Generate combined virtual key presses to get chars like â, ó, ć, and more complex

I’m implementing a virtual keyboard using QML.

So far I literally map the char to print for the different key maps (w/o shift, with shift, with Alt Gr, etc.).

So I have to write down all the chars for different layouts somewhere in the code.

Especially for layouts and languages where two or more key presses generate a more complex char like pressing ` followed by a to get à becomes very nasty.

So I think this is a sub-optimal approach and there is a much cleverer solution to just provide the scan codes (as provided by ‘showkey -s’ in Linux) and to use the systems locale settings to get the correct char printed in let’s say a QQuick LineEdit.

What would be the best/correct way tho pass the scan codes to the system to generate “real” key events and how could I “receive” them in my code.

System is Linux. C++ backend would by fine. Guess QML alone won’t be capable … May It be clever to generate an own keyboard input device for this ?

Or could I inject the scan codes into the existing keyboard device to simulate the presses ?

Answer

Following code derived from

How to Generate Keyboard Input Using libevdev in C

using libevdev seems to point into the right direction.

It allows to send scan codes to a newly created keyboard device (needs root).

#include <stdio.h>
#include <libevdev/libevdev.h>
#include <libevdev/libevdev-uinput.h>
#include <unistd.h>

int main()
{
    int err;
    struct libevdev *dev;
    struct libevdev_uinput *uidev;

    dev = libevdev_new();
    libevdev_set_name(dev, "fake keyboard device");

    libevdev_enable_event_type(dev, EV_KEY);
    for(uint key = 1; key < 255; ++key) {
        libevdev_enable_event_code(dev, EV_KEY, key, nullptr);
    }

    err = libevdev_uinput_create_from_device(dev,
        LIBEVDEV_UINPUT_OPEN_MANAGED,
        &uidev);

    if (err != 0) {
        return err;
    }

    sleep(1);

    libevdev_uinput_write_event(uidev, EV_KEY, KEY_LEFTSHIFT, 1);

    libevdev_uinput_write_event(uidev, EV_KEY, KEY_A, 1);
    //libevdev_uinput_write_event(uidev, EV_SYN, SYN_REPORT, 0);
    libevdev_uinput_write_event(uidev, EV_KEY, KEY_A, 0);
    //libevdev_uinput_write_event(uidev, EV_SYN, SYN_REPORT, 0);

    libevdev_uinput_write_event(uidev, EV_KEY, KEY_B, 1);
    //libevdev_uinput_write_event(uidev, EV_SYN, SYN_REPORT, 0);
    libevdev_uinput_write_event(uidev, EV_KEY, KEY_B, 0);
    //libevdev_uinput_write_event(uidev, EV_SYN, SYN_REPORT, 0);


    libevdev_uinput_write_event(uidev, EV_KEY, KEY_LEFTSHIFT, 0);

    libevdev_uinput_write_event(uidev, EV_SYN, SYN_REPORT, 0);

    sleep(1);

    libevdev_uinput_destroy(uidev);
    printf("nCompleten");
}