IPhone: different system timers?

I have been using mach_absolute_time() for all my timing functions so far. calculating how long between frames etc.

I now want to get the exact time touch input events happen using event.timestamp in the touch callbacks.

the problem is these two seem to use completely different timers. sure, you can get them both in seconds, but their origins are different and seemingly r开发者_如何学编程andom...

is there any way to sync the two different timers?

or is there anyway to get access to the same timer that the touch input uses to generate that timestamp property? otherwise its next to useless.

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Had some trouble with this myself. There isn't a lot of good documentation, so I went with experimentation. Here's what I was able to determine:

mach_absolute_time depends on the processor of the device. It returns ticks since the device was last rebooted (otherwise known as uptime). In order to get it in a human readable form, you have to modify it by the result from mach_timebase_info (a ratio), which will return billionth of seconds (or nanoseconds). To make this more usable I use a function like the one below:

#include <mach/mach_time.h>

int getUptimeInMilliseconds()
{
 static const int64_t kOneMillion = 1000 * 1000;
 static mach_timebase_info_data_t s_timebase_info;

 if (s_timebase_info.denom == 0) {
  (void) mach_timebase_info(&s_timebase_info);
 }

 // mach_absolute_time() returns billionth of seconds,
 // so divide by one million to get milliseconds
 return (int)((mach_absolute_time() * s_timebase_info.numer) / (kOneMillion * s_timebase_info.denom));
}

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Get the initial difference between two i.e what is returned by mach_absolute_time() initally when your application starts and also get the event.timestamp initially at the same time...

store the difference... it would remain same through out the time your application runs.. so you can use this time difference to convert one to another...

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How about CFAbsoluteTimeGetCurrent?