Qt signal emitted but slot never called

Question

I want to connect two object(player and DownloadWebm) using slots and signals, but something strange happens. Here is my code :

  LogInWindow *log_in=new LogInWindow(cstr,centerX,centerY,start_byte,end_byte);
    QWidget::connect(log_in,SIGNAL(editingFinished()),&loop,SLOT(quit()));

    loop.exec();//stop executing bellow code till user input a movie id

    DownloadWebm *download_webm=nullptr;//download_webm is initialized in myThread constructor

    MyThread *DownloadWebm_Thread = new  MyThread(download_webm,cstr,start_byte,end_byte);

    DownloadWebm_Thread->start();//start the thread

    LinuxWebmPlayer *player = new LinuxWebmPlayer("video.webm",0);


    while(download_webm == nullptr){}//wait till download_webm is initialized

    qRegisterMetaType<Video_Bytes_Package>("Video_Bytes_Package");
    qRegisterMetaType<Info>("Info");

    QObject::connect(download_webm,SIGNAL(send_video_info(Info)),player,SLOT(set_video_info(Info)));
    QObject::connect(download_webm,SIGNAL(send_packege(Video_Bytes_Package)),player,SLOT(add_packeges(Video_Bytes_Package)));
    QObject::connect(download_webm,SIGNAL(send_finish_downloading(bool)),player,SLOT(video_is_commplet()));
    QObject::connect(download_webm,SIGNAL(send_one_cluster_recived()),player,SLOT(calculate_frames()));

  const bool connected =  QObject::connect(player,SIGNAL(yes()),download_webm,SLOT(slot()));
  emit player->yes();
  qDebug() << "Connection established?" << connected;//return true
  Q_ASSERT(connected);

When I connect download_webm to the player the slot receives the signal emitted from the download_webm but when I try to connect the player to download_webm, the connection is successful, the signal is emitted but the slot from download_webm is never called.

DownloadWebm is running in another thread.

Here is my declaration of signal in downloadwebm :

 class DownloadWebm : public QObject
{

    Q_OBJECT

public:
    DownloadWebm();
    DownloadWebm(char *link,const char *app_path,long x,long y);
    void start();
public slots:
    void slot();
    void first_package_analyzed(int,long);

 private:
  ................
  ................ 

  signals:
   void send_packege(Video_Bytes_Package package);
   void send_video_info(Info info);
   void send_finish_downloading(bool) ;

And player header :

   class LinuxWebmPlayer : public QMainWindow
{
    Q_OBJECT
public:

 LinuxWebmPlayer(char* argv,int pos);

signals:
    void send_cluster_size_frames(long);
    void yes();


public slots:
   void add_packeges(Video_Bytes_Package);
   void set_video_info(Info);

Does the thread where your DownloadWebm lives in is running an event loop. Doesn't look like it's the case. — chehrlic
Yes, you are right, I used a infinte loop in my object but that causes my object to freeze, after more research I find that I can use QEventLoop to avoid blocking the object, tanks! — Silviu Petrut

Silviu Petrut Silviu Petrut · Accepted Answer · 2020-04-23T18:43:55

I find the problem, I don't have an event loop in my DownloadWebm object so when the signal is emitted the object has finish executing .

Qt signal emitted but slot never called

1 Answers