Processing Header-Control Notifications
In your view or dialog class, use the Properties window to create an OnChildNotify handler function with a switch statement for any header-control (CHeaderCtrl) notification messages you want to handle (see Mapping Messages to Functions). Notifications are sent to the parent window when the user clicks or double-clicks a header item, drags a divider between items, and so on.
The notification messages associated with a header control are listed in Header Control Reference in the Windows SDK.