The EnumChildWindows function enumerates the child windows that belong to the specified parent window by passing the handle to each child window, in turn, to an application-defined callback function. EnumChildWindows

 

主框架调用EnumChildWindows让每个子窗口都去执行SendPrepareToClose去做每个子窗口的清理工作。

BOOL CMainFrame::DestroyWindow()
{
    ::EnumChildWindows(m_hWnd, SendPrepareToClose, 0);     return CMDIFrameWnd::DestroyWindow();
}  
static BOOL CALLBACK SendPrepareToClose(HWND hWnd, LPARAM)
{
    CWnd* pWnd = CWnd::FromHandlePermanent(hWnd);
    if (pWnd != NULL)
    {
        pWnd->SendMessage(WM_USER_PREPARE_TO_CLOSE);
        if (pWnd->GetWindow(GW_CHILD) != NULL)
            ::EnumChildWindows(pWnd->m_hWnd, SendPrepareToClose, 0);
    }
    return TRUE;
}