QT Code for Reddit post: https://www.reddit.com/r/Qt5/comments/b6gppp/need_help_with_qobjectconnect_getting_started/?utm_source=share&utm_medium=web2x

backend.cpp

#include "backend.hpp"
#include "serialportreader.h"

#include <QtDebug>
#include <QSerialPortInfo>
#include <QSerialPort>



void Backend::connBtnPressed(const QString msg){
    qDebug() << msg;
    if(conBtnTxt == "Disconnect"){
        serial.close();
        conBtnTxt = "Connect";
        conBtnTxtChanged();
        return;
    }
    serial.setPortName(msg);
    serial.setBaudRate(QSerialPort::Baud9600);
    serial.setDataBits(QSerialPort::Data8);
    serial.setStopBits(QSerialPort::OneStop);
    serial.setParity(QSerialPort::NoParity);
    serial.setFlowControl(QSerialPort::NoFlowControl);

    if (!serial.open(QIODevice::ReadOnly))
    {
        qDebug() << "!!! Failed to open port " << msg <<
                     ". Error: " << serial.errorString();
        return;
    }
    qDebug() << "Port opened" ;

    SerialPortReader reader(&serial);

    conBtnTxt = "Disconnect";
    conBtnTxtChanged();

}


void Backend::updateList(){
    const auto infos = QSerialPortInfo::availablePorts();
    QStringList dataList;
    for (const QSerialPortInfo &info : infos)
        dataList.append(info.portName());
    if(!infos.count())
        dataList.append("No serial ports");

    portList = dataList;
    portListChanged();
}

backend.hpp

#ifndef BACKEND_HPP
#define BACKEND_HPP

#include <QObject>
#include <QtDebug>
#include <QtSerialPort>

#include "serialportreader.h"

class Backend : public QObject
{
    Q_OBJECT
    Q_PROPERTY(QStringList portList MEMBER portList NOTIFY portListChanged)
    Q_PROPERTY(QString conBtnTxt MEMBER conBtnTxt NOTIFY conBtnTxtChanged)
    QStringList portList;
    QString conBtnTxt;
    QStringList terminalText;


public:
    // explicitBackend(QObject *parent = nullptr);
    Backend() {
        conBtnTxt = "Connect";
        updateList();
    }

public slots:
    void connBtnPressed(const QString msg) ;
    void updateList();

signals:
    void portListChanged();
    void conBtnTxtChanged();

private:
    QSerialPort serial;

};

#endif // BACKEND_HPP

Debug output with lambda function

QML debugging is enabled. Only use this in a safe environment.
Styles ("Default", "Fusion", "Imagine", "Material", "Universal")
"ttyACM1"
Port opened
Read Callback
Data size: 36
"[CO2,6549, tVOC,936, millis,755262]\n"
Read Callback
Data size: 1
"\n"
Read Callback
Data size: 36
"[CO2,6207, tVOC,884, millis,756252]\n"
Read Callback
no serial port object QObject(0x0)
Read Callback
no serial port object QObject(0x0)

Program crashes after a few seconds

main.cpp

#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QQmlComponent>
#include <QDebug>
#include <QObject>

#include "backend.hpp"

#include <QQmlContext>
#include <QtQuickControls2>

int main(int argc, char *argv[])
{
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
    QGuiApplication app(argc, argv);
    QQuickStyle::setStyle("Fusion");
    qDebug() << "Styles" << QQuickStyle::availableStyles();

    QQmlApplicationEngine engine;
    
    /***** expose backend *****/
    Backend backend;
    engine.rootContext()->setContextProperty("backend", &backend);

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

    
    return app.exec();
}

serialportreader.cpp

/* Copyright (C) 2013-2015 Karl Phillip Buhr <karlphillip@gmail.com>
 *
 * This work is licensed under the Creative Commons Attribution-ShareAlike License.
 * To view a copy of this license, visit:
 *      https://creativecommons.org/licenses/by-sa/2.5/legalcode
 *
 * Or to read the human-readable summary of the license:
 *      https://creativecommons.org/licenses/by-sa/2.5/
 */
#include "serialportreader.h"
#include <iostream>

#include <QCoreApplication>
#include <QDebug>
#include <QByteArray>
#include <QTextCodec>
#include <QtDebug>

SerialPortReader::SerialPortReader(QSerialPort* serial, QObject* parent)
: QObject(parent), _serial(serial)
{
    // The magic call that makes QSerialPort outputs from a serial connected with Arduino
    _serial->setDataTerminalReady(true);

    
    // original calls 
//    QObject::connect(_serial, SIGNAL(readyRead()),
//                     SLOT(_read_callback()));

//    QObject::connect(_serial, SIGNAL(error(QSerialPort::SerialPortError)),
//                     SLOT(_error_callback(QSerialPort::SerialPortError)));

    QObject::connect(_serial, &QIODevice::readyRead, [&]{_read_callback();});

    //QObject::connect(_serial, &QSerialPort::errorOccurred,
    //                 &SerialPortReader::_error_callback);
}

SerialPortReader::~SerialPortReader()
{
}

void SerialPortReader::_read_callback()
{
    qDebug() << "Read Callback";
    if (!_serial){
        qDebug() << "no serial port object" << _serial;
        return;
    }

    QByteArray data = _serial->readLine();

    int sz = data.size();
    qDebug() << "Data size:" << sz;

    // To print data from other serial devices
    QString str = QString::fromUtf8(data);
    qDebug() << str;

}

void SerialPortReader::_error_callback(QSerialPort::SerialPortError error)
{
    if (error == QSerialPort::ReadError)
    {
        qDebug() << "!!! An I/O error occurred while reading the data from port " << _serial->portName() <<
                     ". Error: " << _serial->errorString();
        //QCoreApplication::exit(1);
    }
}

serialportreader.h

/* Copyright (C) 2013-2015 Karl Phillip Buhr <karlphillip@gmail.com>
 *
 * This work is licensed under the Creative Commons Attribution-ShareAlike License.
 * To view a copy of this license, visit:
 *      https://creativecommons.org/licenses/by-sa/2.5/legalcode
 *
 * Or to read the human-readable summary of the license:
 *      https://creativecommons.org/licenses/by-sa/2.5/
 */
#pragma once
#include <QByteArray>
#include <QObject>
#include <QtSerialPort/QSerialPort>


class SerialPortReader : public QObject
{
    Q_OBJECT

public:
    SerialPortReader(QSerialPort* serial, QObject* parent = nullptr);
    ~SerialPortReader();

private slots:
    void _read_callback();
    void _error_callback(QSerialPort::SerialPortError error);

private:
    QSerialPort* _serial;
};