repaint doesn't call paintEvent

DrZod

I am trying to write a Paint like program and for a start i just want to draw simple rectangles.So I Inherited from QGraphicsView . The Idea is to just click on Canvas drag the mouse and draw a rect. But repaint() in mouseMoveEvent doesn't call/trigger paintEvent. I also checked viewports paintEvent and that doesn't get called either.

Qt version : 5.7.1
OS : Windows 7

class Canvas : public QGraphicsView {
    Q_OBJECT
public:
    Canvas(QWidget* parent = 0);
protected:
    void mousePressEvent(QMouseEvent* e) override;
    void mouseMoveEvent(QMouseEvent* e) override;
    void paintEvent(QPaintEvent* e) override;
private:
    QPointF firstPoint_;
    QPointF secondPoint_;
};

Canvas::Canvas(QWidget* parent ) : QGraphicsView(parent),m_drawingRectangle(false) ,m_currentColor(Qt::red){
    customViewport_ = new CustomViewport(this);
    setViewport(customViewport_);
}

void Canvas::mousePressEvent(QMouseEvent* e) {
    switch (e->button()) {
        case Qt::LeftButton: {
            customViewport_->setFirstPoint(e->pos());
            firstPoint_ = mapToScene(e->pos());
        }
    }
    QGraphicsView::mousePressEvent(e);
}

void Canvas::mouseMoveEvent(QMouseEvent* e) {
    secondPoint_ = mapToScene(e->pos());
    repaint();
    QGraphicsView::mouseMoveEvent(e);
}

void Canvas::paintEvent(QPaintEvent* e){
    QGraphicsView::paintEvent(e);
    QPainter painter(viewport());    //I have also tried with QPainter(this)

    painter.drawRect(QRectF(firstPoint_,secondPoint_));
}

jsulm

@DrZod How did you check that paintEvent() is not called?
Also, did you try update() instead of repaint()?

KroMignon

@DrZod said in repaint doesn't call paintEvent:

mouseMoveEvent doesn't call/trigger paintEvent.

Maybe a silly question, but are you sure the item is visible?
And, as written in documentation, it is better to use update() instead of repaint() to avoid possible recursive calls:

Warning: If you call repaint() in a function which may itself be called from paintEvent(), you may get infinite recursion. The update() function never causes recursion.

J.Hilk

From what I understand from the Documentation, you're not supposed to do your manual painting on a QGraphicsView from the paint event, but rather use the drawForeground method.

I also found this old topic:
https://forum.qt.io/topic/93327/how-can-i-use-qpainter-to-paint-on-qgraphicsview
should still be relevant for your case.

DrZod

@jsulm I put a breakpoint in my paintEvent implementation.
I have tried both of them,neither one triggers paintEvent

DrZod

@KroMignon What item are you referring? and to answer your question yes I have tried update().

KroMignon

@DrZod I am not aware about QGraphicsView, but I think your Canvas class should sub-class QGraphicsItem. I don't be sure it made sense to sub-class QGraphicsView... But I am not a specialist in this domaine.