How to get QTextBrowser to always insert text at the end

Question

I am trying to make a serial terminal program by using QTextBrowser to display incoming data from a serial port. I have set a QTimer to call the paintEvent every 100ms, and show characters on the QTextBrowser widget if anything was received on the serial port.

My problem is that every time I click say in the middle of the QTextBrowser, it is as if though the cursor moves and then on all subsequent ui->tbOutput->insertPlainText(QString(buf));, only half of the QTextBrowser gets updated.

When I click on the bottom of the QTextBrowser widget, the whole QTextBrowser is updated again.

This is the code that I have, where from various other articles, I have tried to scroll to the bottom, and move the text cursor to the end, but it does not do what I want.

void MainWindow::paintEvent(QPaintEvent *event)
{
    Q_UNUSED(event);

    static char buf[10240];

    if (terminal->serialport.bytesAvailable() > 0)
    {
        // sizeof(buf)-1 so that there is space for zero termination character
        qint64 numread = terminal->serialport.read(buf,sizeof(buf)-1); 

        if ((numread > 0) && (numread < sizeof(buf)))
        {
            buf[numread] = 0; // set zero termination
            ui->tbOutput->insertPlainText(QString(buf));
            ui->tbOutput->verticalScrollBar()->setValue(
                ui->tbOutput->verticalScrollBar()->maximum());

            ui->tbOutput->textCursor().setPosition(QTextCursor::End);
        }
    }
}

cbuchart · Accepted Answer · 2017-05-08T10:11:01.860

A few things:

QTextBrowser::textCursor returns a copy, so any modification is not applied to the document
QTextBrowser::setPosition moves the cursor to an absolute position, therefore you are always moving to position 11 (int value to QTextCursor::End). Use QTextBrowser::movePosition instead
finally, it would be better to move the cursor before adding the text, so you are sure it will be added at the end of the document.

Here the modified code:

void MainWindow::paintEvent(QPaintEvent *event)
{
    Q_UNUSED(event);

    static char buf[10240];

    if (terminal->serialport.bytesAvailable() > 0)
    {
        // sizeof(buf)-1 so that there is space for zero termination character
        qint64 numread = terminal->serialport.read(buf,sizeof(buf)-1); 

        if ((numread > 0) && (numread < sizeof(buf)))
        {
            buf[numread] = 0; // set zero termination
            auto textCursor = ui->tbOutput->textCursor();
            textCursor.movePosition(QTextCursor::End);
            ui->tbOutput->setTextCursor(textCursor);
            ui->tbOutput->insertPlainText(QString(buf));
            ui->tbOutput->verticalScrollBar()->setValue(
                ui->tbOutput->verticalScrollBar()->maximum());
        }
    }
}

On the other hand, some additional considerations:

QIODevice::read(char* data, qint64 maxSize) will read at most maxSize bytes, so checking if the number of read bytes is smaller than your buffer is unnecessary.
Do not do it in the paintEvent, it is not the place to read data but to display it. Instead, connect the timer with a slot and read data there and re-paint your console (ui->tbOutput->update()) only if new data has arrived.

Thank you. Your answer got me on to the right track. I agree with your additional considerations and have implemented them in my code. — Armand Jordaan, May 08 '17 at 10:09

How to get QTextBrowser to always insert text at the end

1 Answers1