PreSubclassWindow函数

PreSubclassWindow是在CWnd::SubclassWindow中调用的，其目的是允许程序在动态子类化控件或窗口之前做一些其它所需要的动作

BOOL CWnd::SubclassWindow(HWND hWnd)
{
    if (!Attach(hWnd))
        return FALSE;

    // allow any other subclassing to occur
    PreSubclassWindow();

    // now hook into the AFX WndProc
    WNDPROC* lplpfn = GetSuperWndProcAddr();
    WNDPROC oldWndProc = (WNDPROC)::SetWindowLongPtr(hWnd, GWLP_WNDPROC,
        (INT_PTR)AfxGetAfxWndProc());
    ASSERT(oldWndProc != AfxGetAfxWndProc());

    if (*lplpfn == NULL)
        *lplpfn = oldWndProc;   // the first control of that type created
#ifdef _DEBUG
    else if (*lplpfn != oldWndProc)
    {
        TRACE(traceAppMsg, 0, "Error: Trying to use SubclassWindow with incorrect CWnd\n");
        TRACE(traceAppMsg, 0, "\tderived class.\n");
        TRACE(traceAppMsg, 0, "\thWnd = $%08X (nIDC=$%08X) is not a %hs.\n", (UINT)(UINT_PTR)hWnd,
            _AfxGetDlgCtrlID(hWnd), GetRuntimeClass()->m_lpszClassName);
        ASSERT(FALSE);
        // undo the subclassing if continuing after assert
      ::SetWindowLongPtr(hWnd, GWLP_WNDPROC, (INT_PTR)oldWndProc);
    }
#endif

    return TRUE;
}