Advanced MFC Programming

Chapter 8. DC, Pen, Brush and Palette
void CGDIView::OnDraw(CDC* pDC)
{
int nNumOfPts;
int nMode;
int i;
CPen pen;
CPen *ptrPenOld;
CPoint pt[4];
}
CGDIDoc* pDoc = GetDocument();
ASSERT_VALID(pDoc);
pen.CreatePen(PS_SOLID, 1, RGB(0, 255, 255));
ptrPenOld=pDC->SelectObject(&pen);
nMode=pDC->SetROP2(R2_COPYPEN);
nNumOfPts=pDoc->GetNumOfPts();
for(i=0; iGetOnePt(4*i);
pt[1]=pDoc->GetOnePt(4*i+1);
pt[2]=pDoc->GetOnePt(4*i+2);
pt[3]=pDoc->GetOnePt(4*i+3);
pDC->PolyBezier(pt, 4);
}
pDC->SetROP2(nMode);
pDC->SelectObject(ptrPenOld);
Since we need 4 control points to draw a curve, the number of curves we can draw should be equal to
the number of points stored in array CGDIDoc::m_dwaPts divided by 4. In the function, a loop is used to
draw each single curve. Within each loop, four control points are retrieved one by one, and stored in a local
CPoint type array pt. After all four control points are retrieved, function CDC::PolyBezier(…) is called to
draw the curve. Here, we also need to create a pen and select it into the DC before any drawing operation is
performed.
The rest thing we need to implement is recording control points. In order to do this, we need to handle
two mouse related messages: WM_LBUTTONUP and WM_MOUSEMOVE. In the sample, their message handlers are
added through using Class Wizard, the corresponding member functions are CGDIView::OnLButtonUp(…)
and CGDIView::OnMouseMove(…) respectively.
Since a curve needs four control points, we use mouse’s left button up event to record them. In the
application, a counter is implemented to count how many control points have been added. Before a new
curve is added, this counter is set to 0. As we receive message WM_LBUTTONUP, the counter will be
incremented by 1. As it reaches 4, we finish recording the control points, store the data in the document and
update the client window.
In the sample, to implement curve drawing, some new variables are declared in class CGDIView as
follows:
……
……
class CGDIView : public CView
{
protected:
CPoint m_ptCurve[4];
int m_nCurrentPt;
BOOL m_bNeedErase;
BOOL m_bCapture;
};
We are familiar with variables m_bCapture and m_bNeedErase. Here, variable m_ptCurve will be used
to record temporary control points, and m_nCurrentPt will act as a counter.
Some of the variables are initialized in the constructor:
CGDIView::CGDIView()
{
m_nCurrentPt=0;
m_bNeedErase=FALSE;
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