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.

Don't attempt to show UI from the background/worker threads. Instead show that UI from the main GUI thread. Arrange for your background/worker to signal the GUI thread in order to get this done. This example shows how to do it: http://doc.qt.nokia.com/4.7-snapshot/thread-basics.html#example-3-clock. — David Heffernan, Mar 19 '12 at 21:48

score 24 · Accepted Answer · edited Apr 18 '16 at 23:35

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, :)

Hello I made some small improvement just for fun template class gui_launcher : public QObject { mygui *w; // other components public: void add2Gui() { this->moveToThread( QApplication::instance()->thread() ); QCoreApplication::postEvent( this, new QEvent( QEvent::User ) ); } virtual bool event( QEvent *ev ) { if( ev->type() == QEvent::User ) { w = new mygui; w->show(); return true; } return false; } }; — jamk, Dec 11 '12 at 12:44

score 7 · Answer 2 · edited May 23 '17 at 12:26

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.

Here is a full example of calling a QMessageBox from a QThread. It is very similar to your example. https://stackoverflow.com/a/23992291/999943 — phyatt, Sep 14 '17 at 17:03

score 1 · Answer 3 · answered Mar 19 '12 at 21:17

1

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.

answered Mar 19 '12 at 21:17

Tom Kerr

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Masood Khaari · Answer 4 · 2013-03-11T09:38:13.523

0

The above answers can be combined with a QAction object (or custom class objects) to transfer any action to the main GUI thread to be executed, not just creating widgets or displaying a message box. (e.g. by emitting sendAction(QAction*), or implementing a custom QEvent class embodying a QAction*.)

edited Mar 11 '13 at 09:38

answered Mar 10 '13 at 07:42

Masood Khaari

2,911
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How do I create a Window in different QT threads?

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