Can work QPrinter in a thread?

gronerth

Hello guys,

I need to generate pdf files but i need to generate it in background because it may freeze the application. I have used the following functions in a thread:

@
editor = new QTextDocument();
printer = new QPrinter(QPrinter::ScreenResolution);
printer->setOutputFormat(QPrinter::PdfFormat);

editor->clear();
editor->setHtml(dataHtml);

QDir path("./Reports/");
printer->setOutputFileName(path.absolutePath() + QString("/")+fileName);
editor->print(printer);

@

All the objects were created in the thread instance. When i try to execute the code, i have always the following error: QPainter::begin(): Returned false. So, i was wondering if QPrinter can't work in a thread, if it is so, does someone have any suggestions to achieve the same result?

Thanks in advance...

gronerth

As far as i can see, if all those operations were made in the run() function it works, but if it is made in another function doesnt works...

I mean..This kind of code works:

@
void run()
{
//Create the objects
//Generates the PDF file...
}
@

But this doesnt:

@
void run()
{
//create the objects
generatePDFs();
}

void generatePDfs() // <--- this function is part of the class..
{
//generates the pdf file <-- here i get the error of the QPainter...
}
@

I can't see where is the problem, maybe i am using wrong the QThread class...

giesbert

That sounds strange. So it's not a problem of doing it inside a thread, it#s a problem of function separation?

Then please give aus more code. a small, (not) working example with all needed code. What exactly is done in run, what in generatePdfs?

goetz

This could be an instance of the "GUI only in the main thread" problem (painting on a QPrinter is GUI, as far as I know).

Is there any output on the console? As far as I know, QPainter prints some debugging messages when it fails.

giesbert

What iritated me is, he said uif everything is in run, it works....
run is the thread function...

goetz

[quote author="Gerolf" date="1302169254"]What iritated me is, he said uif everything is in run, it works....
run is the thread function...[/quote]

An instance of the "run in the context where the method is called" problem?

giesbert

That's why I asked for more code...

alexKI

[quote author="gronerth" date="1302133641"]As far as i can see, if all those operations were made in the run() function it works, but if it is made in another function doesnt works...[/quote]
do you use the constructor to initialize objects for this functions (parent thread for this objects will be thread where class was created)?

gronerth

To give an small example, this kind of code doesn't work:

@
class reportGeneratorThread : public QThread
{
Q_OBJECT
public:
void run();
private:
QPrinter *printer;
QTextDocument *editor;
public slots:
void makeReports();
}

reportGeneratorThread::reportGeneratorThread(QObject *parent) :
QThread(parent)
{

}

void reportGeneratorThread::run()
{
editor = new QTextDocument();
editor->setHtml("<h1>hola</h1>");
printer = new QPrinter(QPrinter::HighResolution);
printer->setOutputFormat(QPrinter::PdfFormat);
printer->setOutputFileName("report.pdf");
makeReports();
}

void reportGeneratorThread::makeReports()
{
editor->print(printer);
}
@

[quote author="Volker" date="1302167511"]This could be an instance of the "GUI only in the main thread" problem (painting on a QPrinter is GUI, as far as I know).

Is there any output on the console? As far as I know, QPainter prints some debugging messages when it fails.[/quote]

The debug message is: QPainter::begin(): Returned false

but if i put the code editor->print(printer) inside the run function it works....

webmaster.skelton

I believe that Volker is correct, aa QPrinter inherits from QPaintDevice, though I have not found anything that explicitly says it is always "thunked" back into the GUI thread. This may also be a stupid question, but when you call the thread you call it by using "start()" and not "run()" correct? As this will also cause strange behavior.

gronerth

[quote author="webmaster.skelton" date="1302199366"]I believe that Volker is correct, aa QPrinter inherits from QPaintDevice, though I have not found anything that explicitly says it is always "thunked" back into the GUI thread. This may also be a stupid question, but when you call the thread you call it by using "start()" and not "run()" correct? As this will also cause strange behavior. [/quote]

yes, i call it with "start()"...