QLabel not updating from slot call

Question

My QLabel is not updating from slot. I run sender class in separate thread using QObject::moveToThread:

QThread* serviceThread = new QThread;
service = new ExportService();
connect(service,SIGNAL(stateChanged(Service::ServiceState)),
        this,SLOT(statusChanged(Service::ServiceState)));
service->moveToThread(serviceThread);
serviceThread->start();

Service object send states by emiting signal with ServiceState enum value, this signal is captured by QDialog slot:

void Dialog::statusChanged(Service::ServiceState s)
{
    switch (s) {
    case Service::IDLE:
        qDebug() << "Idle";
        ui->label->setText("Service send response succesfully.");
        break;
    case Service::REQUESTING:
        qDebug() << "Requesting";
        ui->label->setText("Requesting response from service...");
        break;
    case Service::ERROR:
        qDebug() << "Error";
        ui->label->setText("Error. Cannot get response from service.");
        break;
    default:
        break;
    }
}

After operation on the object which emits a signal twice, the first time with value of Service::REQUESTING and second time with value of Service::IDLE my QLabel change text only to "Service send response succesfully.". In the console I can see that qDebug() << "Requesting"; works so the state changed successfully.

After comment out ui->label->setText("Service send response succesfully."); label has changed to requesting state but after the whole operation was done ie i see "Requesting" in console then "Idle" and after that QLabel has changed.

What should I do if I want to see QLabel changing in realtime?

How long is it in "requesting" state? Maybe it goes just too fast? Is the UI responsive in between? — Frank Osterfeld
I think the same like @FrankOsterfeld . It seems QLabel updates too fast and you can't fixed changes. — t3ft3l--i
You also need to explicitly specify Qt::QueuedConnection In your connect() call for objects moved to another thread. — anonymous
@FrankOsterfeld It is in "Requesting" state for ~1 second, UI is responsive when requesting response. I will try with more complex request and see if it works then. — Sajmplus
You may want to try and process the events in the event loop after setting the text of the label. See QApplication::processEvents(). — erl

user3528438 user3528438 · Accepted Answer · 2015-05-05T11:20:24

First, try adding update() after setText(), chances are setText() doesn't automatically schedule a repaint() for the QLabel, if it works then problem solved.

However, for update() function:

http://doc.qt.io/qt-4.8/qwidget.html#update

void QWidget::update()

This function does not cause an immediate repaint; instead it schedules a paint event for processing when Qt returns to the main event loop. This permits Qt to optimize for more speed and less flicker than a call to repaint() does.

> Calling update() several times normally results in just one paintEvent() call.

which basically says, if you call them too frequently and some of them will be optimized away.

If this is not the desired behaviour, try adding a forced repaint() after setText(), or using a timer to schedule periodical forced repaints.

http://doc.qt.io/qt-4.8/qwidget.html#repaint

UPDATE

As is mentioned in the comment, forcing repaint() isn't good solution.

This answer is intended to provide an analysis of the cause of the code's behaviour and the "forced repaint()" suggestion is more of a way to verify this analysis than a solution to the problem.

However, without further information on the purpose of the program, it's very difficult to provide further suggestions.

QLabel not updating from slot call

2 Answers