CWND to QWidget*

Massi

Good day,

I'm using MFC to build a GUI on vs2012 and I recently integrated Qt5.4.1 library in my project. Does anyone know how to connect an CWND pointer to QWidget pointer? I can get the HWND from a widget using winId() but how to get a widget from a HWND or a CWND?

CWnd* pMfc to the argument of myQtfunction(QWidget* pwidget)

Thanks in advance!

Massi

JKSH

Hello @Massi,

A QWidget is quite a complex class; many of its internals are managed by QApplication. Therefore, you cannot convert a CWND/HWND window directly to a QWidget, because those are managed outside of QApplication.

Now, it is possible to create a QWindow object to manipulate external windows. It is also possible to embed a QWindow inside a QWidget. You can kind of combine them like this:

WId externalWindow = (WId)FindWindow(NULL, L"Untitled - Notepad"); // Get HWND by window title
QWindow *myWindow = QWindow::fromWinId(externalWindow);
QWidget *myWidget = QWidget::createWindowContainer(myWindow);

However, QWidget::createWindowContainer() was not really designed to handle external windows. You will likely encounter strange issues. See https://bugreports.qt.io/browse/QTBUG-40320 for details.

Massi

Thanks for your reply @JKSH

Unfortunately it didn't work and even if I have a pointer to the window as shown below, it crashes.

BOOL myFMCclass::OnFileOpen(LPCTSTR szPathname, CWnd* pParent)
{
	QFile file(szPathname);
	if (!file.open(QIODevice::ReadOnly | QIODevice::Text))
		return false;
	//Embed a QWindow inside a QWidget
	QWindow* myWindow = QWindow::fromWinId((WId)pParent);
	QWidget* myWidget = QWidget::createWindowContainer(myWindow);

	myQtclass* m_widgetptr = new myQtclass(myWidget);

   return true;
}

Well I'm trying to display a QGraphicView within an FMC GUI. and if I create a new QWidget instead of using QWidget::createWindowContainer() I get the following error:
QWidget: Must construct a QApplication before a QWidget

Again thanks for you help!!!

Massi

JKSH

@Massi said in CWND to QWidget*:

even if I have a pointer to the window as shown below, it crashes.
BOOL myFMCclass::OnFileOpen(LPCTSTR szPathname, CWnd* pParent)

My example is for a HWND, not a CWnd*. You need to convert your pointer to a HWND first. See http://forums.codeguru.com/showthread.php?328037-Converting-CWND*-to-HWND

Well I'm trying to display a QGraphicView within an FMC GUI...

                            **QWidget: Must construct a QApplication before a QWidget**

As the error message says, you need to create a QApplication before you can create a QWidget. Furthermore, you must call QApplication::exec() before you can interact with any QWidgets -- this requirement makes it tricky to add QWidgets to an existing non-Qt program, because QApplication::exec() blocks the calling thread.

You could work around this by creating your QApplication and QWidgets in a different thread (see http://stackoverflow.com/questions/22289423/how-to-avoid-qt-app-exec-blocking-main-thread/22290909#22290909 ). However, I have only done that with "native" QWidgets, not using QWidget::createWindowContainer(). it sounds like you want to embed an MFC window inside a QWidget, and then use that QWidget inside the external MFC application...? This is very convoluted, and I have a feeling it won't work nicely.

Massi

Hello @JKSH ,

Well I want to embed a QGraphicView widget inside a MFC application. I don't know if QWidget class is enough to handle CWnd and use it as a simple QWidget. should I create a Qdialog window and insert the QGraphicView in it and then try to connect it with my MFC GUI?

BOOL myFMCclass::OnFileOpen(LPCTSTR szPathname, CWnd* pParent)
{
	QFile file(szPathname);
	if (!file.open(QIODevice::ReadOnly | QIODevice::Text))
		return false;

	//Embed a QWindow inside a QWidget
	HWND hWind = pParent->GetSafeHwnd();
	if(hWind != NULL)
	{	
		QWindow* myWindow = QWindow::fromWinId((WId)hWind);
		QWidget* myWidget = QWidget::createWindowContainer(myWindow);
		if(myWidget)
			myQtclass* m_widgetptr = new myQtclass(myWidget);
	}
   return true;
}

Many thanks!

Massi

EmmanuelC

Hello,
I have to do exactly the same as you, so I tried to code it the same way. but when I run
QWindow *myWindow = QWindow::fromWinId(externalWindow);
I am getting an access violation error. Have you had this problem? Do you know how to fix it?
Thank you,
Emmanuel

JonB

@EmmanuelC
If your code is also like:

	QWindow* myWindow = QWindow::fromWinId((WId)pParent);
	QWidget* myWidget = QWidget::createWindowContainer(myWindow);

are you sure that your access violation comes during fromWinId() rather than from createWindowContainer? If it's the latter it might be worth looking at https://stackoverflow.com/questions/30162091/drawing-errors-and-crash-with-qwidgetcreatewindowcontainer.

EmmanuelC

Hello JonB, thank you for your answer. Yes I am sure because I didn't even include QWidget::createWindowContainer yet. (I wanted to test first if the four first functions worked.)

For now my code is like this:

CWnd* cwnd= GetDlgItem(IDC_STATIC);
HWND hWnd = cwnd->GetSafeHwnd();
WId externalWindow = (WId)hWnd;
QWindow *myWindow = QWindow::fromWinId(externalWindow);

I don't call any other function after that, and really don't understand why I have an access violation here!

JonB

@EmmanuelC
I know nothing about this, but...

The example above (@JKSH) using fromWinId() called it on the result of a FindWindow() for a window from another process, externalWindow. https://doc.qt.io/qt-5/qwindow.html#fromWinId says:

Creates a local representation of a window created by another process or by using native libraries below Qt.

You have a CWnd, from an internal window I believe. I don't know how that plays.

As I said, I know nothing, so I probably can't say much more than this....