I'm using QThread
for printing work via QPrinter
My PrintWorker
look like this :
class PrintWorker : public QObject {
Q_OBJECT
public:
PrintWorker(QThread*, QPrinter*, QPicture*, QPainter*, QObject *parent = 0);
private:
QPicture *_picture = nullptr;
QPrinter *_printer = nullptr;
QPainter *_painter = nullptr;
public slots:
void print();
signals:
void done();
};
PrintWorker::PrintWorker(QThread *thread, QPrinter *printer, QPicture *picture, QPainter *painter, QObject *parent) :QObject(parent),
_picture(picture), _printer(printer), _painter(painter)
{
moveToThread(thread);
QObject::connect(thread, &QThread::started, this, &PrintWorker::print);
QObject::connect(this, &PrintWorker::done, thread, &QThread::quit);
QObject::connect(this, &PrintWorker::done, this, &PrintWorker::deleteLater);
QObject::connect(thread, &QThread::finished, thread, &QThread::deleteLater);
}
void PrintWorker::print() {
// do some print job with painter and picture
emit done();
}
And print
Method is this :
void NewService::print() {
if (!_printer) { /* _printer : a private member */
_printer = new QPrinter(QPrinter::HighResolution);
_printer->setPageSize(QPrinter::A5);
_printer->setPageOrientation(QPageLayout::Portrait);
_printer->setColorMode(QPrinter::Color);
}
if (!_printDialog) { /* _printDialog : a private member */
_printDialog = new QPrintDialog(_printer);
}
if (_printDialog->exec() == QPrintDialog::Accepted) {
MyWidget *widget = new MyWidget(/* some args*/);
QPainter *painter = new QPainter;
QPicture *picture = new QPicture;
widget->render(picture);
QThread *thread = new QThread;
PrintWorker *worker = new PrintWorker(thread, _printer, picture, painter);
thread->start();
}
}
Now before invoking print()
my app exposes about 9MB of memory after printing and invoking PrintWorker::print()
my app's memory usage get to 26MB
In Another world if we remove emit done
at last part in PrintWorker::print()
it makes no difference.
What we expect after finishing job is memory usage should get down to 26MB - Thread space + _printer
+ _printDialog
objects size ≈ 14MB
So What's wrong with this ?
By calling
QThread::quit()
you are activly exiting the EventLoop in this thread. AQObject::deleteLater()
for objects residing in this thread may not be executed anymore (though they mention onQThread::finished()
-signal the delete later slot will still be called).I personally would just set up the connection using the
finished()
signal or set the parent of the worker-object to be the thread itself, then Qt actually has to remove the object.http://doc.qt.io/qt-5/qthread.html#exit