帮忙看看这个按钮类,为个么会闪.该怎么解决

帮忙看看这个按钮类,为个么会闪.
class   CButtonEx   :   public   CButton
{
public:
CButtonEx(CString   str,int   nFontSize);
CButtonEx();
public:
virtual   ~CButtonEx();
protected:
DECLARE_MESSAGE_MAP()
private:
CBitmap   *m_pBitmap,*m_pBitmapSel;
CString   m_str;
int   m_nFontSize;
public:
void   SetPromptText(CString   str,int   nFontSize);
virtual   void   DrawItem(   LPDRAWITEMSTRUCT   lpDrawItemStruct   );
void   SetBitmap(CBitmap   *pBitmap,CBitmap   *pBitmapSel=NULL);
};

//////////////////////////////////////////////////////////

CButtonEx::CButtonEx(CString   str,int   nFontSize)
{
m_str=str;
m_nFontSize=nFontSize;
}

CButtonEx::~CButtonEx()
{
}


BEGIN_MESSAGE_MAP(CButtonEx,   CButton)
//{{AFX_MSG_MAP(CButtonEx)
//   NOTE   -   the   ClassWizard   will   add   and   remove   mapping   macros   here.
//}}AFX_MSG_MAP
END_MESSAGE_MAP()

/////////////////////////////////////////////////////////////////////////////
//   CButtonEx   message   handlers
void   CButtonEx::DrawItem(   LPDRAWITEMSTRUCT   lpDIS)
{
//   use   the   main   bitmap   for   up,   the   selected   bitmap   for   down
CBitmap*   pBitmap   =   m_pBitmap;
UINT   state   =   lpDIS-> itemState;
if   ((state   &   ODS_SELECTED)   &&   m_pBitmapSel-> m_hObject   !=   NULL)
pBitmap   =   m_pBitmapSel;

//   draw   the   whole   button
CDC*   pDC   =   CDC::FromHandle(lpDIS-> hDC);

CDC   memDC;
memDC.CreateCompatibleDC(pDC);
CBitmap*   pOld   =   memDC.SelectObject(pBitmap);
if   (pOld   ==   NULL)
{
return;           //   destructors   will   clean   up
}
CRect   rect;
rect.CopyRect(&lpDIS-> rcItem);
pDC-> BitBlt(rect.left,   rect.top,   rect.Width(),   rect.Height(),&memDC,   0,   0,   SRCCOPY);
CFont   font;
font.CreatePointFont(m_nFontSize,TEXT( "Arial "),pDC);
CFont   *pOldFont=pDC-> SelectObject(&font);
//pOldFont=pDC-> SelectObject(&font);
pDC-> SetBkMode(TRANSPARENT);
pDC-> DrawText(m_str,rect,DT_CENTER|DT_VCENTER|DT_INTERNAL);
memDC.SelectObject(pOld);
pDC-> SelectObject(pOldFont);

};


void   CButtonEx::SetBitmap(CBitmap   *pBitmap,CBitmap   *pBitmapSel)
{
m_pBitmap   =   pBitmap;
m_pBitmapSel   =   pBitmapSel;
if   (pBitmapSel==NULL)
{
m_pBitmapSel   =   pBitmap;
}
Invalidate();
};

CButtonEx::CButtonEx()
{
m_str=TEXT( " ");
m_nFontSize=1;
}


void   CButtonEx::SetPromptText(CString   str,int   nFontSize)
{
m_str=str;
m_nFontSize=nFontSize;
}


------解决方案--------------------
应该是文字会闪吧，
用双缓存绘图
------解决方案--------------------
你把SetBitmap中的Invalidate()注释掉试下，估计是该函数让按钮一直重绘的原因
------解决方案--------------------