一个很有用的内存DC类

#ifndef _MEMDC_H_
#define _MEMDC_H_

//////////////////////////////////////////////////
// CMemDC - memory DC
//
// Author: Keith Rule
// Email:  keithr@europa.com
// Copyright 1996-2002, Keith Rule
//
// You may freely use or modify this code provided this
// Copyright is included in all derived versions.
//
// History - 10/3/97 Fixed scrolling bug.
//                   Added print support. - KR
//
//           11/3/99 Fixed most common complaint. Added
//                   background color fill. - KR
//
//           11/3/99 Added support for mapping modes other than
//                   MM_TEXT as suggested by Lee Sang Hun. - KR
//
//           02/11/02 Added support for CScrollView as supplied
//                    by Gary Kirkham. - KR
//
// This class implements a memory Device Context which allows
// flicker free drawing.

class CMemDC : public CDC {
private: 
 CBitmap  m_bitmap;  // Offscreen bitmap
 CBitmap* m_oldBitmap; // bitmap originally found in CMemDC
 CDC*  m_pDC;   // Saves CDC passed in constructor
 CRect  m_rect;   // Rectangle of drawing area.
 BOOL  m_bMemDC;  // TRUE if CDC really is a Memory DC.
public:
 
 CMemDC(CDC* pDC, const CRect* pRect = NULL) : CDC()
 {
  ASSERT(pDC != NULL);

  // Some initialization
  m_pDC = pDC;
  m_oldBitmap = NULL;
  m_bMemDC = !pDC->IsPrinting();

  // Get the rectangle to draw
  if (pRect == NULL) {
   pDC->GetClipBox(&m_rect);
  } else {
   m_rect = *pRect;
  }

  if (m_bMemDC) {
   // Create a Memory DC
   CreateCompatibleDC(pDC);
   pDC->LPtoDP(&m_rect);

   m_bitmap.CreateCompatibleBitmap(pDC, m_rect.Width(), m_rect.Height());
   m_oldBitmap = SelectObject(&m_bitmap);

   SetMapMode(pDC->GetMapMode());

   SetWindowExt(pDC->GetWindowExt());
   SetViewportExt(pDC->GetViewportExt());

   pDC->DPtoLP(&m_rect);
   SetWindowOrg(m_rect.left, m_rect.top);
  } else {
   // Make a copy of the relevent parts of the current DC for printing
   m_bPrinting = pDC->m_bPrinting;
   m_hDC       = pDC->m_hDC;
   m_hAttribDC = pDC->m_hAttribDC;
  }

  // Fill background
  FillSolidRect(m_rect, pDC->GetBkColor());
 }
 
 ~CMemDC() 
 {  
  if (m_bMemDC) {
   // Copy the offscreen bitmap onto the screen.
   m_pDC->BitBlt(m_rect.left, m_rect.top, m_rect.Width(), m_rect.Height(),
    this, m_rect.left, m_rect.top, SRCCOPY);   
   
   //Swap back the original bitmap.
   SelectObject(m_oldBitmap);  
  } else {
   // All we need to do is replace the DC with an illegal value,
   // this keeps us from accidently deleting the handles associated with
   // the CDC that was passed to the constructor.   
   m_hDC = m_hAttribDC = NULL;
  } 
 }
 
 // Allow usage as a pointer 
 CMemDC* operator->()
 {
  return this;
 } 

 // Allow usage as a pointer 
 operator CMemDC*()
 {
  return this;
 }
};

#endif