Qt5 multi-threading: signals work in one-way only - c++

Question

Here: Qt multi-thread with GUI I've learned how to create a background worker (the Engine class).
In that class, I have a QSerialPort object, that runs in the main thread (see How to setup QSerialPort on a separate thread?).
I'm using the signal/slot mechanism to send/receive data. But it works in one-way only. The signals emitted from the Engine object ("worker thread") are received by the QSerialPort ("main thread"). The viceversa doesn't work: any signal emitted from QSerialPort is not received by the Engine.
engine.h
#ifndef ENGINE_H
#define ENGINE_H
#include <QObject>
#include <QTimer>
#include "myserial.h"
class Engine : public QObject
{
Q_OBJECT
public:
explicit Engine(QObject *parent = 0);
private:
QTimer m_timer;
MySerial m_serial;
signals:
void serialSendMessage(QByteArray data);
private slots:
void lineReceived(QByteArray line);
void foo();
public slots:
void run();
void open(QString port, quint32 baudrate);
void close();
};
#endif // ENGINE_H
engine.c
#include "engine.h"
#include <QDebug>
Engine::Engine(QObject *parent) : QObject(parent)
{
connect(&m_timer, &QTimer::timeout, this, &Engine::foo);
m_timer.setInterval(200);
}
// THIS IS NEVER EXECUTED!
void Engine::lineReceived(QByteArray line)
{
qDebug() << line;
}
// THIS IS RECEIVED BY QSERIALPORT
void Engine::foo()
{
emit serialSendMessage("Hello World!");
}
void Engine::run()
{
// THIS DOESN'T WORK!
connect(&m_serial, &MySerial::lineReceived, this, &Engine::lineReceived);
// THIS WORK!
connect(this, &Engine::serialSendMessage, &m_serial, &MySerial::sendMessage);
}
void Engine::open(QString port, quint32 baudrate)
{
m_serial.open(port, baudrate);
QTimer::singleShot(0, &m_timer, static_cast<void (QTimer::*)(void)>(&QTimer::start));
}
void Engine::close()
{
m_serial.close();
}
MySerial.h
#ifndef MYSERIAL_H
#define MYSERIAL_H
#include <QObject>
#include <QtSerialPort/QSerialPort>
#include <QtSerialPort/QSerialPortInfo>
class MySerial : public QSerialPort {
Q_OBJECT
public:
explicit MySerial(QObject *parent = 0);
bool open(QString port, quint32 baudrate);
using QSerialPort::open;
QByteArray sendMessage(QByteArray data, bool nmea);
signals:
void lineReceived(QByteArray line);
private slots:
void onReadyRead();
};
#endif // MYSERIAL_H
MySerial.c
#include "myserial.h"
#include <QDebug>
MySerial::MySerial(QObject *parent) : QSerialPort(parent) {
}
bool MySerial::open(QString port, quint32 baudrate)
{
disconnect(this, 0, 0, 0);
connect(this, &FemtoSerial::readyRead, this, &MySerial::onReadyRead);
setPortName(port);
if (!open(QIODevice::ReadWrite)) return false;
setDataBits(QSerialPort::Data8);
setParity(QSerialPort::NoParity);
setStopBits(QSerialPort::OneStop);
setBaudRate(baudrate);
setFlowControl(QSerialPort::NoFlowControl);
return true;
}
void MySerial::onReadyRead() {
static QList<QByteArray> lines;
static QByteArray buffer;
buffer += readAll();
int index = buffer.indexOf("\r");
while (index != -1) {
lines.append(buffer.left(index + 1));
buffer = buffer.mid(index + 1);
index = buffer.indexOf("\r");
}
// THIS SIGNAL IS EMITTED!
while (!lines.isEmpty()) emit lineReceived(lines.takeFirst());
}
QByteArray MySerial::sendMessage(QByteArray data) {
write(data);
return data;
}
EDIT
Trying to add a QEventLoop:
void Engine::run()
{
QEventLoop loop;
connect(&m_serial, &MySerial::lineReceived, this, &Engine::lineReceived);
connect(this, &Engine::serialSendMessage, &m_serial, &MySerial::sendMessage);
loop.exec();
}
The behavior is the same: the data is sent, but the receiving slot is never executed.

Answer 1

bool MySerial::open(QString port, quint32 baudrate)
{
//<s>disconnect(this, 0, 0, 0);</s> // <--- strikeout
connect(this, &FemtoSerial::readyRead, this, &MySerial::onReadyRead);
// ...
}
From the documentation:
Disconnect everything connected to an object's signals
disconnect(myObject, 0, 0, 0);
it means from the signals of myObject not the other way round.
That line prevents the slot in Engine to be executed because it has just disconnected from the source signal.
Engine might disconnect when the port closes, to avoid multiple connections on the next opening.