problem in QPainter::begin

[wagmare]

hi friends ,

usually where this error occurs i cant find it ... when i run my code its giving

QPainter::begin: A paint device can only be painted by one painter at a time.
QPainter::begin: A paint device can only be painted by one painter at a time.
QPainter::begin: A paint device can only be painted by one painter at a time.
QPainter::begin: A paint device can only be painted by one painter at a time.
QPainter::begin: A paint device can only be painted by one painter at a time.
QPainter::begin: A paint device can only be painted by one painter at a time.
QPainter::begin: A paint device can only be painted by one painter at a time.
QPainter::begin: A paint device can only be painted by one painter at a time.
Segmentation fault

QPainter::begin: A paint device can only be painted by one painter at a time.
QPainter::begin: A paint device can only be painted by one painter at a time.
QPainter::begin: A paint device can only be painted by one painter at a time.
QPainter::begin: A paint device can only be painted by one painter at a time.
QPainter::begin: A paint device can only be painted by one painter at a time.
QPainter::begin: A paint device can only be painted by one painter at a time.
QPainter::begin: A paint device can only be painted by one painter at a time.
QPainter::begin: A paint device can only be painted by one painter at a time.
Segmentation fault

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and the core dump occurred ...

can any one please explain where this problem arises ....

[nish]

as the error says.. there are two active painters on same widget... search for... so there may be these cases

1. QPainter p(this);
2. p.begin();
3 new QPainter(this);
4. p->begin();

now search for all those lines and put a qDebug on top of them... next time u will find which line was the second painter

EDIT:
alternatively.. go in qpainter.cpp and put a breakpoint at the line which says the error.. then u can see the call stack. Of course Qt must be in debug mode.
EDIT2:
after writing the above edit... i thought.. is it possible to put a break point in qpainter.cpp? i have not tried it..

[wagmare]

the problem arise here ..

this graphicsItem CentralItem has an ability to add proxywidgets ... and its code

CentralItem::CentralItem(const QRectF &rect, const QBrush &brush, QWidget *embeddedWidget)
          : QGraphicsRectItem(rect),
                brush(brush)
{
        if(embeddedWidget){
                 proxyWidget = new QGraphicsProxyWidget(this);
                proxyWidget->setFocusPolicy(Qt::StrongFocus);
                proxyWidget->setWidget(embeddedWidget);
                proxyWidget->setPos(5, 5);
 
        }
 
}
 
void CentralItem::paint(QPainter *painter, const QStyleOptionGraphicsItem *, QWidget *)
{
        painter->setPen(Qt::NoPen);
        painter->setBrush(QColor(0, 0, 0, 65));
        painter->drawRoundRect(rect().translated(2, 2));
 
        painter->setPen(QPen(Qt::black, 1));
        painter->setBrush(brush);
        painter->drawRect(rect());
 
}

CentralItem::CentralItem(const QRectF &rect, const QBrush &brush, QWidget *embeddedWidget)
          : QGraphicsRectItem(rect),
                brush(brush)
{
        if(embeddedWidget){
                 proxyWidget = new QGraphicsProxyWidget(this);
                proxyWidget->setFocusPolicy(Qt::StrongFocus);
                proxyWidget->setWidget(embeddedWidget);
                proxyWidget->setPos(5, 5);
        
        }

}

void CentralItem::paint(QPainter *painter, const QStyleOptionGraphicsItem *, QWidget *)
{
        painter->setPen(Qt::NoPen);
        painter->setBrush(QColor(0, 0, 0, 65));
        painter->drawRoundRect(rect().translated(2, 2));

        painter->setPen(QPen(Qt::black, 1));
        painter->setBrush(brush);
        painter->drawRect(rect());

}

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and i use it in one graphicsView which is a main window
MainView where i embed a stacked widget ... and it works fine ...

then the problem arise ...
in the stacked widget there are QGraphicsView in individual pages ...

so in one sub GraphicsView i try to use same CentralItem to embed a lineEdit and i receive this error ...



did i confuse a lot ... ? see how i can find that in QPainter problem ...

please help

also take a look at this link ..
https://bugs.launchpad.net/qbzr/+bug/234965

[wagmare]

i learned that

"There is no way to add a widget inside a QGraphicsView that is embedded into another one"

but is there any other way i can resolve this problem ...

[xtfllbl]

Did you call the paintEvent, you need that to paint somthing on the widget. If you just write code like paintXXX and not call in the paintEvent, it`ll not work at all.

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

void MyWidget::paintEvent(QPaintEvent *)
{

    QPainter painter(this);
    paintXXXX(QPainter *);

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[wagmare]

=xtfllbl;109807]Did you call the paintEvent, you need that to paint somthing on the widget. If you just write code like paintXXX and not call in the paintEvent, it`ll not work at all.

actually its a bug in Qt ... we cant add a new widget to a widget that is embedded to graphicsView()

see this code

ProxyBug::ProxyBug(QWidget *parent)                                                                                                  : QDialog(parent)
{
        QVBoxLayout *l = new QVBoxLayout( this );
 
        QGraphicsView * group = new QGraphicsView;
        QGraphicsScene * scene = new QGraphicsScene;
        group->setScene(scene);
 
        QGraphicsView * group2 = new QGraphicsView;
        QGraphicsScene * scene2 = new QGraphicsScene;
        group2->setScene(scene2);
 
        group->scene()->addWidget(group2);
        QLineEdit * lineEdit = new QLineEdit;  
        group2->scene()->addWidget(lineEdit);  // this one crashing *
 
        l->addWidget(group);
        setLayout( l );
 
}

ProxyBug::ProxyBug(QWidget *parent)                                                                                                  : QDialog(parent)
{
        QVBoxLayout *l = new QVBoxLayout( this );

        QGraphicsView * group = new QGraphicsView;
        QGraphicsScene * scene = new QGraphicsScene;
        group->setScene(scene);

        QGraphicsView * group2 = new QGraphicsView;
        QGraphicsScene * scene2 = new QGraphicsScene;
        group2->setScene(scene2);

        group->scene()->addWidget(group2);
        QLineEdit * lineEdit = new QLineEdit;  
        group2->scene()->addWidget(lineEdit);  // this one crashing *

        l->addWidget(group);
        setLayout( l );

}

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the group2 is try to embed another widget where group2 itself is embedded by group ...