CMemDC代碼解

CMemDC是一個很經典的內存DC，實現了MFC的雙緩衝繪圖。主題實現思路是，將要繪製的背景繪製到內存中，然後在CMemDC析構的時候繪製到屏幕上。

class CMemDC : public CDC {
    private:
        CBitmap m_bitmap;
        CBitmap* m_oldBitmap;
        CDC* m_pDC;
        CRect m_rect;
        BOOL m_bMemDC;
    public:
        //內存DC構造函數，根據傳入的pDC判斷是否爲打印機的DC。
                //如果爲打印機的DC（m_bMemDC爲True）,則創建內存m_bitmap內存DC。假如參數bCopyFirst爲真，則直接畫圖。這時就不是雙緩衝。假如
                //參數bCopyFirst爲假，則等到CMemDC析構後才繪圖，實現雙緩衝。
                //如果爲打印機DC（m_bMemDC爲False）,則說明傳入DC非內存DC，簡單複製即可，不需要雙緩衝。
        CMemDC(CDC* pDC, CRect rect = CRect(0,0,0,0), BOOL bCopyFirst = FALSE) : CDC(), m_oldBitmap(NULL), m_pDC(pDC)
        {
            ASSERT(m_pDC != NULL); // 判斷定宏，肯定m_pDC不爲空，若爲空這不繼續進行

            m_bMemDC = !pDC->IsPrinting();

            if (m_bMemDC){
                // Create a Memory DC
                CreateCompatibleDC(pDC); 
//創建內存環境
                if ( rect == CRect(0,0,0,0) )
                    pDC->GetClipBox(&m_rect);
                else
                    m_rect = rect;

                m_bitmap.CreateCompatibleBitmap(pDC, m_rect.Width(), m_rect.Height());
                m_oldBitmap = SelectObject(&m_bitmap);
                SetWindowOrg(m_rect.left, m_rect.top);
                if(bCopyFirst)
                {
                    this->BitBlt(m_rect.left, m_rect.top, m_rect.Width(), m_rect.Height(),
                            m_pDC, m_rect.left, m_rect.top, SRCCOPY);
                }
            } 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;
            }
        }

        //析構函數，假如雙緩衝模式，則繪製圖。
        ~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;}
    };