Using Qt 5, how do I create a user interface (modeless dialog box) in a Qt DLL?

[astodolski]

I understand that the QtGUI has changed significantly from Qt4 to Qt5.

• I want to create a DLL that has a user interface (GUI).
• This DLL must work with third party applications that have their own user interface.
• The third party applications can be written in Qt, .NET, MFC, and other programming languages other than C/C++.
• I created a DLL that displays a modeless dialog box in Qt 4.8.4 and works as designed.
• When I upgraded the DLL for Qt 5.1.1:
  
  • The widgets (combo-box, push button, check box) on the dialog box no longer respond to mouse clicks.
  • The frame is active and the dialog box can be moved around.
  • The window repaints when obscured/exposed by another window.
• The model I am using was provided by the twain consortium and is similar to other sample code I located on this and other forums.

For example, the main entry function in the DLL launches a thread for the QApplication:

// Coerce the QCoreApplication instance if it exists to a QApplication.
	QApplication	*	pApp	= qApp;
	if (pApp)
	{
		// Test for consistency between the application QT version and 
		// the one for the DS.
		int		nV1App	= 0;
		int		nV2App	= 0;
		int		nV3App	= 0;
		SSCANF(qVersion(),"%d.%d.%d",&nV1App, &nV2App, &nV3App);
		int		nV1DS	= (BYTE)(QT_VERSION>>16);
		int		nV2DS	= (BYTE)(QT_VERSION>>8);
//		int		nV3DS	= (BYTE)QT_VERSION;
		if (nV1App < nV1DS || (nV1App == nV1DS && nV2App < nV2DS))
			return TWRC_FAILURE;
	}
	if (!pApp)
	{
#ifdef TWNDS_OS_WIN
		_get_pgmptr(&pWinAppName);
		m_pUIThread		= new UIThread(this, UIInfo);
		m_pUIThread->start();
		// Wait on the semaphore for the thread to start running
		// and that the thread has launched the dialog box.
		ScanXGUIEnabledSem.acquire(1);
#endif

// Coerce the QCoreApplication instance if it exists to a QApplication.
	QApplication	*	pApp	= qApp;
	if (pApp)
	{
		// Test for consistency between the application QT version and 
		// the one for the DS.
		int		nV1App	= 0;
		int		nV2App	= 0;
		int		nV3App	= 0;
		SSCANF(qVersion(),"%d.%d.%d",&nV1App, &nV2App, &nV3App);
		int		nV1DS	= (BYTE)(QT_VERSION>>16);
		int		nV2DS	= (BYTE)(QT_VERSION>>8);
//		int		nV3DS	= (BYTE)QT_VERSION;
		if (nV1App < nV1DS || (nV1App == nV1DS && nV2App < nV2DS))
			return TWRC_FAILURE;
	}
	if (!pApp)
	{
#ifdef TWNDS_OS_WIN
		_get_pgmptr(&pWinAppName);
		m_pUIThread		= new UIThread(this, UIInfo);
		m_pUIThread->start();
		// Wait on the semaphore for the thread to start running
		// and that the thread has launched the dialog box.
		ScanXGUIEnabledSem.acquire(1);
#endif

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The thread (m_pUIThread) creates the QApplication in its run method:

void UIThread::run()
{
	// Launch a QApplciation. (Windows only)
	// There are no command line arguments.
	int		argc	= 1;
	m_pQtApp		= new QApplication(argc, &pWinAppName);
	if (m_pQtApp)
	{
#ifdef TWNDS_OS_WIN
		// Create the dlg and make it visible.
		m_pDlg		= new ScanXGUI(m_pUI, m_pChildWnd);
		if (m_pDlg)
		{
			pScanXDlg	= m_pDlg;
			// Register to receive events.
			m_pQtApp->installEventFilter(m_pDlg);
			// Display the UI.
			m_pDlg->show();
			ScanXGUIEnabledSem.release(1);
			// Launch the message loop and do not return until thread closes.
			m_pQtApp->exec();
			// Ensure that the dialog box and class are destroyed.
			// done() causes the local event loop to finish.
			int		CloseResult	= 1;
			m_pDlg->done(CloseResult);
			delete m_pDlg;
			m_pDlg	= NULL;
		}
#endif
		delete m_pQtApp;
	}
	m_pQtApp	= NULL;
}

void UIThread::run()
{
	// Launch a QApplciation. (Windows only)
	// There are no command line arguments.
	int		argc	= 1;
	m_pQtApp		= new QApplication(argc, &pWinAppName);
	if (m_pQtApp)
	{
#ifdef TWNDS_OS_WIN
		// Create the dlg and make it visible.
		m_pDlg		= new ScanXGUI(m_pUI, m_pChildWnd);
		if (m_pDlg)
		{
			pScanXDlg	= m_pDlg;
			// Register to receive events.
			m_pQtApp->installEventFilter(m_pDlg);
			// Display the UI.
			m_pDlg->show();
			ScanXGUIEnabledSem.release(1);
			// Launch the message loop and do not return until thread closes.
			m_pQtApp->exec();
			// Ensure that the dialog box and class are destroyed.
			// done() causes the local event loop to finish.
			int		CloseResult	= 1;
			m_pDlg->done(CloseResult);
			delete m_pDlg;
			m_pDlg	= NULL;
		}
#endif
		delete m_pQtApp;
	}
	m_pQtApp	= NULL;
}

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