Managing keyboard events between Win32 app and QWinMigrate

Question

I have integrated a Qt dialog into a traditional Win32 application, and am now a bit stumped on how to manage the keyboard events propagating from Qt->Win32. Is there any way test if Qt is 'handling' events (eg, input going to an editbox), and prevent these events propagating to the host application?

The Win32 application has its own very-complex accelerator system, and when working with native editboxes generally we disable accelerators manually. I donb't have the ability to do this for the Qt dialog, as its a shared widget amongst several applications.

Currently I disable the hosts accelerators on the dialog as a whole gaining focus, but would it be possible to tell Qt to prevent kbd events from editboxes from propagating? Ideally without modifying the QtDialogs code (although I can do that if necessary?)

If the Qt dialog is a modal dialog, it disables its owner, and disabled windows don't receive keyboard or mouse input. If QWinMigrate doesn't offer automatic support for this, you have to implement modality manually by calling [EnableWindow](https://msdn.microsoft.com/en-us/library/windows/desktop/ms646291.aspx). — IInspectable, Dec 15 '15 at 22:46
Thanks for the suggestion, but its not a modal dialog, and I cannot make it one. It should live in the host application and be as 'native' as possible. There are a lot of dialogs in the application for editing various properties (think Photoshop) and this should be just one more of those dialogs. — FrozenKiwi, Dec 18 '15 at 19:19

score 3 · Accepted Answer · answered Dec 20 '15 at 18:27

I have no idea if this will actually work but you might give this a go:

class KeyEventPropagetionPreventer: public QObject
{
public:
    KeyEventPropagetionPreventer( QWidget * widget ) 
    : QObject( widget ), widget( widget ), instercept_events( true )
    {
        widget->installEventFilter( this )
    }

    bool eventFilter(QObject *obj, QEvent *event)
    {
        if ( intercept_events && event->type() == QEvent::KeyPress) // or other types if needed
        {
            intercept_events = false; // prevents eating your own events
            QKeyEvent new_event( *event ); // Might be that you need to implement your own copying function if the copyconstructor is disabled
            QApplication::sendEvent(this->widget, &new_event);
            instercept_events = true;
            return true;
        } 
        else 
        {
            return QObject::eventFilter(obj, event);
        }
    }

private:
    QWidget * widget;
    bool instercept_events;
}

Then you add this line where you create the dialog:

new KeyEventPropagetionPreventer( your_qt_dialog ); // will be deleted by the Qt parent/child system when the dialog is deleted.

The idea is to intercept all Keyboard events, but then also create a copy of it and send that to the widget. Hopefully the intercepting actually prevents the event from propagating (and is not qt-eventsystem-only or something) and the QApplication::sendEvent() does not propagate itself.

I hope this works, good luck!

(ps. This code has not been tested or compiled)

Thanks - I'll try this out shortly (next few days) and get back to you — FrozenKiwi, Dec 21 '15 at 11:19
So, after many attempts I got something quite similar to this going - using a global event filter on the qApplication then selectively sending events to Qt or the host application. — FrozenKiwi, Sep 12 '16 at 21:43

score 1 · Answer 2 · answered Dec 25 '15 at 03:04

Is there any way test if Qt is 'handling' events (eg, input going to an editbox)

Just in case you haven't used it yet, you should definitely take a look at Spy++ (included in Visual Studio) which enables you to track every message.

I am not familiar with Qt, but if you want to catch all messages in Win32, you could use GetMessage(), for example:

BOOL bRet;

while( (bRet = GetMessage( &msg, hWnd, 0, 0 )) != 0)
{ 
    if (bRet == -1)
    {
        // handle the error and possibly exit
    }
    else  // you could do whatever you want with the message before it is dispatched
    {
        TranslateMessage(&msg); 
        DispatchMessage(&msg); 
    }
}

Managing keyboard events between Win32 app and QWinMigrate

2 Answers2