OnMouseWheel的通常处理

BOOL CMainWindow::OnMouseWheel(UINT nFlags,
                             short zDelta,
                             CPoint point)
{
    BOOL bUp = TRUE;
    int nDelta = zDelta;
    if(zDelta < 0) {
        bUp = FALSE;
        nDelta = -nDelta;
    }
    UINT nWheelScrollLines;
    ::SystemParametersInfo(SPI_GETWHEELSCROLLLINES, 0,
        &nWheelScrollLines, 0);
    int nLines = (nDelta * nWheelScrollLines) / WHEEL_DELTA;
    while(nLines--)
        SendMessage(WM_VSCROLL, MAKEWPARAM(bUp ? SB_LINEUP : SB_LINEDOWN, 0), 0);
    return TRUE;
}