How to implement QProgressDialog? - c++

Question

I try to use a QProgressDialog to give the user some information on the progress of a long task, while allowing him to cancel this task.
Basically, I have a QDialog with a button Compute. By clicking on it, a time consuming method is called on a member of my QDialog's parent. This method takes a callback to tell the caller the progress of work.
The problem is that the progress dialog takes some time before appearing, and doesn't take into account immediately a click on its Cancel button.
It's clear that there is a glitch in my code, but I'm not accustomed with Qt, and I tried many things. I probably need a separate thread.
An extract of my code:
void MyDialog::callbackFunction(int value, void * objPtr) {
((QProgressDialog *)(objPtr))->setValue(value);
QCoreApplication::processEvents();
}
void MyDialog::on_mComputeBtn_clicked()
{
Compute();
}
void MyDialog::Compute()
{
QProgressDialog progressDialog("Optimization in progress...", "Cancel", 0, 100, this);
progressDialog.setMinimumDuration(500); // 500 ms
progressDialog.setWindowModality(Qt::WindowModal);
progressDialog.setValue(0);
connect(&progressDialog, SIGNAL(canceled()), this, SLOT(Cancel()));
QCoreApplication::processEvents();
parentMember->LongComputation(callbackFunction);
// probably useless
progressDialog.reset();
progressDialog.hide();
QCoreApplication::processEvents();
}

Answer 1

The dialog is not appearing immediately because you set a minimum duration of 500ms. Set it to 0 to make the dialog show immediately on progress change or call its show function manually.
In order to make the cancel button work, move your long computation to another thread ( e.g. use QThread or std::async ) and let your main event loop continue its execution.
Actually there are a lot of problems with your code but these two points should point you to the right direction. In my experience every manual call to processEvents is a big code smell.

Answer 2

What you do is trying to use modal paradigm of QProgressdialog, not letting main event pump to fire, also you set a 0.5 s timeout, minimal duration would ensure the pause. Maybe modeless variant is more appropriate for your case.
if process got discrete steps, you can do it without separate thread
class MyTask : public QObject
{
Q_OBJECT
public:
explicit MyTask(QObject *parent = 0);
signals:
public slots:
void perform();
void cancel();
private:
int steps;
QProgressDialog *pd;
QTimer *t;
};
...
void MyDialog::on_mComputeBtn_clicked()
{
myTask = new MyTask;
}
...
MyTask::MyTask(QObject *parent) :
QObject(parent), steps(0)
{
pd = new QProgressDialog("Task in progress.", "Cancel", 0, 100000);
connect(pd, SIGNAL(canceled()), this, SLOT(cancel()));
t = new QTimer(this);
connect(t, SIGNAL(timeout()), this, SLOT(perform()));
t->start(0);
}
void MyTask::perform()
{
pd->setValue(steps);
//... perform one percent of the operation
steps++;
if (steps > pd->maximum())
t->stop();
}
void MyTask::cancel()
{
t->stop();
//... cleanup
}
QThread example existed here: Qt 5 : update QProgressBar during QThread work via signal