A Better CenterWindow() Function

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Figure 1. The main CenterSample sample program screen.

Introduction

Centering windows on the screen is something which you can normally do with the CWnd::CenterWindow() function in MFC. CenterWindow() takes a pointer to a CWnd as its argument, and supposedly the function will center the window on which it's called against the window to which you pass it a pointer:

pDlg->CenterWindow(AfxGetMainWnd()); // Centers pDlg against the main window?

Listing 1. Demonstrating a use of CWnd::CenterWindow() to center a dialog.

However, a question posed to the MFC Mailing List recently asked, "I have a dialog-based program, where the user can click a button and have a sub-dialog pop up. If I call CWnd::CenterWindow() in the sub-dialog's OnInitDialog() handler, the dialog will always be centered in the center of the screen, not centered with respect to the main dialog. How do I do this?"

So I came up with a "Brute Force" centering function which actually works better than CWnd::CenterWindow(). It's called CSubDialog::CenterWindowOnOwner(), and I added to my sample program's sub-dialog's class, CSubDialog:

void CSubDialog::CenterWindowOnOwner(CWnd* pWndToCenterOn)
{
    // Get the client rectangle of the window on which we want to center
    // Make sure the pointer is not NULL first

    if (pWndToCenterOn == NULL)
        return;
 
    CRect rectToCenterOn;
    pWndToCenterOn->GetWindowRect(&rectToCenterOn);

    // Get this window's area
    CRect rectSubDialog;
    GetWindowRect(&rectSubDialog);

    // Now rectWndToCenterOn contains the screen rectangle of the window 
    // pointed to by pWndToCenterOn.  Next, we apply the same centering 
    // algorithm as does CenterWindow()

    // find the upper left of where we should center to

    int xLeft = (rectToCenterOn.left + rectToCenterOn.right) / 2 - 
        rectSubDialog.Width() / 2;
    int yTop = (rectToCenterOn.top + rectToCenterOn.bottom) / 2 - 
        rectSubDialog.Height() / 2;

    // Move the window to the correct coordinates with SetWindowPos()
    SetWindowPos(NULL, xLeft, yTop, -1, -1,
        SWP_NOSIZE | SWP_NOZORDER | SWP_NOACTIVATE);
}

Listing 2. Our brute-force CenterWindowOnOwner() function.

Then I added code to CSubDialog::OnInitDialog() to center it with respect to the main dialog, which is the main window of the application:

BOOL CSubDialog::OnInitDialog()
{
    CDialog::OnInitDialog();

    ...

    CWnd* pMainWnd = AfxGetMainWnd();
    CenterWindowOnOwner(pMainWnd);

    return TRUE;
}

Listing 3.How to call CenterWindowOnOwner().

And voila! The sub-dialog will always center itself on the main dialog (or main application window), no matter where on the screen that window is placed.