Take action after the main form is shown in a Qt desktop application

Question

In Delphi I often made an OnAfterShow event for the main form. The standard OnShow() for the form would have little but a postmessage()

Answer 1

I solved it without a timer using Paint event. Works for me at least on Windows.

// MainWindow.h
class MainWindow : public QMainWindow
{
    ...
    bool event(QEvent *event) override;
    void functionAfterShown();
    ...
    bool functionAfterShownCalled = false;
    ...
}

// MainWindow.cpp
bool MainWindow::event(QEvent *event)
{
    const bool ret_val = QMainWindow::event(event);
    if(!functionAfterShownCalled && event->type() == QEvent::Paint)
    {
        functionAfterShown();
        functionAfterShownCalled = true;
    }
    return ret_val;
}

Answer 2

You can override showEvent() of the window and call the function you want to be called with a single shot timer :

void MyWidget::showEvent(QShowEvent *)
{
    QTimer::singleShot(50, this, SLOT(doWork());
}

This way when the windows is about to be shown, showEvent is triggered and the doWork slot would be called within a small time after it is shown.

You can also override the eventFilter in your widget and check for QEvent::Show event :

bool MyWidget::eventFilter(QObject * obj, QEvent * event)
{
    if(obj == this && event->type() == QEvent::Show)
    {
        QTimer::singleShot(50, this, SLOT(doWork());
    }

    return false;
}

When using event filter approach, you should also install the event filter in the constructor by:

this->installEventFilter(this);