QLabel not refreshing with QPixmap "loadFromData"

[polch]

Hi.

I have this kind of code :

@
QLabel * label;
QPixmap * pixmap;
uint8_t data[42];

/* init */
[...]

/* loop */
{
update_my(data);
pixmap->loadFromData(data, sizeof(data));
label->setPixmap(*pixmap);
}
@

The problem is that the label don't refresh. I suspect a problem of cache but i thought that there wasn't any implicit cache managment...

My current workaround is calling pixmap->fill() before reload the pixmap (or if i paint on the pixmap, the refresh is ok too).

I could also declare the QPixmap inside the loop, but i would like to avoid memory allocations as far as possible.

I would like to simply raise a kind of dirty flag to force the label redrawing.

What do you suggest please ?

Regards.

Paul.

[tobias.hunger]

You create a pixmap A. You set that pixmap A on the label which keeps a copy of it B. Then you change A. Why should the B change?

You use a pointer to the pixmap A instead of using it directly, but that does not matter at all to the Label: It gets a QPixmap and makes its copy. QPixmaps are implicitly shared, so making copies is pretty cheap.

Hmm... reading your post again I think I misunderstood it a bit in the first round.

[polch]

Here is a test case :

@/* Test.hpp*/
#include <QtGui>

class Test : public QLabel
{
Q_OBJECT;

public:

Test();

protected:

void timerEvent(QTimerEvent *event);

private:

QPixmap * m_Pixmap;

QBasicTimer * m_Timer;

int m_cpt;

char m_data[9];
};
@

@/* Test.cpp */
#include "Test.hpp"

static const char data_black[] = "P1 1 1\n1";
static const char data_white[] = "P1 1 1\n0";

Test::Test() :
m_Pixmap(new QPixmap(1, 1)),
m_Timer(new QBasicTimer),
m_cpt(0)
{
this->setScaledContents(true);
m_Timer->start(1000, this);
}

void Test::timerEvent(QTimerEvent event)
{
if(event->timerId() == m_Timer->timerId())
{
// update label
memcpy(m_data, (m_cpt++%2)?data_white:data_black, sizeof(m_data));
m_Pixmap->loadFromData((const uchar)(m_data), sizeof(m_data));
this->setPixmap(*m_Pixmap);

  fprintf(stdout, "update data : \"%s\"\n", m_data);

  m_Timer->start(1000, this);
} 

QWidget::timerEvent(event);
}
@

@/* Main.cpp */
#include <QtGui>

#include "Test.hpp"

int main(int argv, char **args)
{
QApplication app(argv, args);

Test test;

QPushButton quitButton("Quit");
QObject::connect(&quitButton, SIGNAL(clicked()), qApp, SLOT(quit()));

QVBoxLayout layout;
layout.addWidget(&test);
layout.addWidget(&quitButton);

QWidget window;
window.setLayout(&layout);

window.show();

return app.exec();
}
@