How to subclass QIODevice and read /dev/input/eventX - qt

Question

I have read the QIODevice doc, but still don't know how to archive that.
What I want to do is to create a KeyBoard class deriving from QIODevice. which opens /dev/input/eventX. I hope that my code can use the readyRead() signal of KeyBoard. (QFile does not emit readyRead() signal)
class KeyBoard : public QIODevice {
public:
KeyBoard();
~KeyBoard();
protected:
qint64 readData(char *data, qint64 size);
qint64 writeData(const char *data, qint64 size);
};
What do I need to do in readData() and writeData()?
And how does my code use this class? (I just use the QCoreApplication, no gui)

Answer 1

Use QSocketNotifier on the open file handle. You can read from the device using QFile,or abuse QSerialPort, i.e. QSerialPort m_port{"input/eventX"}. See this answer for an example of using QSocketNotifier with stdin; /dev/input/eventX requires a similar approach.
Here's an example that works on /dev/stdio, but would work identically on /dev/input/eventX.
// https://github.com/KubaO/stackoverflown/tree/master/questions/dev-notifier-49402735
#include <QtCore>
#include <fcntl.h>
#include <boost/optional.hpp>
class DeviceFile : public QFile {
Q_OBJECT
boost::optional<QSocketNotifier> m_notifier;
public:
DeviceFile() {}
DeviceFile(const QString &name) : QFile(name) {}
DeviceFile(QObject * parent = {}) : QFile(parent) {}
DeviceFile(const QString &name, QObject *parent) : QFile(name, parent) {}
bool open(OpenMode flags) override {
return
QFile::isOpen()
|| QFile::open(flags)
&& fcntl(handle(), F_SETFL, O_NONBLOCK) != -1
&& (m_notifier.emplace(this->handle(), QSocketNotifier::Read, this), true)
&& m_notifier->isEnabled()
&& connect(&*m_notifier, &QSocketNotifier::activated, this, &QIODevice::readyRead)
|| (close(), false);
}
void close() override {
m_notifier.reset();
QFile::close();
}
};
int main(int argc, char **argv) {
QCoreApplication app{argc, argv};
DeviceFile dev("/dev/stdin");
QObject::connect(&dev, &QIODevice::readyRead, [&]{
qDebug() << "*";
if (dev.readAll().contains('q'))
app.quit();
});
if (dev.open(QIODevice::ReadOnly))
return app.exec();
}
#include "main.moc"