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Unable to draw onto a scrollarea, with all code pasted



  • 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));
    }
    

  • Lifetime Qt Champion

    Hi,

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



  • 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()));
    
    

  • Lifetime Qt Champion

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


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