Scrollbars part 12: Applying WM_NCCALCSIZE to our scrollbar sample


Now that we have learned about the intricacies of the WM_NCCALCSIZE message, we can use it to get rid of the flicker in our resizing code. We just take the trick we used above and apply it to the scroll program.

First, we need to get rid of the bad flickery resize, so return the OnWindowPosChanging function to the version before we tried doing metaphor work:

BOOL OnWindowPosChanging(HWND hwnd, LPWINDOWPOS pwp)
    if (!(pwp->flags & SWP_NOSIZE)) {
        RECT rc = { 0, 0, pwp->cx, pwp->cy };
        AdjustSizeRectangle(hwnd, WMSZ_BOTTOM, &rc);
        pwp->cy = rc.bottom;
    return 0;

Instead, our work will happen in the WM_NCCALCSIZE handler.

UINT OnNcCalcSize(HWND hwnd, BOOL fCalcValidRects,
                  NCCALCSIZE_PARAMS *pcsp)
                        fCalcValidRects, pcsp, DefWindowProc);
    if (fCalcValidRects) {
        //  Give names to these things
        RECT *prcClientNew = &pcsp->rgrc[0];
        RECT *prcValidDst  = &pcsp->rgrc[1];
        RECT *prcValidSrc  = &pcsp->rgrc[2];
        int dpos;
        int pos;
        // Did we drag the top edge enough to scroll?
        if (prcClientNew->bottom == prcValidSrc->bottom &&
            g_cyLine &&
            (dpos = (prcClientNew->top - prcValidSrc->top)
                                            / g_cyLine) != 0 &&
            (pos = ClampScrollPos(g_yOrigin + dpos)) != g_yOrigin) {
            *prcValidDst = *prcClientNew;
            ScrollTo(hwnd, pos, FALSE);
            prcValidDst->top -= dpos * g_cyLine;
            uRc = WVR_VALIDRECTS;
    return uRc;
    /* Add to WndProc */
    HANDLE_MSG(hwnd, WM_NCCALCSIZE, OnNcCalcSize);

This uses a new helper function which we extracted from the ScrollTo function. (If I had planned this better, this would have been factored out when we first wrote ScrollTo.)

int ClampScrollPos(int pos)
     *  Keep the value in the range 0 .. (g_cItems - g_cyPage).
    pos = max(pos, 0);
    pos = min(pos, g_cItems - g_cyPage);
    return pos;

I am not entirely happy with this code, however. It is my personal opinion that the WM_NCCALCSIZE handler should be stateless. Notice that this one modifies state (by calling ScrollTo). If I had more time (sorry, I’m rushed now – you’ll learn why soon), I would have put the state modification into the WM_WINDOWPOSCHANGING message. So I will leave that as another exercise.

Exercise: Keep the WM_NCCALCSIZE message stateless.


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