How to call OnEraseBkgnd() to repaint the window in another function? (MFC) - c++

Question

I am trying to call OnEraseBkgnd() to repaint the window in another function.
For example, like the code below:
...
CBitmap Background;
BITMAP bm;
CDC dcMemory;
int width, height;
...
BOOL CSplashDlg::OnInitDialog()
{
CDialog::OnInitDialog();
CDC* dc;
Background.LoadBitmap(IDB_COVER); //Load Bitmap
Background.GetBitmap(&bm); //Load Bitmap into handle
width = 0;
height = 0;
while(width < 300)
{
width += 10;
height += 10;
OnEraseBkgnd(dc); //<--- Here I want to call OnEraseBkgnd()
Sleep(5); //Delay 5 millisecond
}
return TRUE;
}
BOOL CSplashDlg::OnEraseBkgnd(CDC* pDC)
{
///////////////////////////////////
Invalidate(); //I don't know where I should put this function
///////////////////////////////////
dcMemory.CreateCompatibleDC(pDC);
CBitmap *pOldbmp = dcMemory.SelectObject(&Background);
pDC->SetStretchBltMode(HALFTONE);
pDC->StretchBlt(0, 0, width, height, &dcMemory, 0, 0, bm.bmWidth, bm.bmHeight, SRCCOPY);
return TRUE;
}
In the above example, I want to call OnEraseBkgnd() to repaint the window while executing inside OnInitDialog(). I have searched the Internet and it always says using Invalidate(); or RedrawWindow(); to repaint it and it'll also call the OnEraseBkgnd() function. The question is: How should I use the Invalidate(); or the RedrawWindow();? Where should I put those function?
I have put those two functions in anywhere but it still not works.
EDIT:
I have modified it. Though now Invalidate() and UpdateWindow() are success to call the OnEraseBkgnd() function. But I found another problem: Why didn't the StretchBlt work when I used Invalidate() or UpdateWindow() to repaint it but did the FillSolidRect work?
...
BOOL CMainDlg::OnInitDialog()
{
CSplashDlg Frame;
Frame.width = 0;
Frame.height = 0;
while(Frame.width <= 300)
{
Frame.width += 10;
Frame.height += 10;
Frame.Invalidate(); //<---Here I use both Invalidate() and UpdateWindow()
Frame.UpdateWindow(); //<---Here I use both Invalidate() and UpdateWindow()
Sleep(5); //Delay 5 millisecond
}
CDialog::OnInitDialog();
return TRUE;
}
BOOL CSplashDlg::OnInitDialog()
{
CDialog::OnInitDialog();
width = 0;
height = 0;
Background.LoadBitmap(IDB_COVER); //Load Bitmap
Background.GetBitmap(&bm); //Load Bitmap into handle
return TRUE;
}
BOOL CSplashDlg::OnEraseBkgnd(CDC* pDC)
{
dcMemory.CreateCompatibleDC(pDC);
CBitmap *pOldbmp = dcMemory.SelectObject(&Background);
///////////////////////////////////////////////////////////////
pDC->SetStretchBltMode(HALFTONE);
pDC->StretchBlt(0, 0, width, height, &dcMemory, 0, 0, bm.bmWidth, bm.bmHeight, SRCCOPY); //It doesn't work when using this one (StretchBlt)
///////////////////////////////////////////////////////////////
pDC->FillSolidRect(0, 0, width, height, RGB(255,0,0)); //But it works when using this one (FillSolidRect)
///////////////////////////////////////////////////////////////
return TRUE;
}
...

Answer 1

You should never call the likes of OnEraseBkgnd( ) and OnPaint( ). MFC will call these when drawing is needed. Your job is to handle the need to draw in OnEraseBkgnd( ).
Invalidate( ) causes MFC to update the drawing area and then it will call OnEraseBkgnd( ) and OnPaint( ). If you Invalidate( ) in OnPaint( ) or OnEraseBkgnd( ) the program will likely hang as it would cause an endless loop of redrawing the window.
You can call Invalidate( ) in OnInitDialog( ) but it is likely unnecessary.
So take the Invalidate out of OnEraseBkgnd( ), don't call OnEraseBkgnd( ) in OnInitDialog( ) and go from there.
You also have to have
ON_WM_PAINT( )
ON_WM_ERASEBKGND( )
in your message map for those to be called by MFC.
NOTE:
I'm not a believer in reinventing wheels. It has mostly been done before. I have not used a splash screen in my projects, but if I were to, I would start here: Splash Screen C++ Class using MFC.
I didn't download and review the code but with four stars, it should be a good place to start. MFC is not an overnight learn. When I started I read books and tons of searches to learn. You just can not make guesses about how the infrastructure works. Hope this helps...