the study of programmng windows with mfc--timer

BEGIN_MESSAGE_MAP (CMainWindow, CFrameWnd)
    ON_WM_CREATE ()
    ON_WM_TIMER ()
END_MESSAGE_MAP ()

int CMainWindow::OnCreate (LPCREATESTRUCT lpcs)
{
    if (CFrameWnd::OnCreate (lpcs) == -1)
        return -1;

    if (!SetTimer (ID_TIMER_ELLIPSE, 100, NULL) ¦¦
        !SetTimer (ID_TIMER_RECTANGLE, 100, NULL)) {
        MessageBox (_T ("Error: SetTimer failed"));
        return -1;
    }
    return 0;
}

void CMainWindow::OnTimer (UINT nTimerID)
{
    CRect rect;
    GetClientRect (&rect);

    int x1 = rand () % rect.right;
    int x2 = rand () % rect.right;
    int y1 = rand () % rect.bottom;
    int y2 = rand () % rect.bottom;

    CClientDC dc (this);
    CBrush brush (RGB (rand () % 255, rand () % 255, rand () % 255));
    CBrush* pOldBrush = dc.SelectObject (&brush);
    if (nTimerID == ID_TIMER_ELLIPSE)
        dc.Ellipse (min (x1, x2), min (y1, y2), max (x1, x2),
            max (y1, y2));
    else // nTimerID == ID_TIMER_RECTANGLE
        dc.Rectangle (min (x1, x2), min (y1, y2), max (x1, x2),
            max (y1, y2));
    dc.SelectObject (pOldBrush);
}