Qt-interest Archive, December 2006
QPainter and widget painting error
Message 1 in thread
I'm trying to build a sample app to demonstrate a different problem
that I think may be a bug in Qt or how I'm using Qt.
I have implemented a class derived from QWidget. I'm overriding
paintEvent() to paint some different colors.
void CustomView::paintEvent(QPaintEvent *)
{
QPainter p(this);
++color;
switch(color) {
case 1:
p.fillRect(rect(), QBrush(Qt::green));
break;
case 2:
p.fillRect(rect(), QBrush(Qt::red));
break;
case 3:
p.fillRect(rect(), QBrush(Qt::blue));
break;
case 4:
default:
p.fillRect(rect(), QBrush(Qt::cyan));
color = 0;
break;
}
}
When this method gets called and the QPainter is created, the message
"QPainter::begin: Widget painting can only begin as a result of a
paintEvent". However, it doesn't ALWAYS do this. It paints first time
OK and if I resize the window, it repaints OK.
What I'm trying to do is to have a separate thread that is calling
qApp->postEvent(myCustomView, new QPaintEvent(myCustomView->rect()).
This thread just loops, posts a paint event, then calls usleep() for
1/2 second. The error only happens due to paint events that were
posted in this manner.
What I don't get is why it complains that I'm drawing a widget from
outside of paintEvent when I'm actually responding to a paint event.
This has got to be something stupid. Any ideas what I'm doing wrong?
--
[ signature omitted ]
Message 2 in thread
Brant,
This came up during the Qt 4 Beta cycle on the feedback list. The strange thing about the only paint inside paintevent rule is that the event also has to be a "spontaneous" event. You might be better off doing a cross thread sig/slot to update(rect) although that might cost you two laps through the event loop.
--Justin
-----Original Message-----
From: Brant Sears <brant@xxxxxxx>
Subj: QPainter and widget painting error
Date: Thu Nov 30, 2006 6:58 pm
Size: 1K
To: QT Interest List <qt-interest@xxxxxxxxxxxxx>
I'm trying to build a sample app to demonstrate a different problem
that I think may be a bug in Qt or how I'm using Qt.
I have implemented a class derived from QWidget. I'm overriding
paintEvent() to paint some different colors.
void CustomView::paintEvent(QPaintEvent *)
{
QPainter p(this);
++color;
switch(color) {
case 1:
p.fillRect(rect(), QBrush(Qt::green));
break;
case 2:
p.fillRect(rect(), QBrush(Qt::red));
break;
case 3:
p.fillRect(rect(), QBrush(Qt::blue));
break;
case 4:
default:
p.fillRect(rect(), QBrush(Qt::cyan));
color = 0;
break;
}
}
When this method gets called and the QPainter is created, the message
"QPainter::begin: Widget painting can only begin as a result of a
paintEvent". However, it doesn't ALWAYS do this. It paints first time
OK and if I resize the window, it repaints OK.
What I'm trying to do is to have a separate thread that is calling
qApp->postEvent(myCustomView, new QPaintEvent(myCustomView->rect()).
This thread just loops, posts a paint event, then calls usleep() for
1/2 second. The error only happens due to paint events that were
posted in this manner.
What I don't get is why it complains that I'm drawing a widget from
outside of paintEvent when I'm actually responding to a paint event.
This has got to be something stupid. Any ideas what I'm doing wrong?
--
[ signature omitted ]
