Win32 MFC OnPaint Resizing and Redrawing handling - c++

Question

how to handle this problem everytime i resize my drawing seems to not draw right.
i think i need to call Invalidate() everytime i resize the window but doesnt WM_PAINT automatically called every time i move or resize the window?
CPaintDC dc(this); // device context for painting
CRect rect;
GetClientRect(rect);
if (IsIconic())
{
//CPaintDC dc(this); // device context for painting
SendMessage(WM_ICONERASEBKGND, reinterpret_cast<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
{
for(int ndx(0); ndx < rect.Width() - 150; ndx += 10)
{
dc.MoveTo( rect.left + 50, rect.bottom / 2 );
dc.LineTo( rect.left + 50 + ndx, rect.bottom / 2 );
}
CBrush mybrush(RGB(30,30,30));
dc.FillRect( CRect(rect.left + 10, rect.top + 10, rect.Width() / 4, rect.Height() / 4),&mybrush );
CDialogEx::OnPaint();
}
Before Resizing:
After Resizing:

Answer 1

When you start painting and use CPaintDC it doesn't make sense to call the base class that tries this again and may again erase the background...
What happens.
Your CPaintDC ist created
BeginPaint is called and WM_ERASEBKGND is sent.
You paint your stuff.
You call the base class anbd a new CPaintDC calles BeginPaint.
Because EndPaint isn't called the paint area isn't validated. So BeginPaint is executed and the WM_ERASEBKGND is called again.
Finally the CPaintDC's destructors are called and the client area is validated.
Never call the OnPaint baseclass function if you start using CPaintDC/BeginPaint!

Answer 2

I do not know if this is the optimal solution.
In your paint code, before painting, fill the entire client area with the background color:
dc.FillSolidRect(&rect, ...);
Remove CDialogEx::OnPaint().
Add an OnEraseBkgnd handler:
BEGIN_MESSAGE_MAP(CMFCApplication1Dlg, CDialogEx)
//...
ON_WM_ERASEBKGND()
END_MESSAGE_MAP()
//...
BOOL OnEraseBkgnd(CDC * pDC)
{
RedrawWindow();
return TRUE;
}
If the dialog is flickering, use MFC's CMemDC (undocumented).

Answer 3

this is fixes my problem
step 1: BeginPaint()
// If you add a minimize button to your dialog, you will need the code below
// to draw the icon. For MFC applications using the document/view model,
// this is automatically done for you by the framework.
void CTestDrawDlg::OnPaint()
{
CPaintDC dc(this); // device context for painting
CRect rect;
GetClientRect(rect);
// draw background manually
EraseBkgnd(&dc);
if (IsIconic())
{
//CPaintDC dc(this); // device context for painting
SendMessage(WM_ICONERASEBKGND, reinterpret_cast<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
{
for(int ndx(0); ndx < rect.Width() - 150; ndx += 10)
{
dc.MoveTo( rect.left + 50, rect.bottom / 2 );
dc.LineTo( rect.left + 50 + ndx, rect.bottom / 2 );
}
CBrush mybrush(RGB(30,30,30));
dc.FillRect( CRect(rect.left + 10, rect.top + 10, rect.Width() / 4, rect.Height() / 4),&mybrush );
}
}
STEP: 2 RedrawWindow() on WM_ERASEBACKGND
BOOL CTestDrawDlg::OnEraseBkgnd(CDC* pDC)
{
// TODO: Add your message handler code here and/or call default
RedrawWindow();
return TRUE;
}
and Manually draw the background
void CTestDrawDlg::EraseBkgnd( CDC * pDC )
{
CRect rect;
GetClientRect(rect);
pDC->FillSolidRect(rect, RGB(255,255,255));
// paint whatever you want in the background
}
STEP 3: EndPaint()