The operating system sends the UDN_DELTAPOS notification message to the parent window of an up-down control when the position of the control is about to change. This happens when the user requests a change in the value by pressing the controlís up or down arrow. The UDN_DELTAPOS message is sent before the WM_VSCROLL or WM_HSCROLL message that actually changes the controlís position. This lets you examine, allow, modify, or disallow the change. The UDN_DELTAPOS message is sent in the form of a WM_NOTIFY message.
UDN_DELTAPOS pnmud = (NM_UPDOWN FAR *) lParam
The iPos member of this structure contains the current position of the control.
The iDelta member of the structure is a signed integer that contains the proposed change in position. If the user has clicked on the up button, this is a positive value. If the user has clicked on the down button, this is a negative value.
Return TRUE in response to this message to prevent the change in the controlís position.
Return FALSE to allow the change in the controlís position.
NM_UPDOWN, WM_COMMAND, WM_VSCROLL
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