The system keeps an internal list of appbars and maintains information about each bar in the list. The system uses the information to manage appbars, perform services for them, and send them notification messages.
An application must register an appbar (that is, add it to the internal list) before it can receive appbar services from the system. To register an appbar, an application sends the ABM_NEW message. The accompanying APPBARDATA structure includes the handle of the appbar's window and an application-defined message identifier. The system uses the message identifier to send notification messages to the window procedure of the appbar window. For more information about appbar notification messages, see Appbar Notification Messages.
An application unregisters an appbar by sending the ABM_REMOVE message. Unregistering an appbar removes it from the system's internal list of appbars. The system no longer sends notification messages to the appbar nor prevents other applications from using the screen area occupied by the appbar. An application should always send ABM_REMOVE before destroying an appbar.