Visual C++ and MFC Programming 2nd Edition

Visual C++ and MFC Fundamentals Chapter 18: Progress-Based Controls

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18.4.6..Scroll Bar Events.................................................................................

As described already, to use a scroll bar, the user clicks either one of its buttons, a thumb

or the scrolling region. When the user clicks, the scroll bar sends a message to its parent

or host, which can be a view, a dialog box, or a form. If the scroll bar is horizontal, the

message sent is WM_HSCROLL, which fires an OnHScroll() event. The CWnd class,

as the parent of all MFC Windows controls and views, carries this event and makes it

available to the child classes (and controls) that can perform the scrolling. The syntax of

the OnHScroll() event is:

afx_msg void OnHScroll(UINT nSBCode, UINT nPos, CScrollBar* pScrollBar);

The nSBCode argument is the type of scrolling that is being performed. Its possible

values are:

Value Description Role

SB_THUMBTRACK The thumb is being dragged to a

specific position

SB_LEFT The thumb has been positioned to the

minimum value of the scroll bar, to

the extreme left.

Used to position the thumb to the

lowest value of the range

SB_LINELEFT The thumb is scrolling to the left one,

character or column at a time

Used to decrease the position of the

thumb, usually by subtracting 1 to the

current value, unless the thumb is

already positioned there

SB_PAGELEFT The thumb is scrolling to the left, one

page at a time

Refers to the page size of the

SCROLLINFO value to decrease the

current position by one page

SB_PAGERIGHT The thumb is scrolling to the right,

one page at a time

Refers to the page size of the

SCROLLINFO value to increase the

current position by one page

SB_LINERIGHT The thumb is scrolling to the right

one, character or column at a time

Used to increase the position of the

thumb, usually by adding 1, unless

the thumb is already positioned there

SB_RIGHT The thumb has been positioned to the

maximum value of the scroll bar, to

the extreme right

Used to position the thumb to the

highest value of the range, unless the

thumb is already positioned there

SB_THUMBPOSITION Absolute postion

SB_ENDSCROLL End Scroll Used to determine the end of the

scrolling

The nPos argument is used in connection with the SB_THUMBTRACK and the

SB_THUMBPOSITION values of the nSBCode argument. The value of nPos specifies

how much scrolling should be done.

The pScrollBar argument can be used to identify the particular scroll bar control needs to

be dealt with on this OnHScroll() event.

If your scroll bar control is vertical, when the user clicks it, it sends a WM_VSCROLL

message which fires the OnVScroll() event. Its syntax is:

afx_msg void OnVScroll(UINT nSBCode, UINT nPos, CScrollBar* pScrollBar);