Proper way to open a QDialog

casperbear

Hello! I'm trying to figure out a way to properly open a QDialog immediately after QMainWindow is shown. In examples below

class WMain : public QMainWindow
...
class WNew : public QDialog

So far I've tried three approaches:

void WMain::showEvent(QShowEvent *event)
{
    QMainWindow::showEvent(event);

    WNew window(this);
    window.exec();
}

QMainWindow is not shown before QDialog is shown. Internet says it's because QDialog::exec is a synchronous call. Changing showEvent to the first paintEvent doesn't solve the problem (QMainWindow is shown before QDialog is shown, but some QMainWindow controls may be black). Using timer-based solutions doesn't look right.

void WMain::showEvent(QShowEvent *event)
{
    QMainWindow::showEvent(event);

    WNew window(this);
    window.open();
}

QDialog is not shown at all. I assume it's because QDialog::open is an asynchronous call and WNew destructor is called when code exits showEvent function.

void WMain::showEvent(QShowEvent *event)
{
    QMainWindow::showEvent(event);

    WNew *window = new WNew(this);
    window->open();
}

This seems to work, but there is a memory leak. WNew destructor is not called when the dialog is closed. Can you suggest please where I should call the destructor? Or some different approach?

SGaist

Hi,

Can you explain your use case ?

As for the memory leak, you can set the ”WA_DeleteOnClose” widget attribute.

mpergand

You can use show() :

void show()
        {
        QMainWindow::show();
        WNew dlg(this);
        dlg.exec();
        }

casperbear

My use case is that I have a dialog for creating a new document. This dialog should be initiated by a user click and also opened on application start.
WA_DeleteOnClose worked for me. Thank you. I would have never guessed this attribute.

It's particularly convenient that I can programmatically control when to use it. (Use it when dialog object is created in heap and don't use it when dialog object is created in stack)

// on application start
void WMain::showEvent(QShowEvent *event)
{
    QMainWindow::showEvent(event);

    WNew *window = new WNew(this);
    window->setAttribute(Qt::WA_DeleteOnClose);
    window->open();
}

// on user click
void WMain::on_actionNew_triggered()
{
    WNew window(this);
    window.exec();
}

SGaist

The you should rather use a single shot QTimer with 0 as timeout value at the end of your constructor to call a slot that will show your dialog. This way the event loop will have started properly and your WMain object should be visible.

casperbear

SGaist, yes this approach is even better. It solved problem with dialog centering (alignment relative to parent) and initial dialog 'active' status. I wouldn't come up with '0' argument in a lifetime. I thought about very small numbers but not '0'. :) I guess '0' works because (if I recall) timer events are processed in event loop only when there are no other events (in win32 at least).

void WMain::showEvent(QShowEvent *event)
{
    QMainWindow::showEvent(event);

    QTimer::singleShot(0, [this]() {
        WNew window(this);
        window.exec();
    });
}