Question

My program creates random rectanges on the screen. Each time the left mouse button is clicked a new rectange of random size and colour is added to the window. However, when I resize the window, all of the rectanges are deleted – I don’t want this to happen. I don’t understand what the program does when it is resized, is it calling the case WM_PAINT? If so, what do I need to include to ensure the current window state remains when resized.

Answer 1

See my answer to a similar question here about painting using GDI.

https://stackoverflow.com/a/12764607/682404

Basically, when your windows is resized, you should receive a WM_PAINT message from Windows. In your message handler, you need to repaint your form. The answer I linked in has some details about the painting process. Your painting code would look rougly like this:

PAINTSTRUCT stPaintStruct;
HDC hPaintDC = BeginPaint(hWnd, &stPaintStruct);

if (hPaintDC != HANDLE_NULL)
{
    // establish clipping rect using stPaintStruct.rcPaint

    if (!m_bRendering)
    {
        m_bRendering = TRUE;

        // Render() knows the how to create the output.
        Render ();

        m_bRendering = FALSE;
    }

    EndPaint (hWnd, &stPaintStruct);
    return (TRUE);
}

Every time you need to paint your window, you always need to paint from scratch, so you need to store somewhere the coordinates of your rectangles.