Re: NSTimer Question
Re: NSTimer Question
- Subject: Re: NSTimer Question
- From: Andy Lee <email@hidden>
- Date: Mon, 16 Feb 2009 09:09:40 -0500
On Feb 16, 2009, at 7:49 AM, Graham Cox wrote:
On 16 Feb 2009, at 11:44 pm, Andreas Grosam wrote:
how does a NSTimer object that has been setup with a repeating time
interval calculate the time when it fires an event?
There may be two possibilities:
Say, the initial time is at t0, the interval is T, and the time
when it fires the first event is at t1, the second at t2, etc.
The time when it fires the ith event will be ti:
A)
ti = t (i-1) + T
B)
ti = i * T + t0
Reason:
If I need a "tick" that corresponds to a current time, I would
prefer method B (mean error remains stable).
In case A the firing time may fade away with respect to the initial
time due to inherent inaccuracies. Although this error is certainly
small compared to the error that we have when we measure the time
when we actually do handle the event, but it may accumulate over a
long duration. That is the error increases with time.
I may be wrong, but I believe it's A.
That's what I thought too (and was pretty sure), but a quick RTFD
indicates otherwise:
<http://developer.apple.com/documentation/Cocoa/Reference/Foundation/Classes/NSTimer_Class/Reference/NSTimer.html
>
A repeating timer reschedules itself based on the scheduled firing
time, not the actual firing time. For example, if a timer is
scheduled to fire at a particular time and every 5 seconds after
that, the scheduled firing time will always fall on the original 5
second time intervals, even if the actual firing time gets delayed.
If the firing time is delayed so far that it passes one or more of
the scheduled firing times, the timer is fired only once for that
time period; the timer is then rescheduled, after firing, for the
next scheduled firing time in the future.
--Andy
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