How do I create a Window in different QT threads?

Question

I have an application in which each thread (except the main thread) needs to create its own window. I tried creating a thread and then calling this->exec() in the run function. However, I get an error before I even get to that call: ASSERT failure in QWidget: "Widgets must be created in the GUI thread."

I want to popup a message window. The problem is that the source has multiple threads each of which may need to popup its own message.

Answer 1

If you need to create QWidget(or some other gui component(s)) in different(non-main) thread(s) you can implement it in such way:

• Create simple wrapper which holds gui component:

  // gui component holder which will be moved to main thread
  class gui_launcher : public QObject
  {
    QWidget *w;
    // other components
    //..
  public:
    virtual bool event( QEvent *ev )
    {   
      if( ev->type() == QEvent::User )
      {
        w = new QWidget;
        w->show();
        return true;
      }
      return false;
    }
  };
  

• create QApplication object in main thread

• another thread body:

  ..
    // create holder
    gui_launcher gl;
    // move it to main thread
    gl.moveToThread( QApplication::instance()->thread() );
    // send it event which will be posted from main thread
    QCoreApplication::postEvent( &gl, new QEvent( QEvent::User ) );
  ..
  

• be happy, :)

Answer 2

Qt will only let you create GUI elements in the GUI thread - what is it that you need to display from the other threads? See something like This answer for an example of updating a progress bar with data from a non-GUI thread.

Update:

If you want to show a message for each window, you can have a class like this:

class MyWorkerThread : public QThread
{
  Q_OBJECT
signals:
  void sendMessage(QString msg);
private:
  void run()
  {
    /* do stuff */
    emit sendMessage(QString("This thread is doing stuff!"));
    /* do more stuff */
  }
};

Then connect it up to your GUI via the signal-slot mechanism with something like:

connect(workerThread, SIGNAL(sendMessage(QString)),
        guiController, SLOT(showMessageBox(QString)));

Where the showMessageBox function does what you need it to do.

Answer 3

I don't believe this is possible. Other non-GUI components can run in other threads and will usually communicate via the signal/slot mechanisms.