how to handle global keys in Qt5.7 in ubuntu

Question

How to capture the global keys in Ubuntu in Qt application. I need to handle the keys like Ctrl, Shift even my qt application is minimized state also. Looks like LibQxt supports this. But as per my understanding this library won't have any support from Qt4. I am using Qt5.7. Do we have any other way to do this ?

Answer 1

This can be achieved using x11/xcb. The idea is to listen to a specific keyboard shortcut, system-wide, using the XGrabKey function from xlib, then catching the corresponding xcb event in the overridden nativeEventFilter method of a QAbstractNativeEventFilter subclass.

As an example, let's activate an application minimized window, using the Ctrl-A shortcut from anywhere.

The project must reference the x11extra qt module, and link the x11 library:

QT += x11extras
LIBS += -lX11

This is the filter header:

#include <QAbstractNativeEventFilter>
#include <QWidget>

class EventFilter : public QAbstractNativeEventFilter
{
public:
    void setup(QWidget *target);
    bool nativeEventFilter(const QByteArray &eventType, void *message, long *result);
private:
    int keycode;
    QWidget * target;
};

and this is the implementation:

#include <xcb/xcb.h>
#include <X11/Xlib.h>
#include <X11/Xutil.h>

#include <QtX11Extras/QX11Info>

void EventFilter::setup(QWidget *target)
{
    this->target = target;

    Display * display = QX11Info::display();
    unsigned int modifiers = ControlMask;
    keycode = XKeysymToKeycode(display, XK_A);
    XGrabKey(display, keycode, modifiers, DefaultRootWindow(display), 1, GrabModeAsync, GrabModeAsync);
}

bool EventFilter::nativeEventFilter(const QByteArray &eventType, void *message, long *)
{
    if (eventType == "xcb_generic_event_t")
    {
        xcb_generic_event_t* xcbevent = static_cast<xcb_generic_event_t *>(message);

        switch(xcbevent->response_type)
        {
        case XCB_KEY_PRESS:
            xcb_key_press_event_t * keypress_event = static_cast<xcb_key_press_event_t *>(message);
            if(keypress_event->state & XCB_MOD_MASK_CONTROL)
            {
                if(keypress_event->detail == keycode)
                {
                    qDebug() << "ACTIVATING ...";
                    target->activateWindow();
                }
            }
        }
    }
    return false;
}

In a main, we create a widget on the fly, instantiate, install and setup our filter.

#include "eventfilter.h"
#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    QWidget w;
    w.setGeometry(100, 100, 400, 300);
    w.show();

    EventFilter filter;
    a.installNativeEventFilter(&filter);
    filter.setup(&w);

    return a.exec();
}

If the user minimize the widget, or the widget is deactivated, the Ctrl-A shortcut should prompt it back to the foreground.

Please notice that the XGrabKey call from the setup method could fail if some other x client grabbed the same key combination, already.

Also notice that extra modifier masks will be delivered in the event state field if some lock keys are on (e.g. on my system the pressed caps lock yelds an extra XCB_MOD_MASK_LOCK, and the num lock an extra XCB_MOD_MASK_2).