In MFC, when opening a file or creating a file, is WM_PAINT generated?

[sunnysky]

Hello,

I am learning MFC programming with the book "Programming Windows with MFC (2nd Edition)". I have some questions about the book example from chapter 9.

When I open a file or click new file, the function CSquaresView::OnDraw() is called. I checked the stack, the reason is that CView::OnPaint() calls this function. But I don't understand how CView::OnPaint() is called. Is the message WM_PAINT generated if CSingleDocTemplate::OpenDocumentFile() is called by the default setting? Is it possible to change the behavior?

Thanks,
Brian

[ovidiucucu]

An application does not (or should not) directly send WM_PAINT message. Instead, it calls a function (Invalidate, InvalidateRect, or one of few others) that invalidates an area of a window. WM_PAINT is further sent by the system, when that area needs to be repainted.

In our case, the view window is invalidated in the default implementation of CView::OnUpdate. You can put a breakpoint there, then look on call stack window to see what happens before.

void CView::OnUpdate(CView* pSender, LPARAM /*lHint*/, CObject* /*pHint*/)
{
    ASSERT(pSender != this);
    UNUSED(pSender);     // unused in release builds

    // invalidate the entire pane, erase background too
    Invalidate(TRUE);
}

That's generally OK, because once the document has been changed, the view must be changed, as well.
However, if really need it, is possible to change that by overriding CView::OnUpdate and simply do not call the base class method.

void CSDITestView::OnUpdate(CView* /*pSender*/, LPARAM /*lHint*/, CObject* /*pHint*/)
{
    // Do not call base class OnUpdate.
    // Do something else...
}

[sunnysky]

Thanks a lot. After I set the break point there, I found the reason. When either creating or opening a file, CSingleDocTemplate::OpenDocumentFile() is called. Then it calls InitialUpdateFrame(). Then InitialUpdateFrame() calls CView::OnUpdate().