新手问题：请指教下面的这段动画程序错在那里？

Question

//TestView.h
...
public:
CBitmap *pMemBitmap;
CDC *pMemDC;
...
//TestView.cpp
CTestView::CTestView()
{
// TODO: 
m_ntime    = 0;
pMemBitmap = new CBitmap;
pMemDC     = new CDC;
}CTestView::~CTestView()
{
delete pMemBitmap;
delete pMemDC;
}void CTestView::OnDraw(CDC* pDC)
{
CTestDoc* pDoc = GetDocument();
ASSERT_VALID(pDoc);
// TODO: 
pMemDC->CreateCompatibleDC(NULL);
pMemBitmap->CreateCompatibleBitmap(pDC,nWidth,nHeight); OnTimer(1);
}void CTestView::OnTimer(UINT nIDEvent) 
{
// TODO: 
CClientDC dc(this);
int nx, ny; m_ntime++; nx = dc.GetDeviceCaps(HORZRES)/2;
ny = dc.GetDeviceCaps(VERTRES)/2; CBitmap *pOldBit = dc.SelectObject(pMemBitmap); pMemDC->FillSolidRect(0,0,nWidth,nHeight,RGB(168,168,168));
pMemDC->Ellipse(nx + m_ntime, ny + m_ntime, nx + m_ntime, ny + m_ntime); dc.BitBlt(0,0,nWidth,nHeight,pMemDC,0,0,SRCCOPY); Invalidate(); CView::OnTimer(nIDEvent);
}void CTestView::OnInitialUpdate() 
{
CView::OnInitialUpdate();

// TODO: 
SetTimer(1, 100, NULL);
nWidth = nHeight = 1000;
}

Answer 1

pMemDC->CreateCompatibleDC(NULL);
应该为
pDC->CreateCompatibleDC(pMemDC)

Answer 2

Erro!!!
void CTestView::OnDraw(CDC* pDC)
{
CTestDoc* pDoc = GetDocument();
ASSERT_VALID(pDoc);
// TODO: 
pMemDC->CreateCompatibleDC(pDC);                      
pMemBitmap->CreateCompatibleBitmap(pMemDC,nWidth,nHeight); OnTimer(100);
}void CTestView::OnTimer(UINT nIDEvent) 
{
// TODO: 
CClientDC dc(this);
int nx, ny; m_ntime++; nx = dc.GetDeviceCaps(HORZRES)/2;
ny = dc.GetDeviceCaps(VERTRES)/2; CBitmap *pOldBit = pMemDC->SelectObject(pMemBitmap); pMemDC->FillSolidRect(0,0,nWidth,nHeight,RGB(168,168,168));
pMemDC->Ellipse(nx + m_ntime, ny + m_ntime, nx + m_ntime, ny + m_ntime); dc.BitBlt(0,0,nWidth,nHeight,pMemDC,0,0,SRCCOPY); //Invalidate(); //CView::OnTimer(nIDEvent);
}

Answer 3

void CTestView::OnDraw(CDC* pDC)
{
CTestDoc* pDoc = GetDocument();
ASSERT_VALID(pDoc);
// TODO: 
pMemDC->CreateCompatibleDC(pDC);                      
pMemBitmap->CreateCompatibleBitmap(pMemDC,nWidth,nHeight); //OnTimer(100);
}

Answer 4

it should be like this:
//TestView.h
...
public:
CBitmap *pMemBitmap;
CDC *pMemDC;
         int m_nflag;
...CTestView::CTestView()
{
// TODO: 
m_ntime    = 0;
         m_nflag    = 0;
pMemBitmap = new CBitmap;
pMemDC     = new CDC;
}void CTestView::OnDraw(CDC* pDC)
{
CTestDoc* pDoc = GetDocument();
ASSERT_VALID(pDoc);
// TODO: 
         if (m_nflag == 0)
         {
   pMemDC->CreateCompatibleDC(NULL);
   pMemBitmap->CreateCompatibleBitmap(pDC,nWidth,nHeight);    CBitmap *pOldBit = dc.SelectObject(pMemBitmap);
            m_nflag = 1;
          } OnTimer(1);
}void CTestView::OnTimer(UINT nIDEvent) 
{
// TODO: 
CClientDC dc(this);
int nx, ny; m_ntime++; nx = dc.GetDeviceCaps(HORZRES)/2;
ny = dc.GetDeviceCaps(VERTRES)/2; pMemDC->FillSolidRect(0,0,nWidth,nHeight,RGB(168,168,168));
pMemDC->Ellipse(nx + m_ntime, ny + m_ntime, nx + m_ntime, ny + m_ntime); dc.BitBlt(0,0,nWidth,nHeight,pMemDC,0,0,SRCCOPY); Invalidate(); CView::OnTimer(nIDEvent);
}