

//TaskQueueLibevent 继承自TaskQueueBase()，真正处理Task的函数
class TaskQueueLibevent final : public TaskQueueBase {
public:TaskQueueLibevent(absl::string_view queue_name, rtc::ThreadPriority priority);void Delete() override;void PostTask(std::unique_ptr<QueuedTask> task) override;void PostDelayedTask(std::unique_ptr<QueuedTask> task,uint32_t milliseconds) override;
private:bool is_active_ = true;//输入、输出管道用来唤起线程int wakeup_pipe_in_ = -1;int wakeup_pipe_out_ = -1;event_base* event_base_;event wakeup_event_;//任务队列线程//一个任务队列对象一个线程//对于windows平台内部会执行 CreateThread()创建线程//android调用 pthread_create()创建线程rtc::PlatformThread thread_;Mutex pending_lock_;absl::InlinedVector<std::unique_ptr<QueuedTask>, 4> pending_RTC_GUARDED_BY(pending_lock_);// Holds a list of events pending timers for cleanup when the loop exits.std::list<TimerEvent*> pending_timers_;//具体的实现部分TaskQueueLibevent::TaskQueueLibevent(absl::string_view queue_name,rtc::ThreadPriority priority): event_base_(event_base_new()),thread_(&TaskQueueLibevent::ThreadMain, this, queue_name, priority) {int fds[2];//创建一个管道用于线程之间通信RTC_CHECK(pipe(fds) == 0);//把管道设置诶非阻塞的SetNonBlocking(fds[0]);SetNonBlocking(fds[1]);wakeup_pipe_out_ = fds[0];wakeup_pipe_in_ = fds[1];//绑定管道可读事件，当管道可读时会自动调用 OnWakeup//这块是借用libevent库来实现的EventAssign(&wakeup_event_, event_base_, wakeup_pipe_out_,EV_READ | EV_PERSIST, OnWakeup, this);event_add(&wakeup_event_, 0);thread_.Start();
}
//调用 fcntl把管道设置诶非阻塞
bool SetNonBlocking(int fd) {const int flags = fcntl(fd, F_GETFL);RTC_CHECK(flags != -1);return (flags & O_NONBLOCK) || fcntl(fd, F_SETFL, flags | O_NONBLOCK) != -1;
}//线程函数一直轮训相关事件
void TaskQueueLibevent::ThreadMain(void* context) {TaskQueueLibevent* me = static_cast<TaskQueueLibevent*>(context);{CurrentTaskQueueSetter set_current(me);while (me->is_active_)event_base_loop(me->event_base_, 0);}for (TimerEvent* timer : me->pending_timers_)delete timer;
}

向任务队列中抛入任务

void TaskQueueLibevent::PostTask(std::unique_ptr<QueuedTask> task) {{//向队列投递任务时，先加锁MutexLock lock(&pending_lock_);bool had_pending_tasks = !pending_.empty();//把任务插入到队列中pending_.push_back(std::move(task));// Only write to the pipe if there were no pending tasks before this one// since the thread could be sleeping. If there were already pending tasks// then we know there's either a pending write in the pipe or the thread has// not yet processed the pending tasks. In either case, the thread will// eventually wake up and process all pending tasks including this one.//如果当前线程有未处理的阻塞任务，就暂时不唤醒if (had_pending_tasks) {return;}}// Note: This behvior outlined above ensures we never fill up the pipe write// buffer since there will only ever be 1 byte pending.//写入的内容是KRunTasks，表明是要处理的Task任务char message = kRunTasks;//向管道中写入数据，唤起线程RTC_CHECK_EQ(write(wakeup_pipe_in_, &message, sizeof(message)),sizeof(message));
}

管道中写入数据后，会触发OnWakeup()函数进行处理

void TaskQueueLibevent::OnWakeup(int socket,short flags,  // NOLINTvoid* context) {TaskQueueLibevent* me = static_cast<TaskQueueLibevent*>(context);RTC_DCHECK(me->wakeup_pipe_out_ == socket);char buf;// 调用 read 函数从管道中读入数据//读到数据后基于相应的数据类型进行处理RTC_CHECK(sizeof(buf) == read(socket, &buf, sizeof(buf)));switch (buf) {case kQuit:me->is_active_ = false;event_base_loopbreak(me->event_base_);break;case kRunTasks: {//要处理的任务absl::InlinedVector<std::unique_ptr<QueuedTask>, 4> tasks;{//加锁//并取出要处理的任务MutexLock lock(&me->pending_lock_);tasks.swap(me->pending_);}RTC_DCHECK(!tasks.empty());//遍历所有的task，并调用各自的Run()函数进行处理//最终就是回调传进来任务函数for (auto& task : tasks) {if (task->Run()) {task.reset();} else {// |false| means the task should *not* be deleted.task.release();}}break;}default:RTC_NOTREACHED();break;}
}

使用例子，向任务队列投入任务

向队列中投入一个lambda 任务,
当任务线程接收到该任务后，就会回调该lambda函数

 video_encoder_queue_.PostTask([this,&video_data]{....encoder(video_data);....});

视频编码队列 encoder_queue_

encoder_queue_ 就是利用前面讲到TaskQueue进行视频编码的

具体实现：

//声明编码队列
rtc::TaskQueue encoder_queue_;
//构造函数列表中创建该编码队列
VideoStreamEncoder::VideoStreamEncoder()://创建编码队列encoder_queue_(task_queue_factory->CreateTaskQueue("EncoderQueue",TaskQueueFactory::Priority::NORMAL)){}//向编码队列中投入视频数据进行编码
//所以真正的编码线程，就是encoder_queue所拥有的线程了
void VideoStreamEncoder::OnFrame(const VideoFrame& video_frame) {encoder_queue_.PostTask([this, incoming_frame, post_time_us, log_stats, post_interval_us]() {if (posted_frames_waiting_for_encode == 1 && !cwnd_frame_drop) {MaybeEncodeVideoFrame(incoming_frame, post_time_us);}}
}

析构函数会主动关闭任务队列

析构函数中会自动关闭该任务队列，并停止对应任务线程，不需要我们干预

TaskQueue::~TaskQueue() {// There might running task that tries to rescheduler itself to the TaskQueue// and not yet aware TaskQueue destructor is called.// Calling back to TaskQueue::PostTask need impl_ pointer still be valid, so// do not invalidate impl_ pointer until Delete returns.impl_->Delete();
}void TaskQueueLibevent::Delete() {RTC_DCHECK(!IsCurrent());struct timespec ts;char message = kQuit;//向管道中写入关闭消息while (write(wakeup_pipe_in_, &message, sizeof(message)) != sizeof(message)) {// The queue is full, so we have no choice but to wait and retry.RTC_CHECK_EQ(EAGAIN, errno);ts.tv_sec = 0;ts.tv_nsec = 1000000;nanosleep(&ts, nullptr);}//停止线程thread_.Stop();event_del(&wakeup_event_);IgnoreSigPipeSignalOnCurrentThread();//关闭管道close(wakeup_pipe_in_);close(wakeup_pipe_out_);wakeup_pipe_in_ = -1;wakeup_pipe_out_ = -1;event_base_free(event_base_);delete this;
}