请大家看这样一段代码(解释一下清除的原因和如何实现我想象的那种效果）：

[pdswb]

其实是技术内幕第四版p84页的，
1，为什么代码中没有类似对橡皮筋那样的处理，就使椭圆矩形无效了？
2，如果我想得到这样的效果，当用鼠标拖动椭圆的时候，黑色方块向右移动，如何处理？
我用的是在全局指定i=100,j=200,然后，
在void CEx05cView::OnDraw(CDC* pDC)
{
CBrush brushHatch(HS_DIAGCROSS, RGB(255, 0, 0));
CPoint point(0, 0); // logical (0, 0)

pDC->LPtoDP(&point); // In device coordinates,
pDC->SetBrushOrg(point); // align the brush with
// the window origin
pDC->SelectObject(&brushHatch);
pDC->Ellipse(CRect(m_pointTopLeft, m_sizeEllipse));
pDC->SelectStockObject(BLACK_BRUSH); // Deselect brushHatch
InvalidateRec(CRect(i, -100, j, -200),true);//加入这一行，这是类似于本例对椭圆的处

//理，可是不行啊！！！
pDC->Rectangle(CRect(++i, -100, ++j, -200)); // Test invalid rect
}
*************************************************************
#include "stdafx.h"
#include "ex05c.h"

#include "ex05cDoc.h"
#include "ex05cView.h"

#ifdef _DEBUG
#define new DEBUG_NEW
#undef THIS_FILE
static char THIS_FILE[] = __FILE__;
#endif

/////////////////////////////////////////////////////////////////////////////
// CEx05cView

IMPLEMENT_DYNCREATE(CEx05cView, CScrollView)

BEGIN_MESSAGE_MAP(CEx05cView, CScrollView)
//{{AFX_MSG_MAP(CEx05cView)
ON_WM_LBUTTONDOWN()
ON_WM_LBUTTONUP()
ON_WM_MOUSEMOVE()
//}}AFX_MSG_MAP
END_MESSAGE_MAP()

/////////////////////////////////////////////////////////////////////////////
// CEx05cView construction/destruction

CEx05cView::CEx05cView() : m_sizeEllipse(100, -100),
m_pointTopLeft(0, 0),
m_sizeOffset(0, 0)
{
m_bCaptured = FALSE;
}

CEx05cView::~CEx05cView()
{
}

BOOL CEx05cView:reCreateWindow(CREATESTRUCT& cs)
{
// TODO: Modify the Window class or styles here by modifying
// the CREATESTRUCT cs

return CScrollView:reCreateWindow(cs);
}

/////////////////////////////////////////////////////////////////////////////
// CEx05cView drawing

void CEx05cView::OnDraw(CDC* pDC)
{
CBrush brushHatch(HS_DIAGCROSS, RGB(255, 0, 0));
CPoint point(0, 0); // logical (0, 0)

pDC->LPtoDP(&point); // In device coordinates,
pDC->SetBrushOrg(point); // align the brush with
// the window origin
pDC->SelectObject(&brushHatch);
pDC->Ellipse(CRect(m_pointTopLeft, m_sizeEllipse));
pDC->SelectStockObject(BLACK_BRUSH); // Deselect brushHatch
pDC->Rectangle(CRect(100, -100, 200, -200)); // Test invalid rect
}

void CEx05cView::OnInitialUpdate()
{
CScrollView::OnInitialUpdate();

CSize sizeTotal(800, 1050); // 8-by-10.5 inches
CSize sizePage(sizeTotal.cx / 2, sizeTotal.cy / 2);
CSize sizeLine(sizeTotal.cx / 50, sizeTotal.cy / 50);
SetScrollSizes(MM_LOENGLISH, sizeTotal, sizePage, sizeLine);
}

/////////////////////////////////////////////////////////////////////////////
// CEx05cView diagnostics

#ifdef _DEBUG
void CEx05cView::AssertValid() const
{
CScrollView::AssertValid();
}

void CEx05cView:ump(CDumpContext& dc) const
{
CScrollView:ump(dc);
}

CEx05cDoc* CEx05cView::GetDocument() // non-debug version is inline
{
ASSERT(m_pDocument->IsKindOf(RUNTIME_CLASS(CEx05cDoc)));
return (CEx05cDoc*)m_pDocument;
}
#endif //_DEBUG

/////////////////////////////////////////////////////////////////////////////
// CEx05cView message handlers

void CEx05cView::OnLButtonDown(UINT nFlags, CPoint point)
{
CRect rectEllipse(m_pointTopLeft, m_sizeEllipse); // still logical
CRgn circle;

CClientDC dc(this);
OnPrepareDC(&dc);
dc.LPtoDP(rectEllipse); // Now it's in device coordinates
circle.CreateEllipticRgnIndirect(rectEllipse);
if (circle.PtInRegion(point)) {
// Capturing the mouse ensures subsequent LButtonUp message
SetCapture();
m_bCaptured = TRUE;
CPoint pointTopLeft(m_pointTopLeft);
dc.LPtoDP(&pointTopLeft);
m_sizeOffset = point - pointTopLeft; // device coordinates
// New mouse cursor is active while mouse is captured
::SetCursor(::LoadCursor(NULL, IDC_CROSS));
}
}

void CEx05cView::OnLButtonUp(UINT nFlags, CPoint point)
{
if (m_bCaptured) {
::ReleaseCapture();
m_bCaptured = FALSE;
}
}

void CEx05cView::OnMouseMove(UINT nFlags, CPoint point)
{
if (m_bCaptured) {
CClientDC dc(this);
OnPrepareDC(&dc);
CRect rectOld(m_pointTopLeft, m_sizeEllipse);
dc.LPtoDP(rectOld);
InvalidateRect(rectOld, TRUE);
m_pointTopLeft = point - m_sizeOffset;
dc.DPtoLP(&m_pointTopLeft);
CRect rectNew(m_pointTopLeft, m_sizeEllipse);
dc.LPtoDP(rectNew);
InvalidateRect(rectNew, TRUE);
}
}

[flyelf]

pDC->SelectClipRgn来限制画图区域