QTextCursor::mergeBlockFormat() doesn't set format.

[Khamza]

I have this function for setting block format:

void PasswordShowArea::updateBlockFormat()
{
    insertPlainText("Some text");
    QTextBlockFormat fmt = textCursor().blockFormat();

    fmt.setTextIndent(g_lineNumberAreaWidth + g_lineNumberRightMargin);

    fmt.setLineHeight(fontMetrics().height() * 2, QTextBlockFormat::LineDistanceHeight);

    textCursor().mergeBlockFormat(fmt);
}

Idea was to se block format in constructor but it doesn't work as expected. The function itself is called but neither text is inserted nor block format changed:

PasswordShowArea::PasswordShowArea(QWidget *parent)
    : QTextEdit(parent)
{
    init();

    //doesn't work
//  updateBlockFormat();
    // works - gpt4 suggest
    QTimer::singleShot(0, this, &PasswordShowArea::updateBlockFormat);
}

void PasswordShowArea::init()
{
    QObject::connect(document()->documentLayout(), &QAbstractTextDocumentLayout::update,
                     this, &PasswordShowArea::updatePasswordShowArea);

    setTextColor(palette().color(QPalette::Text));

}

Any ideas why this happen?
P.S. gpt suggested that the problem may be that the object at the time the updateBlockFormat() is called is not fully initialized

[JonB]

@Khamza
Yes, if it does not work in constructor but does work on QTimer::singleShot(0) then you have you answer. There are other places in Qt where this trick is necessary.

[Khamza]

@JonB Thanks for your answer
I found another solution:

void PasswordShowArea::showEvent(QShowEvent *event)
{
    QTextEdit::showEvent(event);
    updateBlockFormat();
}

Could you please give a brief overview of why both of this approach works?