How can I check if the Shift key is pressed in Qt in real time?

Donald Duck

I have a QTimer object and I need to test if the Shift key is pressed in Qt every 30 milliseconds. I tried to use if(QGuiApplication::keyboardModifiers().testFlag(Qt::ShiftModifier)):

QObject::connect(timer, &QTimer::timeout, [=](){
    if(QGuiApplication::keyboardModifiers().testFlag(Qt::ShiftModifier)){
        //Do stuff
    }
    else{
        //Do something different
    }
}

The problem is that the result of the QGuiApplication::keyboardModifiers() isn't updated very often. After some testing, I noticed that it only updates when I press another key. So I tried to simulate pressing the Enter key:

QKeyEvent *event = new QKeyEvent(QKeyEvent::KeyPress, Qt::Key_Enter, Qt::NoModifier);
QCoreApplication::postEvent(someWidget, event);

This didn't change anything.

I would like a working way of checking in real time if the Shift key is pressed. I can't use events because I need this to work no matter what widget is visible. Any solution?

mrjj

@Donald-Duck said in How can I check if the Shift key is pressed in Qt in real time?:

I need this to work no matter what widget is visible. Any solution?

Hi
I was wondering if an event filter would work for you ?
http://doc.qt.io/qt-4.8/eventsandfilters.html#event-filters
You still have to install it to the widgets but no subclassing or extra code is otherwise
needed.

Donald Duck

No, it doesn't work. I tried the following code to see if it works:

bool SomeWidget::eventFilter(QObject *watched, QEvent *event){
    QMessageBox::information(NULL, "", "");
    return true;
}

As far as I've understood, the above code is supposed to open a message box whenever an event is triggered, but it doesn't do anything. I also tried with return false, but with no success. What's the problem?

mrjj

@Donald-Duck
Did you assign/ install it on the widgets ?
http://doc.qt.io/qt-4.8/qobject.html#installEventFilter

http://www.informit.com/articles/article.aspx?p=1405544&seqNum=2

Donald Duck

@mrjj Thanks, it worked.

Chris Kawa

You don't need to do all that. Use QGuiApplication::queryKeyboardModifiers() instead of QGuiApplication::keyboardModifiers().