Intercept QML keys on C++ side.

egor.utsov

Hi all. Is it possible to intercept keyboard events on C++ side with UI in QML? I've already tried to install event filter on whole QApplication::instance() and on QQMLEngine particulary, it didn't help to catch keyboard events.

My main.cpp looks like:

int main(int argc, char* argv[])
{
    QApplication app(argc, argv);
    QQmlApplicationEngine engine;
    engine.installEventFilter(new KeysInterceptor());

    engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
    if (engine.rootObjects().isEmpty())
        return -1;
    return app.exec();
}

Interceptor:

class KeysInterceptor : public QObject
{
    Q_OBJECT
public:
    explicit KeysInterceptor(QObject *parent = nullptr);


    // QObject interface
public:
    virtual bool event(QEvent *event) override;
    virtual bool eventFilter(QObject *watched, QEvent *event) override;
};

dheerendra

You can do it. You can install filter on the app itself & try to do something. You can do is one part. I'm wondering any reason to get into this hack ?

egor.utsov

@dheerendra As i said, i tried already to install event filter for whole application, but QKeyEvents not appear. I need to black the screen by timer, that count after last key was pressed. In other words - if device is idle for some time, screen should be shutdown.

dheerendra

How do you find that device is idle ? Can you explain ? Based on this there can be another solution.
Coming to filters -
We can make it work. Show me your code inside the eventFilter. Did you install the filter for QApplication object ?

egor.utsov

@dheerendra Every key press should reset timer inside filter object. Once timer fired - signal generated and device black out screen.

Interceptor:

bool KeysInterceptor::event(QEvent *event)
{
    qDebug() << __FUNCTION__ << ":" << event->type();
    return QObject::event(event);
}


bool KeysInterceptor::eventFilter(QObject *watched, QEvent *event)
{
    qDebug() << __FUNCTION__ << ":" << watched << " - " << event->type();
    return QObject::eventFilter(watched, event);
}

dheerendra

with this code for which object you have installed the filter ? Is it for engine or app object ?

egor.utsov

@dheerendra I tried both ways below:

QApplication::instance()->installEventFilter(new KeysInterceptor);

QQmlApplicationEngine engine;
engine.installEventFilter(new KeysInterceptor);

dheerendra

@egor.utsov said in Intercept QML keys on C++ side.:

app

Just try..

KeysInterceptor *keyI = new KeysInterceptor;
app.installEventFilter(keyI);

Remove event(...) method.

egor.utsov

@dheerendra
main.cpp:

QApplication app(argc, argv);
KeysInterceptor *ki = new KeysInterceptor;
app.installEventFilter(ki);

KeyInterceptor.cpp:

bool KeysInterceptor::eventFilter(QObject *watched, QEvent *event)
{
    if (event->type() == QEvent::Type::KeyPress || event->type() == QEvent::KeyRelease)
        qDebug() << __FUNCTION__ << ":" << watched << " - " << event->type();

    return QObject::eventFilter(watched, event);
}

egor.utsov

@dheerendra i just checked it on desktop and it works like a charm

dheerendra

This should work. What is the issue ? Sorry i missed seeing your response.
Question - To blackout your screen are you dependent on the key events ?

egor.utsov

@dheerendra yes. Blackout should happen if no activity on keyboard is observed for ten minutes for example

dheerendra

You can do this with simple timer which fires after 10 minutes. Inside the handler just check whether key event happened. I'm not in favor of event filter in QApplication becz, every event goes there, checks & then responds. It is risky & performance hit.

egor.utsov

@dheerendra how i can save time of last key event in this case?

dheerendra

you can use the following

Window {
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")
    property date startDate :new Date();
    property int mils;
    Timer{
        id : tim
        interval: 10000
        repeat: true
        running: true
        onTriggered: {
            var end  = new Date();
            var elapsed = end.getTime() - startDate.getTime()
            if (elapsed>=10000){
                console.log("Blackout")
            }else {
                console.log("No blackout")
                startDate = new Date()
            }
        }
    }

    Rectangle{
        anchors.fill: parent
        focus: true
        objectName: "Dheerendra"
        Keys.onPressed: {
            console.log("Key is pressed")
            startDate = new Date();
        }
    }
}

egor.utsov

@dheerendra i have many places, where key processed. It is hard to put such code everywhere

dheerendra

It can be componetized as well. If it is everywhere, catch at the source only with qApp.

egor.utsov

@dheerendra once again - key events does not appear in filter. Think it is worth to mention that i use linuxfb plugin withOUT libinput.

dheerendra

basic question - In your program are you able to catch/handle the keyboard events ?

egor.utsov

@dheerendra qml ui react to key presses.