CMemDC代码解

CMemDC是一个很经典的内存DC，实现了MFC的双缓冲绘图。主题实现思路是，将要绘制的背景绘制到内存中，然后在CMemDC析构的时候绘制到屏幕上。

class CMemDC : public CDC {private:CBitmap m_bitmap; CBitmap* m_oldBitmap; CDC* m_pDC; CRect m_rect; BOOL m_bMemDC; public://内存DC构造函数，根据传入的pDC判断是否为打印机的DC。                //如果为打印机的DC（m_bMemDC为True）,则创建内存m_bitmap内存DC。假如参数bCopyFirst为真，则直接画图。这时就不是双缓冲。假如                //参数bCopyFirst为假，则等到CMemDC析构后才绘图，实现双缓冲。                //如果为打印机DC（m_bMemDC为False）,则说明传入DC非内存DC，简单复制即可，不需要双缓冲。CMemDC(CDC* pDC, CRect rect = CRect(0,0,0,0), BOOL bCopyFirst = FALSE) : CDC(), m_oldBitmap(NULL), m_pDC(pDC){ASSERT(m_pDC != NULL); // If you asserted here, you passed in a NULL CDC.m_bMemDC = !pDC->IsPrinting();if (m_bMemDC){// Create a Memory DCCreateCompatibleDC(pDC);if ( rect == CRect(0,0,0,0) )pDC->GetClipBox(&m_rect);elsem_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 printingm_bPrinting = pDC->m_bPrinting;m_hDC = pDC->m_hDC;m_hAttribDC = pDC->m_hAttribDC;}}//析构函数，假如双缓冲模式，则绘制图。~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 pointerCMemDC* operator->() {return this;}// Allow usage as a pointeroperator CMemDC*() {return this;}};