Editing Carbon (API) (section)

==Timers==
In the classic Mac OS, there was no operating system support for application level timers (the lower level Time Manager was available, but it executed timer callbacks at interrupt time, during which calls could not be safely made to most Toolbox routines). Timers were usually left to application developers to implement, and this was usually done by counting elapsed time during the ''idle'' event - that is, an event that was returned by ''WaitNextEvent'' when any other event wasn't available. In order for such timers to have reasonable resolution, developers could not afford ''WaitNextEvent'' to delay too long, and so low "sleep" parameters were usually set. This results in highly inefficient scheduling behavior, since the thread will not sleep for very long, instead repeatedly waking to return these idle events. Apple added timer support to Carbon to address this problem—the system can schedule timers with great efficiency.