ScrollWindow()

Hi,

I have a few questions about this scroll bar example from Charles Petzold's "Programming Windows." The code is at the bottom of the post.

1) Why there's no UpdateWindow(hwnd) in the horizontal scrollbar message handler when there's one in vertical scrollbar message handler?

2) Can you explain in more detail how ScrollWindow() works? I've read MSDN's explanation but it doesn't help much.

It scrolls the client area, but invalidates the parts that are uncovered and they then need to be drawn on screen, right? This parts can only be located on the top or or on the bottom of the client area? (Since this is where uncovered parts appear)

Code:

LRESULT CALLBACK WndProc (HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam) { static int cxChar, cxCaps, cyChar, cxClient, cyClient, iMaxWidth ; HDC hdc ; int i, x, y, iVertPos, iHorzPos, iPaintBeg, iPaintEnd ; PAINTSTRUCT ps ; SCROLLINFO si ; TCHAR szBuffer[10] ; TEXTMETRIC tm ; switch (message) { case WM_CREATE: hdc = GetDC (hwnd) ; GetTextMetrics (hdc, &tm) ; cxChar = tm.tmAveCharWidth ; cxCaps = (tm.tmPitchAndFamily & 1 ? 3 : 2) * cxChar / 2 ; cyChar = tm.tmHeight + tm.tmExternalLeading ; ReleaseDC (hwnd, hdc) ; // Save the width of the three columns iMaxWidth = 40 * cxChar + 22 * cxCaps ; return 0 ; case WM_SIZE: cxClient = LOWORD (lParam) ; cyClient = HIWORD (lParam) ; // Set vertical scroll bar range and page size si.cbSize = sizeof (si) ; si.fMask = SIF_RANGE | SIF_PAGE ; si.nMin = 0 ; si.nMax = NUMLINES - 1 ; si.nPage = cyClient / cyChar ; SetScrollInfo (hwnd, SB_VERT, &si, TRUE) ; // Set horizontal scroll bar range and page size si.cbSize = sizeof (si) ; si.fMask = SIF_RANGE | SIF_PAGE ; si.nMin = 0 ; si.nMax = 2 + iMaxWidth / cxChar ; si.nPage = cxClient / cxChar ; SetScrollInfo (hwnd, SB_HORZ, &si, TRUE) ; return 0 ; case WM_VSCROLL: // Get all the vertical scroll bar information si.cbSize = sizeof (si) ; si.fMask = SIF_ALL ; GetScrollInfo (hwnd, SB_VERT, &si) ; // Save the position for comparison later on iVertPos = si.nPos ; switch (LOWORD (wParam)) { case SB_TOP: si.nPos = si.nMin ; break ; case SB_BOTTOM: si.nPos = si.nMax ; break ; case SB_LINEUP: si.nPos -= 1 ; break ; case SB_LINEDOWN: si.nPos += 1 ; break ; case SB_PAGEUP: si.nPos -= si.nPage ; break ; case SB_PAGEDOWN: si.nPos += si.nPage ; break ; case SB_THUMBTRACK: si.nPos = si.nTrackPos ; break ; default: break ; } // Set the position and then retrieve it. Due to adjustments // by Windows it may not be the same as the value set. si.fMask = SIF_POS ; SetScrollInfo (hwnd, SB_VERT, &si, TRUE) ; GetScrollInfo (hwnd, SB_VERT, &si) ; // If the position has changed, scroll the window and update it if (si.nPos != iVertPos) { ScrollWindow (hwnd, 0, cyChar * (iVertPos - si.nPos), NULL, NULL) ; UpdateWindow (hwnd) ; } return 0 ; case WM_HSCROLL: // Get all the horizontal scroll bar information //GetScrollInfo(hwnd, SB_HORZ, &si); si.cbSize = sizeof (si) ; si.fMask = SIF_ALL ; GetScrollInfo (hwnd, SB_HORZ, &si) ; // Save the position for comparison later on iHorzPos = si.nPos ; switch (LOWORD (wParam)) { case SB_LINELEFT: si.nPos -= 1 ; break ; case SB_LINERIGHT: si.nPos += 1 ; break ; case SB_PAGELEFT: si.nPos -= si.nPage ; break ; case SB_PAGERIGHT: si.nPos += si.nPage ; break ; case SB_THUMBPOSITION: si.nPos = si.nTrackPos ; break ; default : break ; } // Set the position and then retrieve it. Due to adjustments // by Windows it may not be the same as the value set. si.fMask = SIF_POS ; SetScrollInfo (hwnd, SB_HORZ, &si, TRUE) ; GetScrollInfo (hwnd, SB_HORZ, &si) ; // If the position has changed, scroll the window if (si.nPos != iHorzPos) { ScrollWindow (hwnd, cxChar * (iHorzPos - si.nPos), 0, NULL, NULL) ; //UpdateWindow(hwnd); } return 0 ; case WM_PAINT : hdc = BeginPaint (hwnd, &ps) ; // Get vertical scroll bar position si.cbSize = sizeof (si) ; si.fMask = SIF_POS ; GetScrollInfo (hwnd, SB_VERT, &si) ; iVertPos = si.nPos ; // Get horizontal scroll bar position GetScrollInfo (hwnd, SB_HORZ, &si) ; iHorzPos = si.nPos ; // Find painting limits iPaintBeg = max (0, iVertPos + ps.rcPaint.top / cyChar) ; iPaintEnd = min (NUMLINES - 1, iVertPos + ps.rcPaint.bottom / cyChar) ; for (i = iPaintBeg ; i <= iPaintEnd ; i++) { x = cxChar * (1 - iHorzPos) ; y = cyChar * (i - iVertPos) ; TextOut (hdc, x, y, sysmetrics[i].szLabel, lstrlen (sysmetrics[i].szLabel)) ; TextOut (hdc, x + 22 * cxCaps, y, sysmetrics[i].szDesc, lstrlen (sysmetrics[i].szDesc)) ; SetTextAlign (hdc, TA_RIGHT | TA_TOP) ; TextOut (hdc, x + 22 * cxCaps + 40 * cxChar, y, szBuffer, wsprintf (szBuffer, TEXT ("%5d"), GetSystemMetrics (sysmetrics[i].iIndex))) ; SetTextAlign (hdc, TA_LEFT | TA_TOP) ; } EndPaint (hwnd, &ps) ; return 0 ; case WM_DESTROY : PostQuitMessage (0) ; return 0 ; } return DefWindowProc (hwnd, message, wParam, lParam) ; }

You are right that Petzold's code uses UpdateWindow() in one, and not the other. However, this is not really a bug, although I am not sure why he did not choose to use it in both places, or neither. From the MSDN page for ScrollWindow(), it states:

"The area uncovered by ScrollWindow is not repainted, but it is combined into the window's update region. The application eventually receives a WM_PAINT message notifying it that the region must be repainted. To repaint the uncovered area at the same time the scrolling is in action, call the UpdateWindow function immediately after calling ScrollWindow."

If you'll look at the MSDN page for UpdateWindow(), it states:

"The UpdateWindow function updates the client area of the specified window by sending a WM_PAINT message to the window if the window's update region is not empty. The function sends a WM_PAINT message directly to the window procedure of the specified window, bypassing the application queue. If the update region is empty, no message is sent."

So, MSDN is pretty good at explaining what is going on.

If you wish to process a WM_PAINT message immediately, use UpdateWindow(). WM_PAINT is a low priority message, so it does not get processed until almost all other messages are processed. In this case, if the CPU is doing a lot, the window will appear to be painted in two steps - ScrollWindow() scrolls the window, and at a later time WM_PAINT redraws the invalid portion that is missing. Since UpdateWindow() forces any invalid portions to be drawn immediately, you can use this to fix this problem. I would call it every time I use ScrollWindow().

OK, thanks. Can you explain this part for me?

Code:

x = cxChar * (1 - iHorzPos) ;

Why the 1-iHorzPos?

Ok, but first, explain to me what the three variables stand for (i.e. what do their values mean, and what are they used for?). I have a feeling you'll answer your own question if you do this.

Quote:

Originally posted by Ariod
1) Why there's no UpdateWindow(hwnd) in the horizontal scrollbar message handler when there's one in vertical scrollbar message handler?

Quote:

Originally posted by JasonD
...although I am not sure why he did not choose to use it in both places, or neither.

I looked at all of Petzold's examples that use ScrollWindow(), and I know why he chose to use it for vertical scrolling, and not horizontal scrolling. You'll notice that he chooses to process SB_THUMBTRACK for vertical scrolling (which updates the window as the user moves the mouse), and only processes SB_THUMBPOSITION for the horizontal scroll (which updates the window after the user stops moving the mouse). Shrink the window so that both scroll bars appear, and you'll see what I mean. You'll see that it is not so important to force a window update after the scroll when it is only drawn once. However, for a scroll that continually updates the window, the WM_PAINT messages are pushed to the back of the priority queue, and you could have a situation where the window continues to scroll, but the missing portion is not painted until you let go of the mouse.

Of course, this begs the question: Why not process SB_THUMBTRACK for both scroll bars? I had noticed he did not do this before, but I had not included it in my errata page, since it was not really a bug. However, there is no reason to have one scroll bar update as the mouse moves, and the other one not (even if it is the scroll bar that is unlikely to be used). Also, because this makes readers wonder why UpdateWindow() isn't used in both places, I have decided to add these errata to my page.

I hope this helps explain his code.

Quote:

Originally posted by JasonD
Ok, but first, explain to me what the three variables stand for (i.e. what do their values mean, and what are they used for?). I have a feeling you'll answer your own question if you do this.

x - coordinates
cxChar - average character width
iHorzPos - position of the horizontal scroll bar

If client area is scrolled to the left, part of text is being drawn in the negative area of the x axis, outside the client area?

But why 1 - iHorzPos, why not just -iHorzPos? To avoid negative value when iHorzPos is 0?

It's the way it is to insert a space to the left of the first column of text. Try removing the 1, and you'll see how it differs. From Petzold's code, he has specified that there should be 1 space to the left of the first column (with this statement you are inquiring about), and 2 spaces to the right of the last column (inside the WM_SIZE message handler, where the scroll bar's range is set). This is purely personal taste, but you usually do not want your text flush against the client area's edge.

Thanks again Jason :)