解决MFC闪烁的CMemDC类

翻译来源：解决MFC闪烁的CMemDC类

https://www.codeproject.com/Articles/33/Flicker-Free-Drawing-In-MFC

我在网上看到几篇很好的文章 CmemDC类 的使用方法，利用CMemDC画图的两种方法，然后找到了CMemDC类的来源算是对CMemDC类资源的补充。

从VS2008开始提供了CMemDC Class

这里主要介绍第三方CMemDC类的封装，还能随时得到最新的CMemDC类，源代码请自行到codeproject网站下载。已搬运至优快云。

• Download demo project - 172 Kb
• Download demo ActiveX control - 28 Kb

一个简单的动画示例用于显示CMemDC几种模式

一、介绍

从MFC应用程序中删除闪烁是一个值得深入讨论领域。您可以在书籍和在线查找主题。然而，所提出的技术有些复杂，通常难以添加到现有的应用程序中。一种经常提出的技术称为双缓冲。双缓冲允许在屏幕外存储器中绘制新屏幕，然后将完成的屏幕置于物理屏幕上。

本文介绍了一个类CMemDC，它将与写入相关的大多数问题封装到离屏缓冲区。添加CMemDC到现有应用程序或MFC Active X控件几乎是微不足道的。

二、修改MFC应用程序使用CMemDC

• 在你的项目中添加文件memdc.h。
• 将#include“memdc.h”行添加到stdafx.h。
• 添加一个Windows消息处理程序WM_ERASEBKGND。
• 更改消息处理程序中的代码如下：
  
  

  // Change this code
  BOOL CExampleView::OnEraseBkgnd(CDC* pDC) 
  {
        // TODO: Add your message handler code here and/or call default
        return CView::OnEraseBkgnd(pDC);
  }
   
  // To this code
  BOOL CExampleView::OnEraseBkgnd(CDC* pDC) 
  {
        return FALSE;
  }

• 将您的OnDraw 代码更改为以下内容：
  
  

  void CExampleView::OnDraw(CDC* dc)
  {
      CMemDC pDC(dc);
      CExampleDoc* pDoc = GetDocument();
      ASSERT_VALID(pDoc);
      // TODO: add draw code for native data here - use pDC 
       //as the device context to draw to
  }

在进行这些更改后，编译代码，并注意到您之前看到的闪烁已经消失了。

三、修改MFC Active X控件以使用CMemDC

要添加CMemDC 支持，请按照向应用程序添加支持的说明进行操作，但是您可以对该OnDraw 功能进行一些小的更改。

void CParticleTestCtlCtrl::OnDraw(CDC* pdc, const CRect& rcBounds, 
                                  const CRect& rcInvalid)
{
    CMemDC pDC(pdc, &rcBounds);
    // TODO: add draw code for native data here
    // - use pDC as the device context to draw 
}

唯一实质的区别是将rcBounds传递给CMemDC构造函数。

四、源代码

#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