Advanced MFC Programming

Chapter 8. DC, Pen, Brush and Palette
CDC::LineTo(…) to complete drawing. We don’t need to call CDC::MoveTo(…) each time when drawing
several connected line segments continuously. After function CDC::LineTo(…) is called, the DC’s origin
will always be updated to the end point of that line. The following function shows how the mouse moving
activities are handled in the sample:
void CGDIView::OnMouseMove(UINT nFlags, CPoint point)
{
CClientDC dc(this);
CPen pen;
CPen *ptrPenOld;
int nMode;
}
if(nFlags & MK_LBUTTON)
{
pen.CreatePen(PS_DOT, 1, RGB(0, 0, 0));
ptrPenOld=dc.SelectObject(&pen);
nMode=dc.SetROP2(R2_XORPEN);
if(m_bNeedErase == TRUE)
{
dc.MoveTo(m_ptStart);
dc.LineTo(m_ptEnd);
}
else
{
m_bNeedErase=TRUE;
}
m_ptEnd=point;
dc.MoveTo(m_ptStart);
dc.LineTo(m_ptEnd);
dc.SetROP2(nMode);
dc.SelectObject(ptrPenOld);
}
CView::OnMouseMove(nFlags, point);
We declare CClientDC type variable to obtain the device context of the client window. First function
CPen::CreatePen(…) is called to create a dotted pen with black color. Then this pen is selected into the
device context and the old pen is stored in ptrPenOld. Next, the device context’s drawing mode is set to
R2_XORPEN, and the original drawing mode is stored in nMode. If this is the first time the function is called
after the user pressed mouse’s left button, m_bNeedErase flag must be FALSE. In this case we need to set it
to TRUE. Otherwise if m_bNeedErase is TRUE, we need to first draw the old outline (This will erase the
outline). Next the current mouse position is stored in variable m_ptEnd, and CDC::MoveTo(…),
CDC::LineTo(…) are called to draw the new outline. Finally device context’s old drawing mode is resumed.
Also, the old pen is selected back into the DC (which will also select the new pen out of the DC).
For WM_LBUTTONUP message handler, we also need to erase the old line if necessary. Because the new
line is fixed now, we need to add new data to the document and update the client window. The following
function shows how this message is handled:
void CGDIView::OnLButtonUp(UINT nFlags, CPoint point)
{
CClientDC dc(this);
CPen pen;
CPen *ptrPenOld;
int nMode;
CGDIDoc *ptrDoc;
pen.CreatePen(PS_DOT, 1, RGB(0, 0, 0));
ptrPenOld=dc.SelectObject(&pen);
nMode=dc.SetROP2(R2_XORPEN);
dc.MoveTo(m_ptStart);
dc.LineTo(m_ptEnd);
dc.SetROP2(nMode);
dc.SelectObject(ptrPenOld);
pen.DeleteObject();
ptrDoc=(CGDIDoc *)GetDocument();
206