WM_VSCROLL

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Windows Embedded CE 6.0 R3

4/8/2010

This message is sent to a window when a scroll event occurs in the window's standard vertical scroll bar. This message is also sent to the owner of a vertical scroll bar control when a scroll event occurs in the control.

Syntax

WM_VSCROLL nScrollCode = (int)LOWORD(wParam); 
    nPos = (short int)HIWORD(wParam); 
    hwndScrollBar = (HWND) lParam;

Parameters

• nScrollCode
  Value of the low-order word of wParam. Specifies a scroll bar value that indicates the user's scrolling request. It is one of the following values.

  Value	Description
  SB_BOTTOM	Scrolls to the lower right
  SB_ENDSCROLL	Ends scroll
  SB_LINEDOWN	Scrolls one line down
  SB_LINEUP	Scrolls one line up
  SB_PAGEDOWN	Scrolls one page down
  SB_PAGEUP	Scrolls one page up
  SB_THUMBPOSITION	The user has dragged the scroll box (thumb) and released the mouse button. The nPos parameter indicates the position of the scroll box at the end of the drag operation.
  SB_THUMBTRACK	The user is dragging the scroll box. This message is sent repeatedly until the user releases the mouse button. The nPos parameter indicates the position that the scroll box has been dragged to.
  SB_TOP	Scrolls to the upper left

• nPos
  Value of the high-order word of wParam. Specifies the current position of the scroll box if the nScrollCode parameter is SB_THUMBPOSITION or SB_THUMBTRACK; otherwise, nPos is not used.

• hwndScrollBar
  If the message is sent by a scroll bar, then hwndScrollBar is the handle to the scroll bar control. If the message is not sent by a scroll bar, hwndScrollBar is NULL.

Return Value

An application should return zero if it processes this message.

Remarks

The SB_THUMBTRACK message is typically used by applications that provide feedback as the user drags the scroll box.

If an application scrolls the content of the window, it must also reset the position of the scroll box by using the SetScrollPos function.

When the range of scroll bar uses a non-zero value for the minimum position, SB_THUMBTRACK and SB_THUMBPOSITION notifications report the current position incorrectly. The SB_THUMBPOSITION maps the scroll bar range to 0 - (max-min). The following code example shows how to work around this problem by adding your minimum to the nPos value passed in with this message.

case WM_VSCROLL:
{
    int nScrollCode = (int)LOWORD(wParam);
    int nPos = (short int)HIWORD(wParam);
    SCROLLINFO si = {sizeof(SCROLLINFO), 
                     SIF_PAGE|SIF_POS|SIF_RANGE|SIF_TRACKPOS, 0, 0, 0, 0, 
                     0};
    GetScrollInfo (hWnd, SB_VERT, &si);
    int nNewPos = si.nPos;
    switch (nScrollCode)
    {
        // Include code that checks for other values of nScrollCode.
        // ...
        case SB_THUMBPOSITION:
          nNewPos = nPos + si.nMin; // Adding si.nMin is the workaround.
          break;
    }
    si.fMask = SIF_POS;
    si.nPos = nNewPos;
    SetScrollInfo (hWnd, SB_VERT, &si, TRUE);
}
return TRUE;

Note that the WM_VSCROLL message carries only 16 bits of scroll box position data. Thus, applications that rely solely on WM_VSCROLL (and WM_HSCROLL) for scroll position data have a practical maximum position value of 65,535.

However, because the SetScrollInfo, SetScrollPos, SetScrollRange, GetScrollInfo, GetScrollPos, and GetScrollRange functions support 32-bit scroll bar position data, there is a way to circumvent the 16-bit barrier of the WM_HSCROLL and WM_VSCROLL messages. See GetScrollInfo for a description of the technique.

Requirements

See Also

Reference

GetScrollInfo
SetScrollPos
SetScrollRange
WM_HSCROLL
Scroll Bars Messages