Unable to draw onto a scrollarea, with all code pasted

[qt_fan_4k]

I'm trying to draw many rectangles in a scrollarea, but my tiny demo below did never work
The mainwindow is below. The code below can work, but not scrollable, and when I change 0 to 1, enabling the scrollarea, nothing displayed.

The code below is all I have. I really can't fingure out what's wrong here.

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
{
    if (0) {
        QScrollArea *scrollArea = new QScrollArea(this);
        scrollArea->setBackgroundRole(QPalette::Dark);
        QmQTargetCanvas *targetCanvas = new QmQTargetCanvas(scrollArea);
        scrollArea->setWidget(targetCanvas);
    } else {
        QmQTargetCanvas *targetCanvas = new QmQTargetCanvas(this);
        this->setCentralWidget(targetCanvas);
    }
    this->showMaximized();
}

QmQTargetCanvas::QmQTargetCanvas(QWidget *parent) :
    QWidget(parent)
{
    setBackgroundRole(QPalette::Base);
    setAutoFillBackground(true);
}

void QmQTargetCanvas::paintEvent(QPaintEvent * /* event */)
{
    QRect rect(10, 20, 80, 60);

    QPainter painter(this);
    painter.setPen(QPen(Qt::red, 2, Qt::SolidLine, Qt::RoundCap, Qt::RoundJoin ));
    QRadialGradient radialGradient(50, 50, 50, 80, 20);
    radialGradient.setColorAt(0.0, Qt::white);
    radialGradient.setColorAt(0.2, Qt::darkCyan);
    radialGradient.setColorAt(1.0, Qt::black);
    painter.setBrush(radialGradient);

    for (int x = 0; x < width(); x += 100) {
        for (int y = 0; y < 900; y += 100) {
            painter.save();
            painter.translate(x, y);
            painter.drawRoundedRect(rect, 25, 25, Qt::RelativeSize);
            painter.drawText(rect, Qt::AlignCenter, QString::number(height()));
        }
    }

    painter.setPen(palette().dark().color());
    painter.setBrush(Qt::NoBrush);
    painter.drawRect(QRect(0, 0, width() - 1, height() - 1));
}

[SGaist]

Hi,

You're not setting your scrollArea as your MainWindow central widget.

[qt_fan_4k]

Thank you, SGaist. Yes, I forgot to centralize the scrollarea as I did for the canvas.
What really bothers me is that the scrollarea is not scrollable.

As you can see, I was intending to draw these rectancles as many as to fill a 900-pixel high canvas, but the painter never drew outside the physical area of visible canvas.
I tried resize, but it did not work.

    for (int x = 0; x < width(); x += 100) {
        for (int y = 0; y < ***900***; y += 100) {
            painter.save();
            painter.translate(x, y);
            painter.drawRoundedRect(rect, 25, 25, Qt::RelativeSize);
            painter.drawText(rect, Qt::AlignCenter, QString::number(height()));

[SGaist]

You should also set the height you want on the QmQTargetCanvas instance.