c++ - Performantly appending (rich) text into QTextEdit or QTextBrowser in Qt

Question

QTextEdit can be appended text to simply using append(). However, if the document is rich text, every time you append to the document, it is apparently reparsed. This seems like a bit of a trap in Qt.

If you're using the edit box as a log window and appending text in fast successions as a result of external signals, the appending can easily hang your app with no intermediate appends shown until each of the appends have completed.

How do I append rich text to a QTextEdit without it slowing down the entire UI?

Answer 1

If you want each append to actually show quickly & separately (instead of waiting until they've all been appended before they are shown), you need to access the internal QTextDocument:

void fastAppend(QString message,QTextEdit *editWidget)
{
    const bool atBottom = editWidget->verticalScrollBar()->value() == editWidget->verticalScrollBar()->maximum();
    QTextDocument* doc = editWidget->document();
    QTextCursor cursor(doc);
    cursor.movePosition(QTextCursor::End);
    cursor.beginEditBlock();
    cursor.insertBlock();
    cursor.insertHtml(message);
    cursor.endEditBlock();

    //scroll scrollarea to bottom if it was at bottom when we started
    //(we don't want to force scrolling to bottom if user is looking at a
    //higher position)
    if (atBottom) {
        scrollLogToBottom(editWidget);
    }
}

void scrollLogToBottom(QTextEdit *editWidget)
{

    QScrollBar* bar =  editWidget->verticalScrollBar();
    bar->setValue(bar->maximum());
}

The scrolling to bottom is optional, but in logging use it's a reasonable default for UI behaviour.

Also, if your app is doing lots of other processing at the same time, appending this at the end of fastAppend, will prioritize actually getting the message displayed asap:

    //show the message in output right away by triggering event loop
    QCoreApplication::processEvents();

This actually seems a kind of trap in Qt. I would know why there isn't a fastAppend method directly in QTextEdit? Or are there caveats to this solution?

(My company actually paid KDAB for this advice, but this seems so silly that I thought this should be more common knowledge.)