Native Event Filters for Keyboard Events in C++ Qt Application

testnativefilters.cpp

/* Native Event Filters for Keyboard Events
 * Copyright © 2022 Pedro López-Cabanillas
 * 
 * testnativefilters.pro:
 * TEMPLATE = app
 * QT += core gui widgets
 * 
 * SOURCES += \
 *     testnativefilters.cpp
 * 
 * linux {
 *     CONFIG += link_pkgconfig
 *     PKGCONFIG += xcb
 * }
 */

#include <QAbstractNativeEventFilter>
#include <QApplication>
#include <QCheckBox>
#include <QDebug>
#include <QMainWindow>
#include <QKeyEvent>
#include <QLabel>
#include <QVBoxLayout>
#include <QWidget>

#if defined(Q_OS_WIN)
    #include <windows.h>
#elif defined(Q_OS_LINUX)
    #include <xcb/xcb.h>
#endif

class NativeFilter : public QAbstractNativeEventFilter
{
public:
#if QT_VERSION < QT_VERSION_CHECK(6,0,0)
    virtual bool nativeEventFilter(const QByteArray &eventType, void *message, long*) override
#else
    virtual bool nativeEventFilter(const QByteArray &eventType, void *message, qintptr*) override
#endif
    {
        if (eventType == "xcb_generic_event_t") {
    #if defined(Q_OS_LINUX)
            xcb_generic_event_t* ev = static_cast<xcb_generic_event_t *>(message);
            switch (ev->response_type & ~0x80) {
            case XCB_KEY_PRESS:
                {
                    xcb_key_press_event_t *kp = reinterpret_cast<xcb_key_press_event_t *>(ev);
                    qDebug() << "filtered key pressed:" << kp->detail;
                }
                return true;
            case XCB_KEY_RELEASE:
                {
                    xcb_key_release_event_t *kr = reinterpret_cast<xcb_key_release_event_t *>(ev);
                    qDebug() << "filtered key released:" << kr->detail;
                }
                return true;
            default:
                break;
            }
    #endif
        } else if (eventType == "windows_dispatcher_MSG" || eventType == "windows_generic_MSG") {
    #if defined(Q_OS_WIN)
            bool isRepeat = false;
            MSG* msg = static_cast<MSG *>(message);
            int scanCode = LOBYTE(HIWORD(msg->lParam));
            switch (msg->message) {
            case WM_KEYDOWN:
                qDebug() << "filtered key pressed:" << scanCode;
                return true;
            case WM_KEYUP:
                qDebug() << "filtered key released:" << scanCode;
                return true;
            default:
                break;
            }
    #endif
        }
        return false;
    }
};

class MainWindow : public QMainWindow
{
public:
    MainWindow(QWidget *parent = 0) : QMainWindow(parent)
    {
        QWidget *widget = new QWidget(this);
        this->setCentralWidget(widget);
        QVBoxLayout *layout = new QVBoxLayout(widget);
        QLabel *lbl = new QLabel(this);
        layout->addWidget(lbl);
        lbl->setAlignment(Qt::AlignCenter | Qt::AlignVCenter);
        lbl->setText(QString("Qt: %1<br/>platform: %2").arg(qVersion(), qApp->platformName()));
        QCheckBox *chk = new QCheckBox(this);
        layout->addWidget(chk);
        chk->setText("Use app level event filter");
        connect(chk, &QCheckBox::stateChanged, qApp, [=](int state){
            if (state == Qt::Checked) {
                qApp->installNativeEventFilter(&m_filter);
            } else {
                qApp->removeNativeEventFilter(&m_filter);
            }
        });
    }

protected:
    void keyPressEvent(QKeyEvent *event) override
    {
        qDebug() << "unfiltered key pressed:"
                 << event->nativeScanCode();
    }

    void keyReleaseEvent(QKeyEvent *event) override
    {
        qDebug() << "unfiltered key released:"
                 << event->nativeScanCode();
    }

private:
    NativeFilter m_filter;
};

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();
    return a.exec();
}