Qt5 paintEvent not called inside QScrollArea

Question

I'm having a bit of a problem with Qt. I'm trying to create a 2D drawing of cells, with QRect, by overloading paintEvent for a custom class which inherits QWidget and which is placed inside a QScrollArea. The problem is, paintEvent does not trigger at all (not on resize events, not when I call repaint() or update(), nor when I launch my program). Here is where I overload paintEvent, in GOL.cpp:

void GOL::paintEvent(QPaintEvent *) {

    QPainter painter(this);

    //painter.setPen(Qt::black);

    int x1Rect = rectPaint.x();
    int y1Rect = rectPaint.y();
    int x2Rect = x1Rect + rectPaint.width();
    int y2Rect = y1Rect + rectPaint.height();

    int xCell;
    int yCell = 0;

    for (int i = 0; i < rows; i++) {

        xCell = 0;

        for (int j = 0; j < cols; j++) {

            if (xCell <= x2Rect && yCell <= y2Rect && xCell + cellSize >= x1Rect &&
                    yCell + cellSize >= y1Rect) {

                if (principalMatrix->get(i,j)) {

                    painter.fillRect(xCell, yCell, cellSize - 1, cellSize - 1, cellColourAlive);
                }
                else {

                    painter.fillRect(xCell, yCell, cellSize - 1, cellSize - 1, cellColourDead);
                }
            }

            xCell += cellSize;
        }

        yCell += cellSize;
    }
}

And my layout is as follows, in DisplayGame.cpp:

DisplayGame::DisplayGame(QWidget *parent, int threads_no, int generations, char* file_in, char* file_out) :
    QWidget(parent) {

    gol = new GOL(threads_no, generations, file_in, file_out);

    QHBoxLayout *title = setupTitle();
    QHBoxLayout *buttons = setupButtons();
    QVBoxLayout *layout = new QVBoxLayout();
    scrlArea = new QScrollArea;
    scrlArea->setWidget(gol);
    layout->addLayout(title);
    layout->addWidget(scrlArea);
    layout->addLayout(buttons);
    setLayout(layout);
}

I honestly have no idea why it does not draw anything. Any ideas?

Answer 1

I've fixed it by modifying as follows:

DisplayGame::DisplayGame(QWidget *parent, int threads_no, int generations, char* file_in, char* file_out) :
    QWidget(parent) {

    gol = new GOL(this, threads_no, generations, file_in, file_out);

    QSize *adjustSize = new QSize(gol->cellSize, gol->cellSize); //QSize object that is as big as my QRect matrix
    adjustSize->setWidth(gol->cellSize * gol->rows);
    adjustSize->setHeight(gol->cellSize * gol->cols);
    gol->setMinimumSize(*adjustSize);

    QVBoxLayout *layout = new QVBoxLayout;

    QHBoxLayout *title = setupTitle();
    layout->addLayout(title);

    QHBoxLayout *buttons = setupButtons();
    layout->addLayout(buttons);

    QPalette pal(palette()); //Setting the background black, so the white spaces between QRect items cannot be seen (though I could have modified the margins?)
    pal.setColor(QPalette::Background, Qt::black);

    scrlArea = new QScrollArea(this);
    scrlArea->setAutoFillBackground(true);
    scrlArea->setPalette(pal);
    scrlArea->setWidget(gol);

    layout->addWidget(scrlArea);

    setLayout(layout);
}

And I've left the paintEvent as it was. It was, ultimately, a size problem, as AlexanderVX said.