QPrinter in QThread produce QPixmap error

RazrFalcon

Simple code example:
@void MainWindow::on_btnPrint_clicked()
{
QThread *thread = new QThread(this);
ThreadedPrint *print = new ThreadedPrint();
print->moveToThread(thread);
thread->start();
QMetaObject::invokeMethod(print, "print", Qt::QueuedConnection);
}

ThreadedPrint::ThreadedPrint(QObject *parent) :
QObject(parent)
{
m_printer = new QPrinter(QPrinter::PrinterResolution);
m_printer->setPageSize(QPrinter::A4);
}

void ThreadedPrint::print()
{
QPainter painter(m_printer);
QRect pageRect = m_printer->pageRect();
QImage image(pageRect.width(), pageRect.height(), QImage::Format_RGB32);
image.fill(Qt::black);
painter.drawImage(0, 0, image);
}
@
And i got:
QPixmap: It is not safe to use pixmaps outside the GUI thread
and I got empty page from my printer.
But when I use printing into PDF all works fine, so bug only appears when I try to print on real printer.

The documentation states:
Painting in Threads
QPainter can be used in a thread to paint onto QImage, QPrinter, and QPicture paint devices.

So my question is: can i print in non-gui thread?

ChrisW67

The QPrinter instance allocated on the heap in your ThreadedPrint constructor will be in the main thread (and stay there).

So either:

Make the QPrinter instance a member variable
Put the heap allocation in a slot, called init() for example, and have that slot connected to the started() signal of the QThread wrapper object.

Here is an example Qt 4 program that works without issue here:
@
#include <QtGui>
#include <QDebug>

class ThreadedPrint: public QObject {
Q_OBJECT
QPrinter *m_printer;
public:
ThreadedPrint(QObject *p = 0): QObject(p) {
m_printer = 0;
}
~ThreadedPrint() {
delete m_printer;
}

signals:
void finished();

public slots:
void init() {
qDebug() << Q_FUNC_INFO;
m_printer = new QPrinter;
}
void doPrint() {
qDebug() << Q_FUNC_INFO;
Q_ASSERT(m_printer);
QPainter painter(m_printer);
QRect pageRect = m_printer->pageRect();
QImage image(pageRect.width(), pageRect.height(), QImage::Format_RGB32);
image.fill(Qt::black);
painter.drawImage(0, 0, image);

    // wait a while then finish the thread
    QTimer::singleShot(3000, this, SIGNAL(finished()));
}

};

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

QThread *thread= new QThread(qApp);
ThreadedPrint *worker = new ThreadedPrint;
worker->moveToThread(thread);
QObject::connect(thread, SIGNAL(started()), worker, SLOT(init()));
QObject::connect(worker, SIGNAL(finished()), thread, SLOT(quit()));
QObject::connect(worker, SIGNAL(finished()), worker, SLOT(deleteLater()));
QObject::connect(thread, SIGNAL(finished()), thread, SLOT(deleteLater()));
thread->start();

QTimer::singleShot(10000, qApp, SLOT(quit()));
QTimer::singleShot(1000, worker, SLOT(doPrint()));
return app.exec();

}
#include "main.moc"
@

RazrFalcon

Thanks for explanation, but I still have the same error...

Even when I create QPrinter object in doPrint slot.

ChrisW67

The example code I posted works fine here. Does it fail for you?

RazrFalcon

Yes. I compile it without changes and get QPixmap error.
Qt 4.8.4, Windows 7 x32.