NSTimer accuracy

FROM : Mike Laster
DATE : Mon Nov 18 18:47:03 2002

If I have a timer scheduled to fire at a particular time, is it
possible for Foundation to trigger it early?  I'm running into an
intermittent bug with a server which needs to run a method every
midnight.  The problem is that sometimes I see the event happen twice
at nearly the same instant.  The method is not thread-safe, since only
one is supposed to be running at a time, and it tends to crash the
server.

Maybe I just need a different paradigm for supporting "run this method
every midnight".  What I currently do is:

- (void) timerTick:(NSTimer *)inTimer
{
    NSCalendarDate *alarmTime = nil;
    NSTimer *timer = nil;

    alarmTime = [[[NSCalendarDate date] beginningOfDay]
dateByAddingDays:1];

    [NSThread detachNewThreadSelector:@selector(doDailyStuff:)
                              toTarget:self
                            withObject:nil];

    timer = [NSTimer scheduledTimerWithTimeInterval:[alarmTime
timeIntervalSinceNow]
                                              target:self selector:_cmd
                                            userInfo:[inTimer userInfo]
                                            repeats:NO];
}

beginningofDay: is a category on NSCalendarDate of:

- (NSCalendarDate *) beginningOfDay
{
    int year = [self yearOfCommonEra];
    int month = [self monthOfYear];
    int day = [self dayOfMonth];
    NSTimeZone *tz = [self timeZone];

    return [NSCalendarDate dateWithYear:year month:month day:day
                                    hour:0 minute:0 second:0
timeZone:tz];
}


alarmTime should be "the next nearest midnight in the future".  The
problem is that if timerTick: fires at 11:59:59 that means "in one
second", which is
not my intent.

Is there a better way of going about this?
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