Re: NSTimer Question
Re: NSTimer Question
- Subject: Re: NSTimer Question
- From: Graham Cox <email@hidden>
- Date: Mon, 16 Feb 2009 23:49:20 +1100
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. Timers that get delayed for some
reason (like a callback taking too long) do not attempt to compensate
on subsequent firings.
If you need to accurately measure the elapsed time in a timer
callback, don't assume, ask the system for it and measure it yourself.
--Graham
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