QSerialPort readings refresh too fast for the Qt widget - c++

Question

I am using QSerialPort to read from a device connected to a COM port on my computer, and it sends characters every half a second to my computer. I can read them from the qDebug window, so I know the connection works and Qt receives the data.
However I continuously read from the serial port and refresh a label widget on my GUI. The label becomes blank when I run the app, I think this problem is caused by the label name constantly refreshing.
My QserialPort is managed in the mainwindow constructor, closed in destructor, and the readings are done in a function called serialReceived(), which I believe is called (or causes the label to refresh) too often
MainWindow::MainWindow(QWidget *parent)
: QMainWindow(parent)
, ui(new Ui::MainWindow)
{
ui->setupUi(this);
serial = new QSerialPort(this);
qDebug() << "nb ports: " << QSerialPortInfo::availablePorts().length();
foreach(const QSerialPortInfo &serialPortInfo, QSerialPortInfo::availablePorts())
{
qDebug() << "name" << serialPortInfo.portName();
}
serial->setPortName("COM11");
serial->setBaudRate(QSerialPort::Baud9600);
serial->setDataBits(QSerialPort::Data8);
serial->setParity(QSerialPort::NoParity);
serial->setStopBits(QSerialPort::OneStop);
serial->setFlowControl(QSerialPort::NoFlowControl);
qDebug() << "is " << serial->open(QSerialPort::ReadOnly);
qDebug() << "err " << serial->error();
//serial->write("ok");
// Create the signal and slot
connect(serial, SIGNAL(readyRead()), this, SLOT(serialReceived()));
}
MainWindow::~MainWindow()
{
delete ui;
serial->close(); // instance is closed when mainwindow destroyed
}
void MainWindow::serialReceived()
{
QByteArray ba;
ba = serial->readAll();
ui->label->setText(serial->readAll());
qDebug()<<ba;
}

Answer 1

void MainWindow::serialReceived()
{
QByteArray ba;
ba = serial->readAll();
ui->label->setText(serial->readAll());
qDebug()<<ba;
}
You're first reading the data into ba, then you try to read again but since readAll() already read the data there is nothing left. You want
void MainWindow::serialReceived()
{
QByteArray ba = serial->readAll();
ui->label->setText(ba);
qDebug() << ba;
}

Answer 2

You just can read data at any time you want, not only by readyRead signal. The QSerialPort class will buffer all received data until you read it.
You also can append every received part of data to some scrollable QPlainTextEdit. I recommend this way.
void MainWindow::serialReceived()
{
QByteArray ba;
ba = serial->readAll();
ui->plainTextEdit->appendPlainText(ba);
}
Using timer:
connect(&m_timer, &QTimer::timeout, this, &MyClass::onTimer);
...
m_timer->start(5000);
...
void MyClass::onTimer()
{
if(serial->bytesAvailable() > 0)
{
QByteArray ba;
ba = serial->readAll();
ui->label->setText(ba);
qDebug() << ba;
}
}
You can also temporary disable visual updates of a widget using QWidget::setUpdatesEnabled(), but seems you should not miss part of the data.
Be note, QIODevice (and QSerialPort as its sublass, too) class makes no guarantee that a certain amount of data will be available on the readyRead event. For example, if you wrote 10 bytes to the port at a time on the other end, in some cases you will receive the signal that will allow less data to be available at the monent, that is, before all the transmitted bytes arrives.