WM_PAINT handler in dlg: cpu usage of 100%

good morning everybody!

in my about dialog i need to draw a string dynamically on a bitmap (static control).

to achieve this, i created a WM_PAINT handler for that dialog and in it i draw the string...

so problem is, whenever the WM_PAINT handler is there, regardless if anything is done in it, even if it has an empty body, the cpu usage rises to 100%.

and after a few seconds i get a i/o exception.

i spy(ed)++ the dialog, and if there is a handler for WM_PAINT it will recieve thousands of WM_PAINT msgs.

i don't get what i'm doing wrong... can't i implement a WM_PAINT handler in a dialog?
and if no, how can i draw my string in an other way?

Post some code. You seem to be invoking WM_PAINT message using InvalidateRect or UpdateWindow in your WM_PAINT handler.

as i said: even if OnPaint has an empty body...

but here the two versions i tried:
v1

Code:

---

void CAboutDlg::OnPaint()
{
        CWnd* pwndPic = GetDlgItem(IDC_ABOUT_BMP);

        CDC* pDC = pwndPic->GetDC();
       
        CRect rcPic;
        pwndPic->GetClientRect(rcPic);

        pDC->SelectObject(&m_font);
        pDC->SetBkMode(TRANSPARENT);
        pDC->DrawText(AfxGetAppName(), -1, rcPic, DT_CENTER|DT_VCENTER);

        pwndPic->ReleaseDC(pDC);
}

---

v2: draws the text smoother

Code:

---

void CAboutDlg::OnPaint()
{
        CWnd* pwndPic = GetDlgItem(IDC_ABOUT_BMP);

        PAINTSTRUCT ps;
        CDC* pDC = pwndPic->BeginPaint(&ps);
       
        CRect rcPic;
        pwndPic->GetClientRect(rcPic);

        pDC->SelectObject(&m_font);
        pDC->SetBkMode(TRANSPARENT);
        pDC->DrawText(AfxGetAppName(), -1, rcPic, DT_CENTER|DT_VCENTER);

        pwndPic->EndPaint(&ps)
}

---

and even if i have this:

Code:

---

void CAboutDlg::OnPaint()
{
}

---

cpu usage goes up to 100%, and after some time... error

About without any body, i just checked it. I think you must provide default handling if you are not repainting the dialog itself(removing the CPaintDC dc(this) also) as CPaintDC constructor calls BeginPaint

Code:

---

CPaintDC::CPaintDC(CWnd* pWnd)
{
        ASSERT_VALID(pWnd);
        ASSERT(::IsWindow(pWnd->m_hWnd));

        if (!Attach(::BeginPaint(m_hWnd = pWnd->m_hWnd, &m_ps)))
                AfxThrowResourceException();
}

---

I don't now details of BeginPaint and EndPaint what it seems either they should be handled once WM_PAINT is called or else default called which is CDialog::OnPaint().
Hope this helps.

Note: you can do your painting on a dialog, that's what WM_PAINT is ment for.

You must handle the message one or the other way or tell windows you don't want to handle it (Call default CDialog::OnPaint()).

Quote:

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Originally posted by bigBA
and even if i have this:

Code:

---

void CAboutDlg::OnPaint()
{
}

---

cpu usage goes up to 100%, and after some time... error

---

hm.. yes. default handling... that was the point.

i added this line at the end of OnPaint and all went ok:

Code:

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Default(); //as done in all default handles of mfc classes

---

allright :)

now i have to figure out how to draw text an a static control, without making the control to discard its contents... (the bitmap isn't drawn if i do BeginPaint/EndPaint)

Hi,
Your calls to BeginPaint and EndPaint are pertaining to a child control why don't you handle the WM_PAINT handler of this control itself, just need to declare a new class and subclass your control from it.

simple answer: i don't want to write a new class.. already have about 50 classes in the project :)

my idea for drawing the text:
place a static text control above the static bitmap control..
and make a CPaintDC from the text control... but this doesn't work...
when i do nothing (only Default()) the static text is above the bitmap, but when i get a CPaintDC from the text control it disappears.
actually then it is below the bitmap control (when i set this to invisible my drawn text shows up)...

Quote:

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Originally posted by bigBA
simple answer: i don't want to write a new class.. already have about 50 classes in the project :)

---

Strange reasoning... There are projects with thousands of classes. ;)

Quote:

---

Originally posted by bigBA
my idea for drawing the text:
place a static text control above the static bitmap control..
and make a CPaintDC from the text control... but this doesn't work...
when i do nothing (only Default()) the static text is above the bitmap, but when i get a CPaintDC from the text control it disappears.
actually then it is below the bitmap control (when i set this to invisible my drawn text shows up)...

---

A few things:

• The inifinite repaints in your original code were caused by the fact that your WM_PAINT handler didn't call BeginPaint() / EndPaint(). If EndPaint() is not called, the WM_PAINT message will be sent over and over again. The standard way to do this with MFC is to create a local CPaintDC object in your OnPaint() implementation: Its ctor will call BeginPaint(), and its dtor calls EndPaint().
• You can't create a CPaintDC for a child window in the parent window's OnPaint() - BeginPaint() and EndPaint() must be called exclusively in response to a WM_PAINT message. However, while in your dialog's OnPaint(), you are currently processing the WM_PAINT message sent to the dialog, not the control.
• It is generally never a good idea to implement your own WM_PAINT handler in a dialog: The dialog's WM_PAINT does a lot of things, like scaling the controls from dialog units to screen units, drawing the controls etc. The results when overwriting a control in a dialog's OnPaint() handler are undefined - it might even seem to work, but fail with another Windows version. To do custom drawing in a dialog, you usually create your own control and do the drawing in the control's WM_PAINT handler. In your case, it is probably best to derive your control from CStatic, in other cases (more complex controls which require interaction and scrolling) you create a custom control derived directly from CWnd.

hm... the point with the logical units... don't know.
i'm just waiting for the error-responses ;)

i did it this way now, and it worked fine:

Code:

---

        CWnd* pwndPic = GetDlgItem(IDC_ABOUT_BMP);

        //CPaintDC dc(pwndPic);
        //CPaintDC* pDC = &dc;

        PAINTSTRUCT ps;
        CDC* pDC = pwndPic->BeginPaint(&ps);

        CRect rcPic;
        pwndPic->GetClientRect(rcPic);

        HBITMAP hBmp = ((CStatic*)pwndPic)->GetBitmap();
        if( hBmp )
        {
                pDC->DrawState(CPoint(0,0),CSize(rcPic.Width(), rcPic.Height()),hBmp, DST_BITMAP|DSS_NORMAL);
        }

        CRect rcText(20,115,380,205);

        pDC->SelectObject(&m_fontBig);
        pDC->SetBkMode(TRANSPARENT);
        int iHeight = pDC->DrawText(AfxGetAppName(), -1, rcText, DT_CENTER|DT_VCENTER);

        if( !m_strComments.IsEmpty() )
        {
                rcText.top += iHeight + 5;
                pDC->SetTextColor(RGB(255,0,0));
                pDC->SelectObject(&m_fontSmall);
                pDC->DrawText(m_strComments, -1, rcText, DT_CENTER|DT_VCENTER);
        }

        pwndPic->EndPaint(&ps);

        CDialog::OnPaint();
        //Default(); //without this cpu usage: 100%

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if it won't work on other windows versions, i will go to the solution with deriveing cstatic and subclassing the control.

i know that my excuse with "i already have about 50 classes" is a bit weak, but we are upgrading an very old app and the next release is about tomorrow, and i have alot of other things to do as well (more essential things)

Quote:

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Originally posted by bigBA
i did it this way now, and it worked fine:

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As I said: It might seem to work, by pure luck. It is likely to fail under other circumstances, like another machine or another Windows version. A problem with your code is that you are calling the control's BeginPaint() without that it received a WM_PAINT message. A second problem is that you call CDialog::OnPaint() yourself, that's another thing which is strongly discouraged. And BTW, I really wonder that this code seems to work: When you call CDialog::OnPaint(), I would expect that the original bitmap control is redrawn, overwriting what you previously painted.

Quote:

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Originally posted by bigBA
i know that my excuse with "i already have about 50 classes" is a bit weak, but we are upgrading an very old app and the next release is about tomorrow, and i have alot of other things to do as well (more essential things)

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But that shouldn't force you into sloppy coding practices... ;) The code you posted wouldn't pass any code review, especially if it's for a release, I would never accept that a developer knowingly introduces code which is definitely plain wrong. :rolleyes:

hm... ok...

your are right... to become sloppy isn't the right way :rolleyes:

The thing i'm calling CDialog::OnPaint() is, when i do not, the program freezes, as i said: cpu usage goes up to 100%, and if i call Default() (or CDialog::OnPaint) it will work.

the thing was.. that i have a picture (the background for our logo) and program name and maybe comments from the verion info has to be drawn on it dynamically... so i desided to do it this way.
i also used that static bmp control, because i don't wanted to load the picture myself... (now it is sloppier then ever, right?)

but maybe its better practice to subclass the control... maybe i should this way. it isn't really much work.. just cut/paste the Dialogs OnPaint in the derived class's OnPaint (nearly :) )

now i use this approach, also avoiding new class&subclassing.

i'm setting the Static control to SS_OWNERDRAW
and in OnDrawItem of my Dlg i paint it.

works. and i don't see any problems :) are there any?

Quote:

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Originally posted by bigBA
now i use this approach, also avoiding new class&subclassing.

i'm setting the Static control to SS_OWNERDRAW
and in OnDrawItem of my Dlg i paint it.

works. and i don't see any problems :) are there any?

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No, that's the "cheap" approach, but it's fine... :)