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原文：http://yuantao82.spaces.live.com/Blog/cns!8FC0A772D812A22F!139.entry?owner=1

#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 accidentally 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

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