How to Make the CMiniFrameWnd Window Paint Its Background Area

The information in this article applies to:

• The Microsoft Foundation Classes (MFC) included with: Microsoft Visual C++, 32-bit Edition, versions 2.0, 2.1, 2.2, 4.0, and 4.1

SUMMARY

This article gives you two ways to make the CMiniFrameWnd miniframe window paint its background area.

A miniframe window (CMiniFrameWnd) doesn't paint its background area even though Windows sends a WM_ERASEBKGND message to prepare the invalidated portion of the window for painting. The default window procedure for this message erases the background by using the class background brush specified by the hbrBackground member of the WNDCLASS structure. As this background brush is NULL for a CMiniFrameWnd and MFC doesn't provide a handler for WM_ERASEBKGND to erase the background of a CMiniFrameWnd, the client area of a miniframe window is never erased.

MORE INFORMATION

A CMiniFrameWnd is a half-height/half-caption frame window typically enclosing another child window that takes up the entire client area of the window. The default window class for a CMiniFrameWnd is pre-registered by MFC without a background brush; that is, the hbrBackground member of the WNDCLASS structure is NULL.

The application sends a WM_ERASEBKGND message to prepare the invalidated portion of a window for painting. The window procedure of MFC (AfxWndProc) receives this message first and in turn calls the default window procedure (DefWindowProc), which erases the background by using the class background brush. Because there is no background brush for a CMiniFrameWnd, the background area is never painted.

This default behavior is by design because there is no need for the miniframe window to repaint its background, as the child window typically enclosing the miniframe window will paint over its background area.

Following are two ways to make the CMiniFrameWnd paint its background area.

Method One

This method registers a window class for CMiniFrameWnd with a background brush. To implement this, derive a class from CMiniFrameWnd and override the Create member function. The Create function would be similar to the default implementation of CMiniFrameWnd::Create (in WINMINI.CPP of ~/mfc/src directory), but with the addition of a background brush. The following code demonstrates this:

// Header file of CMiniFrameWnd derived class (mymini.h)

    DECLARE_DYNCREATE(CMyMiniFrameWnd)

    CMyMiniFrameWnd();

    // Add the declaration for the Create function
    BOOL Create(LPCTSTR lpClassName, LPCTSTR lpszWindowName,
         DWORD dwStyle, const RECT& rect,
         CWnd *pParentWnd = NULL, UINT nID = 0);

    // Rest of the class definition
    ...

// Implementation file of CMiniFrameWnd derived class (mymini.cpp)

    LPCTSTR lpszClassName, LPCTSTR lpszWindowName,
    DWORD dwStyle, const RECT& rect,
    CWnd* pParentWnd, UINT nID)

    // Window title
    m_strCaption = lpszWindowName;

    // Register your own window class
    return CreateEx(0,
        lpszClassName ?
            lpszClassName :
            AfxRegisterWndClass(
                CS_DBLCLKS,
                ::LoadCursor(NULL, IDC_ARROW),
                (HBRUSH) (COLOR_WINDOW + 1)), // Specify bkgnd brush
        lpszWindowName,
        dwStyle,
        rect.left, rect.top,
        rect.right - rect.left, rect.bottom - rect.top,
        pParentWnd->GetSafeHwnd(),
        (HMENU)nID);
}

Method Two

This method illustrates how to provide a handler for the WM_ERASEBKGND message for the miniframe window. By implementing this, the miniframe window paints its own background area in response to a WM_ERASEBKGND. Use ClassWizard to provide a handler for WM_ERASEBKGND for a CMiniFrameWnd derived class. Modify the handler as shown in this code:

BOOL CMyMiniFrameWnd::OnEraseBkgnd(CDC* pDC) {

    // Set brush to desired background color
    CBrush backBrush(RGB(192, 192, 192));

    // Save the old brush
    CBrush* pOldBrush = pDC->SelectObject(&backBrush);

    // Get the current clipping boundary
    CRect rect;
    pDC->GetClipBox(&rect);

    // Erase the area needed
    pDC->PatBlt(rect.left, rect.top, rect.Width(), rect.Height(),
         PATCOPY);

    pDC->SelectObject(pOldBrush); // Select the old brush back
    return TRUE;  // message handled

REFERENCES

For more information, please see WM_ERASEBKGND in the Windows API documentation MFC source code.