Qt-jambi-interest Archive, November 2007
Extending an ItemView
Message 1 in thread
Hi,
I'm just experimenting with QtJambi as well as Qt C++ and I was trying
to "port" the C++ Chart example to Jambi when I got into trouble.
When running my "ported" app I get a lot of these messages, and the
widget doesn't resize properly:
QPaintEngine::setSystemRect: Should not be changed while engine is active
QPaintEngine::setSystemClip: Should not be changed while engine is active
I then tried to create a simple class extending QListView in both Java
and C++ to try to figure this one out:
public class TestView extends QListView {
protected void paintEvent(QPaintEvent e) {
QPainter painter = new QPainter(viewport());
super.paintEvent(e);
}
}
And:
void TestView::paintEvent(QPaintEvent *event) {
QPainter painter = QPainter(viewport());
QListView::paintEvent(event);
}
The C++ code works just fine, but changing it to:
void TestView::paintEvent(QPaintEvent *event) {
QPainter *painter = new QPainter(viewport());
QListView::paintEvent(event);
}
produces the same behaviour as the Java code.
Beeing a Java programmer and not a C++ programmer I don't know what the
big difference is between the two C++ versions, but I would guess that
Jambi uses the latter version.
Is this a bug? Or how do I get around this problem in Jambi?
Regards
Trond GjÃlstad Ziarkowski
Message 2 in thread
Trond GjÃlstad Ziarkowski wrote:
> I then tried to create a simple class extending QListView in both Java
> and C++ to try to figure this one out:
> public class TestView extends QListView {
> protected void paintEvent(QPaintEvent e) {
> QPainter painter = new QPainter(viewport());
> super.paintEvent(e);
> }
> }
>
> And:
> void TestView::paintEvent(QPaintEvent *event) {
> QPainter painter = QPainter(viewport());
> QListView::paintEvent(event);
> }
>
> The C++ code works just fine, but changing it to:
> void TestView::paintEvent(QPaintEvent *event) {
> QPainter *painter = new QPainter(viewport());
> QListView::paintEvent(event);
> }
>
> produces the same behaviour as the Java code.
>
>
>
>
Any QPainter opened (begin is called) on a paint device has to also be
closed (end is called.) When you call the QPainter constructor and pass
in a paint device, it automatically calls begin. When the object is
destroyed, it automatically calls end.
The problem with the Java-code is that it does not call the end-method,
and the destructor does not get called until the garbage collector
decides to collect the painter. With the first C++ example, the object
is allocated on the stack, and thus the destructor is called
automatically when the object goes out of scope. With the second C++
example, the object is allocated on the heap, and will have to be
manually deleted/ended. The second C++ example will also leak memory.
So, to fix your Java example, change the code to the following:
protected void paintEvent(QPaintEvent e) {
QPainter painter = new QPainter(viewport());
painter.end();
super.paintEvent(e);
}
That should do the trick.
-- Eskil
