HOWTO: How to Detect Mouse Clicks on Client Area of MDI Frame Windows (133716)

MORE INFORMATION

To manage its MDI child windows, the CMDIFrameWnd class creates a window of the class "mdiclient" to cover its entire client area (also referred to as the Application Workspace). MFC stores a handle to this MDIClient window in a public member variable, m_hWndMDIClient, of the CMDIFrameWnd class.

The MDIClient is a standard Windows window, not an MFC object. However, subclassing it allows you to treat it just as you would any other CWnd, taking advantage of standard MFC features like message maps. For example, you could use this technique to provide a context menu when the user clicks the right mouse button. Or, as the Windows 95 desktop does, you could use it to provide an easy means of selecting a group of iconized MDI child windows, enclosing them within a tracker rectangle as the user drags with the left mouse button button held down.

Steps to Subclass the MDIClient

Subclassing the MDIClient is actually quite easy as summarized in the following three steps. For more detailed information about this process, please see the following article in the Microsoft Knowledge Base:

1. Derive a class from CWnd, called CMyMDIClient for example, and add it to your project. You can use ClassWizard to help you do this.
2. If you are creating a 16-bit application, you need to add a public function called GetSuperWndProcAddr() to your new CMyMDIClient class. Do not do this step when building applications with 32-bit versions of Visual C++.

      WNDPROC* CMyMDIClient::GetSuperWndProcAddr()
          {
          static WNDPROC NEAR pfnSuper = NULL;
          return &pfnSuper;
          }
   						

3. Include your class header file in MAINFRM.H. Embed a public member object of your new class in your CMainFrame class, for example CMyMDIClient m_MyMDIClient. If you have not already done so, override CMDIFrameWnd::OnCreate(). In your CMainFrame::OnCreate(), first call the base class implementation, CMDIFrameWnd::OnCreate(), which creates the MDIClient window itself and stores the handle in m_hWndMDIClient. Then subclass that window to your embedded member object:

      int CMainFrame::OnCreate(LPCREATESTRUCT lpCreateStruct)
          {
          // Call the base class implementation to create the MDIClient
          // window.
          if (CMDIFrameWnd::OnCreate(lpCreateStruct) == -1)
            return -1;
   
          // Subclass the MDIClient window.
          if (!m_MyMDIClient.SubclassWindow(m_hWndMDIClient))
              {
              TRACE ("Failed to subclass MDI client window\n");
              return (-1);
              }
   
          ...
          }
   						

Code Sample

Once you subclass the MDIClient in this way, you can add any needed message handlers directly to your CMyMDIClient class. To illustrate, the following code displays your application's File menu as a context menu when the user clicks the MDIClient using the right mouse button.

   void CMyMDIClient::OnRButtonDown(UINT nFlags, CPoint point)
       {
       POINT ScreenPoint = point;
       CMenu* pMenuTrackPopup;

       // Get a pointer to the app's File menu popup.
       // AfxGetMainWnd() returns a pointer to the main frame window,
       // GetMenu() returns a pointer to the main menu of the application,
       // and GetSubMenu(0) retrieves the submenu at position 0 (here the
       // File menu).
       pMenuTrackPopup = ((AfxGetMainWnd())->GetMenu())->GetSubMenu(0);

       // Convert the mouse point to screen coordinates since that is what
       // TrackPopupMenu() expects.
       ClientToScreen(&ScreenPoint);

       // Draw and track the "floating" popup
       pMenuTrackPopup->TrackPopupMenu(TPM_RIGHTBUTTON,
          ScreenPoint.x, ScreenPoint.y,
           AfxGetMainWnd(),    // Use the 'this' pointer if you want
                               // commands to be handled in your
                               // CMyMDIClient class instead
           NULL);

       // NOTE: Do not destroy this menu here!

       // Call the base class
       CWnd::OnRButtonDown(nFlags, point);
       }