[This is preliminary documentation and subject to change.]
Server applications use "WinEvents" to notify clients about specific operations they perform. Accessible objects send these event notifications by using the NotifyWinEvent function. This function essentially tells Active Accessibility that a specific event occurred, which object the event is associated with, and specifies the child element if necessary. When Active Accessibility is notified, it calls any hook procedure functions that were registered for that event.
Client applications register callback hook procedure functions for these events by using the SetWinEventHook function. Applications can set a single hook function for all possible events, or multiple hooks for discrete ranges of events. Once called, the callback function receives several parameters describing an object, as well as an event constant value identifying the event that the specified object generated. Active Accessibility provides several options for callback functions; for more information, see About the WinEventProc Callback Function.
Note WinEvents only notify clients that an event is occurring or has occurred—clients cannot fail them.