|  | Home

# Qt based Application Extension

This examples shows how to use the QWinWidget and QMfcApp classes to implement a Qt based user interface in a plugin DLL.

The plugin implements and exports a `C` function `showDialog` that can be called by any Windows application to display a modal Qt dialog. Before a Qt based user interface can be created a QApplication object must exist, and the calling application's event loop and the Qt event loop must run together.

     /****************************************************************************
     **
     ** Copyright (C) 2013 Digia Plc and/or its subsidiary(-ies).
     ** Contact: http://www.qt-project.org/legal
     **
     ** This file is part of the Qt Solutions component.
     **
     ** You may use this file under the terms of the BSD license as follows:
     **
     ** "Redistribution and use in source and binary forms, with or without
     ** modification, are permitted provided that the following conditions are
     ** met:
     **   * Redistributions of source code must retain the above copyright
     **     notice, this list of conditions and the following disclaimer.
     **   * Redistributions in binary form must reproduce the above copyright
     **     notice, this list of conditions and the following disclaimer in
     **     the documentation and/or other materials provided with the
     **     distribution.
     **   * Neither the name of Nokia Corporation and its Subsidiary(-ies) nor
     **     the names of its contributors may be used to endorse or promote
     **     products derived from this software without specific prior written
     **     permission.
     **
     ** THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
     ** "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
     ** LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
     ** A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
     ** OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
     ** SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
     ** LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
     ** DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
     ** THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
     ** (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
     ** OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE."
     **
     ****************************************************************************/

     #include <qmfcapp.h>
     #include <qwinwidget.h>

     #include <QtGui/QMessageBox>
     #include <windows.h>

     BOOL WINAPI DllMain( HINSTANCE hInstance, DWORD dwReason, LPVOID /*lpvReserved*/ )
     {
         static bool ownApplication = FALSE;

         if ( dwReason == DLL_PROCESS_ATTACH )
             ownApplication = QMfcApp::pluginInstance( hInstance );
         if ( dwReason == DLL_PROCESS_DETACH && ownApplication )
             delete qApp;

         return TRUE;
     }

The DLL entry point function `DllMain` uses the QMfcApp::pluginInstance function to make sure that exactly one instance of QApplication exists in the process, and that the DLL owning that instance stays loaded in memory.

     extern "C" __declspec(dllexport) bool showDialog( HWND parent )
     {
         QWinWidget win( parent );
         win.showCentered();
         QMessageBox::about( &win, "About QtMfc", "QtMfc Version 1.0\nCopyright (C) 2003" );

         return TRUE;
     }

The `C` function `showDialog` is exported from the DLL and uses the QWinWidget class to provide proper stacking and modality between the native Win32 window and the QMessageBox.