What is the difference between dispatch_time and dispatch_walltime and in what situations is better to use one or the other?

Question

I know that dispatch_time is the time according to device clock and if the device goes to sleep the clock sleeps too. On the other hand the dipatch_walltime is time according to wall clock, which never goes to sleep. My questions is, is there any difference, performance-wise or other, to use one or the other in different situations? Can somebody give me some more details, because the Apple docs are not exhaustive.

For example I'm writing a Timer class which operates on certain intervals. Also the leeway could be 10 to 30 secs. Which one should I use dispatch_time or dispatch_walltime, performance-wise.

Answer 1

dispatch_time stops running when your computer goes to sleep. dispatch_walltime continues running because the real world continues running.

So suppose you want to do an action 1 hour from now, but after 5 minutes your computer goes to sleep for 50 minutes. Then:

• dispatch_walltime will execute an hour from now, 5 minutes after the computer wakes up.

• dispatch_time will execute after the computer is running for an hour, that is 55 minutes after it wakes up, 1 hour and 50 minutes from now.