C / C++ / MFC

1. When the window needs to be repainted, WM_PAINT is sent.
2. OnPaint typically calls BeginPaint, which prepares the window DC, sends WM_ERASEBKGND and returns the DC ready for painting. So, by the time BeginPaint returns, WM_ERASEBKGND has been already processed.
3. All drawing stuff takes place with the DC returned by BeginPaint.
4. Once the drawing is finished, EndPaint releases the DC and the handler for OnPaint exits.

1. What are the origins and purpose of the WM_ERASEBKGND? Is there more to this message than I said above?
   
2. What is the difference between returning TRUE or FALSE in OnEraseBkGnd?

void CNewView::OnDraw(CDC* pDC)
{
  //Custom drawing
  pDC->LineTo(100, 100);

  //Call to draw control over top of current contents
  COldView::OnDraw(pDC);
}

void CMyClass::OnDraw(CDC* pDC)
{
  //No code
}

void CMyClass::OnErase(CDC* pDC)
{
  CMemDC buff(pDC);

  buff.LineTo(100, 100);

  //I can't remember if I called the default OnErase 

  CView::OnErase(pDC);

  return FALSE;
}

BOOL CRulerView::OnEraseBkgnd(CDC* pDC)
{ 
	CMemDC buff(pDC);

	//Drawing area
	CRect rect;	
	GetClientRect(&rect);

	//Draw ruler background
	buff.FillSolidRect(0, 0, m_rulerSize, rect.bottom, ::GetSysColor(COLOR_BTNFACE));
	buff.FillSolidRect(0, 0, rect.right, m_rulerSize, ::GetSysColor(COLOR_BTNFACE));
	
	//window borders width
	UINT nBorderWidth = ::GetSystemMetrics(SM_CXEDGE);

	//Draw 3D border, blends with parent window
	if(m_b3Dborder){
		buff.DrawEdge(CRect(0, 0, rect.right, m_rulerSize), EDGE_RAISED, BF_RIGHT | BF_TOP);
		buff.DrawEdge(CRect(0, 0, m_rulerSize, rect.bottom), EDGE_RAISED , BF_LEFT | BF_BOTTOM);
	}

	//Draw border edge
	buff.DrawEdge(CRect(m_rulerSize-nBorderWidth, m_rulerSize-nBorderWidth, rect.right, rect.bottom), EDGE_SUNKEN, BF_TOP | BF_LEFT);

	//Draw the rulers
	OnDrawRuler(&buff, CRect(nBorderWidth*2, m_rulerSize-nBorderWidth, m_rulerSize-(nBorderWidth*2), rect.bottom-m_nScrSize+nBorderWidth),  RT_VERT);
	OnDrawRuler(&buff, CRect(m_rulerSize-nBorderWidth, nBorderWidth*2, rect.right-m_nScrSize+nBorderWidth, m_rulerSize-(nBorderWidth*2)), RT_HORZ);

	//Draw the cursor tracker out of bounds
	OnMouseMove(NULL, CPoint(-10,-10));	
	
	return FALSE; 
}

void CRulerView::OnDraw(CDC* pDC)
{
}