PRB: GetActiveView Returns NULL for an MDI Frame Window

WINDOWS             | WINDOWS NT

The information in this article applies to:

• The Microsoft Foundation Classes (MFC) included with:

       - Microsoft Visual C++ for Windows, versions 1.0, 1.5, 1.51, 1.52
       - Microsoft Visual C++, 32-bit Edition, versions 1.0, 2.0, 2.1,
         and 4.0
  

SYMPTOMS

The GetActiveView() function returns the active view (if any) attached to a frame window (CFrameWnd). This function returns NULL when called for an MDI main frame window (CMDIFrameWnd).

CAUSE

In an MDI application, the MDI main frame window (CMDIFrameWnd) does not have a view associated with it. Instead, each individual child window (CMDIChildWnd) has one or more views associated with it. Therefore, a call to the GetActiveView() function for an MDI main frame window returns NULL. The "More Information" section of this article gives the layout of the different windows in an MDI application.

RESOLUTION

To resolve the problem, find the active MDI child window (if any), and then call the GetActiveView() function for that child window. The active MDI child window can be found by calling the MDIGetActive() or GetActiveFrame() function, as demonstrated in this code:

   CMDIFrameWnd *pFrame = (CMDIFrameWnd *)AfxGetApp()->m_pMainWnd;
   // Get the active MDI child window.
   CMDIChildWnd *pChild =
                  (CMDIChildWnd *) pFrame->GetActiveFrame();
   /* or CMDIChildWnd *pChild =
                  pFrame->MDIGetActive();
   */
   // Get the active view attached to the active MDI child window.
   CMyView *pView = (CMyView *) pChild->GetActiveView();

MORE INFORMATION

The client area of the MDI frame window is covered by the client window referred to as the MDICLIENT, also known as the Workspace. This client window in turn contains zero or more child windows, each of which can display a view, as in this diagram:

                    ---------------------------
                   |      Frame Window         |  ---> CMDIFrameWnd
                   | (main application window) |
                    ---------------------------
                             |
                    --------------------
                   |   Client Window    |  ---> MDICLIENT window class
                   | (no view attached) |
                    --------------------
                             |
         -----------------------------------------------------------
         |                            |
 ---------------------      --------------------
|   Child Window 1    |    |   Child Window 2   |  ---> CMDIChildWnd
| (one or more views) |    | (one or more views |
 ---------------------      --------------------

Therefore, calling the GetActiveView() function for an MDI main frame window returns NULL, implying that there is no view directly associated with MDICLIENT. The active view in an MDI application can be obtained by first finding the active MDI child window and then finding the active view for that child window as outlined in the Resolution section of this article.

REFERENCES

"Programming Windows 3.1," Charles Petzold, Microsoft Press, 1992.