Question

Unless I make “static CPaintDC dc(this);” the line won’t draw? But this is not good as it will eventually error, also the graphics wont’ keep on the screen.

Not sure what I am doing wrong

Note: I have a Timer that calls to this every 100ms(x and y are incremented)
thx

void CGraphicsDlg::OnPaint() 
{
    CString s;
    CPaintDC dc(this);// device context for painting

    if (IsIconic())
    {
        SendMessage(WM_ICONERASEBKGND, (WPARAM) dc.GetSafeHdc(), 0);

        // Center icon in client rectangle
        int cxIcon = GetSystemMetrics(SM_CXICON);
        int cyIcon = GetSystemMetrics(SM_CYICON);
        CRect rect;
        GetClientRect(&rect);
        int x = (rect.Width() - cxIcon + 1) / 2;
        int y = (rect.Height() - cyIcon + 1) / 2;

        // Draw the icon
        dc.DrawIcon(x, y, m_hIcon);
    }
    else if(x==0)
    {
        s.Format("%d", x);
        edXa->SetWindowText(s);

        dc.MoveTo(20,400);
    }
    else if (x>0){
        s.Format("%d", x);
        edXb->SetWindowText(s);

        dc.LineTo(20 + x, 40);  // doesn't draw unless I make "static CPaintDC dc(this);" <- which will error out 
    }
    CDialog::OnPaint();
}

void CGraphicsDlg::OnTimer(UINT nIDEvent)
{
    if(nIDEvent==1){
        srand( (unsigned)time( NULL ) );

        //y = rand() % 100;
        y++;
        x++;

        OnPaint();
    }
}

Answer 1

LineTo draws a line from one point to another, using the selected pen. You need to use MoveTo to define the start of the line, and you need to select a pen into the DC.

The larger problem is how you’re trying to use the DC. It isn’t meant to be permanent; you’re supposed to acquire it, draw everything to it, then shut it down. When you try to make CPaintDC static, Windows will eventually shut it down and every attempt to use it thereafter will return an error.

The proper way is to save any coordinates that you need for all of the drawing you need to do. Use a combination of MoveTo and LineTo to draw individual line segments, and every time you reenter OnPaint you need to start over.