Migrating from MFC to Qt: Modal window in MFC

[reymor]

Hello everybody,

I have a MFC application which I would like to migrate to MFC. However, in the meantime, both need to work together (MFC main app with Qt). If I understand correctly, there is no need of qtwinmigrate as now we can use the Qt into MFC app. In other words, I can do:

int argc = 0;
auto qtapp = new QApplication(argc, nullptr);
auto widget = new QWidget;
widget->show();

I can call qt code from MFC app. However, when a need a modal window I cannot do it. Is there any way to accomplish that? I see this project QtMFCIntegration in which they use something called " transparent shield" MFC which calls the Qtwidget. I'm not really sure if I need to do something like that.

I tried to get the HWND, and set this parent using the QWidget::find. However, I was not able to accomplish the modal window. I was not sure if this was the correct way.

Example:

class MyQtDialog : public QDialog {
    Q_OBJECT
public:
    explicit MyQtDialog(QWidget* parent = nullptr) : QDialog(parent) {
        setWindowTitle("Qt Modal Dialog");

        QVBoxLayout* layout = new QVBoxLayout(this);
        QPushButton* button = new QPushButton("Close", this);
        connect(button, &QPushButton::clicked, this, &QDialog::accept);

        layout->addWidget(button);
        setLayout(layout);
    }
};

 __declspec(dllexport) void my_about_call(HWND parent)
{
    int argc = 0;
    QApplication a(argc, nullptr);
    WId qtWIdParent = reinterpret_cast<WId>(parent);
    QWidget *qtParent = QWidget::find(qtWIdParent);

    // Show the Qt modal dialog
    MyQtDialog qtDialog(qtParent);
    qtDialog.setWindowModality(Qt::ApplicationModal);
    qtDialog.exec();
}

some_mfc_code() {
    my_about_call(this->m_pMainWnd->GetSafeHwnd());
}

---

I addition:

It is true that I basically add this as DLL because if I mix mfc and qt headers I got this error:

static_assert failed: 'On MSVC you must pass the /permissive- option to the compiler.'

If I use this permissive option. I got a lot of errors from mfc headers.

btw, I'm using Qt6.8.1.

Thanks in advance

[jsulm]

@reymor You are not starting Qt event loop (a.exec()). Without it Qt stuff will not work. But I don't know how to integrate it with MFC.

[JonB]

@reymor
I don't know about your whole solution. But I notice that you create a new QApplication for each call (show a widget, show a dialog). Are you sure this is the best way? Normally you create a Qt application object once and it persists through out your application lifetime. Looks odd to me (might be just me?) to be creating and destroying QApplication afresh each time.

[JKSH]

@jsulm said in Migrating from MFC to Qt: Modal window in MFC:

You are not starting Qt event loop (a.exec()). Without it Qt stuff will not work.

In this case, MFC is already running the event loop, so Qt simply uses that.

@reymor said in Migrating from MFC to Qt: Modal window in MFC:

I see this project QtMFCIntegration in which they use something called " transparent shield" MFC which calls the Qtwidget. I'm not really sure if I need to do something like that.

Try it. Does it work?

(I have briefly added QWidget-based windows to an existing MFC app, but I have not tried to use modal dialogs in a hybrid Qt-MFC app)

[jsulm]

@JKSH OK, didn't know Qt would use the event loop from MFC automatically.

[JKSH]

@jsulm said in Migrating from MFC to Qt: Modal window in MFC:

@JKSH OK, didn't know Qt would use the event loop from MFC automatically.

I didn't know either, until I tried to port QtWinMigrate to Qt 6.

Volker (our Chief Maintainer) said:

Based on what I'm seeing in Qt 6's (and Qt 5's) Win32 event dispatcher, QtWinMigrate isn't needed at all, at least not for hooking a Qt UI into a Win32 event loop. The code touched by https://codereview.qt-project.org/c/qt/qtbase/+/387409 is designed to make delivery of posted QEvents work even if Qt is not running the event loop.