Subclassing QWidget not working

[guidupas]

Hello all!

I need to subclass QWidget, but its not working. When I call the function desenhaFluxo(), it returns the error:
QWidget:aintEngine: Should no longer be called
QPainter::begin: Paint device returned engine == 0, type: 1
QPainter::setPen: Painter not active

How can I do that?

#include "fluxocaixawidget.h"
 
fluxoCaixaWidget::fluxoCaixaWidget(QWidget *parent) :
    QWidget(parent)
{
 
}
 
void fluxoCaixaWidget::paintEvent(QPaintEvent *)
{
    QPainter painter(this);
 
    QPen caneta(Qt::black);
    caneta.setWidth(4);
    painter.setPen(caneta);
 
 
 
    painter.drawLine(20, this->parentWidget()->height() / 2 - 20, this->parentWidget()->width()-40, this->parentWidget()->height() / 2 - 20);
}
 
void fluxoCaixaWidget::desenhaFluxo()
{
    QPainter painter(this);
 
    QPen caneta(Qt::black);
    caneta.setWidth(4);
    painter.setPen(caneta);
 
    painter.drawLine(10,10,50,10);
}

#include "fluxocaixawidget.h"

fluxoCaixaWidget::fluxoCaixaWidget(QWidget *parent) :
    QWidget(parent)
{

}

void fluxoCaixaWidget::paintEvent(QPaintEvent *)
{
    QPainter painter(this);

    QPen caneta(Qt::black);
    caneta.setWidth(4);
    painter.setPen(caneta);



    painter.drawLine(20, this->parentWidget()->height() / 2 - 20, this->parentWidget()->width()-40, this->parentWidget()->height() / 2 - 20);
}

void fluxoCaixaWidget::desenhaFluxo()
{
    QPainter painter(this);

    QPen caneta(Qt::black);
    caneta.setWidth(4);
    painter.setPen(caneta);

    painter.drawLine(10,10,50,10);
}

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[anda_skoa]

You cannot paint on a widget outside its paintEvent() method,

Either store the line parameters in your class and use the in paintEvent() and trigger an update by calling update() in desenhaFluxo()
or
Paint into a QPixmap and let paintEvent() only paint that pixmap.

Cheers,
_