Using NSTimer to get accurate timings

Question

I need to run my game loop with very accurate timing. I am trying to use NSTimer to do this, and I am getting ok results, but there is a bit of drift. When NSTimer fires does the next time event start counting when the handler finishes or does it start counting straight away. If the former is it reasonable for me to use setFireDate to try to offset the next timer firing - when I tried this things seemed worse.

My timer is set to fire every 44ms and I would like to stop it drifting by more than 20ms. If it does drift I would like to correct it for the next fire. Is this a reasonable thing to try and do with NSTimer?

Answer 1

I don't think NSTimer will give you "very" accurate timing. It fires on the run loop, so if it's in your main thread then it'll get delayed by everything from UI updates to event handling.

Answer 2

You might try creating a thread and scheduling your timer on that thread's run loop. Having that timer as the only thing on that run loop should limit the number of things that can interfere with it.

If that doesn't work, well, you'll already have it working on a thread, so you may as well switch to a usleep() loop.

Answer 3

Also keep in mind that the NSTimer documentation states:

Because of the various input sources a typical run loop manages, the effective resolution of the time interval for a timer is limited to on the order of 50-100 milliseconds. If a timer’s firing time occurs while the run loop is in a mode that is not monitoring the timer or during a long callout, the timer does not fire until the next time the run loop checks the timer. Therefore, the actual time at which the timer fires potentially can be a significant period of time after the scheduled firing time.