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


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());

}



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 *);

[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 );

}


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