how to get HWNDs of all view displayed?

You must get a pointer to your main app and then get a pointer to your DocTemplate.
This code below do something like this:

    POSITION viewpos;
    POSITION pos = m_pDocTemplate->GetFirstDocPosition();

    while (pos != NULL)
    {
        CDocument *pDoc = m_pDocTemplate->GetNextDoc(pos);

        viewpos = pDoc->GetFirstViewPosition();
        CMDIView* tmpView = (CMDIView *) pDoc->GetNextView(viewpos);
                   
        tmpView->Invalidate(TRUE);
    }           

This code repaints all views in a MDI app.

Hope this helps,
Bruce Sinner