// This class implements a memory Device Contextclass CMemDC : public CDC { public: // constructor sets up the memory DC CMemDC(CDC* pDC) : CDC() { ASSERT(pDC != NULL); m_pDC = pDC; m_pOldBitmap = NULL; m_bMemDC = !pDC->IsPrinting(); if (m_bMemDC) // Create a Memory DC { pDC->GetClipBox(&m_rect); CreateCompatibleDC(pDC); m_bitmap.CreateCompatibleBitmap(pDC, m_rect.Width(), m_rect.Height()); m_pOldBitmap = SelectObject(&m_bitmap); 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; } } // Destructor copies the contents of the mem DC to the original DC ~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_pOldBitmap); } 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;}private: CBitmap m_bitmap; // Offscreen bitmap CBitmap* m_pOldBitmap; // 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. };
参考MSDN中的DrawCli示例(MFC)
bm2=new CBitmap;
bm2->CreateBitmap ();
mem.SelectObject(bm2);
m_pDC->BitBlt(0,0,rctClient.Width(),rctClient.Height(),&mem,0,0,SRCCOPY);
{
public: // constructor sets up the memory DC
CMemDC(CDC* pDC) : CDC()
{
ASSERT(pDC != NULL);
m_pDC = pDC;
m_pOldBitmap = NULL;
m_bMemDC = !pDC->IsPrinting(); if (m_bMemDC) // Create a Memory DC
{
pDC->GetClipBox(&m_rect);
CreateCompatibleDC(pDC);
m_bitmap.CreateCompatibleBitmap(pDC, m_rect.Width(), m_rect.Height());
m_pOldBitmap = SelectObject(&m_bitmap);
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;
}
} // Destructor copies the contents of the mem DC to the original DC
~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_pOldBitmap);
} 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;}private:
CBitmap m_bitmap; // Offscreen bitmap
CBitmap* m_pOldBitmap; // 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.
};