User Event Timers

Windows provides standard API functions for tracking the passing of time, with notifications issued on expiry of a timeout. However, this functionality measures time by the clock, without regard to whether the user was in any sense active. It is sometimes desirable that idle time, free of user input, should not count as time passed. SHELL32 provides just this facility, going by the name of User Event Timers, through an essentially undocumented COM object class and interface.

The object class is UserEventTimer, with instances to be created in any of the usual ways. The class implements the IUserEventTimer interface, with support from an IUserEventTimerCallback interface. The relevant GUIDs are

The object and the related interfaces are declared in SHOBJIDL.H, but they are not otherwise documented, at least as late as the Windows Vista SDK dated January 2007.

UserEventTimer Implementation

For each UserEventTimer object, SHELL32 creates a window with the class name “UserEventWindow”. Once created, this window exists only to handle WM_TIMER messages. To receive such messages, the window sets at most one timer, in the sense of the standard Windows API function SetTimer. This timer is here called the internal timer. It is set to trigger periodically, making for a sequence of ticks. At each tick, the internal timer checks whether there was user input since the tick before. If there was not, then the interval between ticks is treated as idle time and is disregarded.

Through the IUserEventTimer interface, users of a UserEventTimer object set any number of user event timers, each with their own timeout period and their own directions for notification of the timeout’s expiry. The internal timer is set only while at least one user event timer is set. At each tick when the internal timer detects that there has been user input since the previous tick, the interval between ticks is counted against the time remaining for each user event timer. Where a deduction causes a user event timer to expire, that timer’s notification is triggered and its timeout is restored for counting down again.