Detect that "I'm running" in Qt GUI event thread

Question

I have this function to update some GUI stuff:

void SavedConnections::renderList()
{
  // Do GUI stuff! Must run in Qt thread!!!
    ...
}

I need to ensure that this function isn't called from other threads. What I plan to do is to defer it into event loop and raise a warning:

void SavedConnections::renderList()
{ 
  if(!this_thread_is_Qt_GUI_thread()) {
    qDebug()<< "Warning: GUI operation attempted from non GUI thread!\n";
    QCoreApplication::postEvent(this, new UpdateGUIEvent());
    return;
  }
  // Do GUI stuff! Must run in Qt thread!!!
    ...
}

This pattern is also very convenient to make methods that are guaranteed to run asynchronously in GUI thread without any ugly syntax. I already asked similar question about Java's ExecutorService.

Answer 1

You can check if the current thread is the thread your object lives in:

if (QThread::currentThread() != this->thread()) {
   // Called from different thread
}

Note that this might not be the main GUI-Thread! It is the thread this lives in (see QObject Thread affinity). If you don't change it using QObject::moveToThread, it is the thread the object was created in.

This is also what QCoreApplication::postEvent uses to determine into which thread the event should be posted. The targeted Thread must run a QEventLoop to respond to the event.

So checking for the main-GUI-Thread (qApp->thread()), but posting to this's thread might not work, if your object does not live in the main-GUI-Thread. However, if you do GUI stuff there, it should anyway live in the GUI-Thread