NsTimer

1) scheduled timer & using selector

NSTimer *t = [NSTimer scheduledTimerWithTimeInterval: 2.0

                      target: self
                      selector:@selector(onTick:)
                      userInfo: nil repeats:NO];

if you set repeats to NO, the timer will wait 2 seconds before running the selector and after that it will stop;
if repeat: YES, the timer will start immediatelly and will repeat calling the selector every 2 seconds;
to stop the timer you call the timer's -invalidate method: [t invalidate];
As a side note, instead of using a timer that doesn't repeat and calls the selector after a specified interval, you could use a simple statement like this:

[self performSelector:@selector(onTick:) withObject:nil afterDelay:2.0];

this will have the same effect as the sample code above; but if you want to call the selector every nth time, you use the timer with repeats:YES

2) self-scheduled timer
   NSDate *d = [NSDate dateWithTimeIntervalSinceNow: 60.0];

NSTimer *t = [[NSTimer alloc] initWithFireDate: d
                              interval: 1
                              target: self
                              selector:@selector(onTick:)
                              userInfo:nil repeats:YES];

NSRunLoop *runner = [NSRunLoop currentRunLoop];
[runner addTimer:t forMode: NSDefaultRunLoopMode];
[t release];

• this will create a timer that will start itself on a custom date specified by you (in this case, after a minute), and repeats itself every one second 

 

3) unscheduled timer & using invocation

NSMethodSignature *sgn = [self methodSignatureForSelector:@selector(onTick:)];
NSInvocation *inv = [NSInvocation invocationWithMethodSignature: sgn];
[inv setTarget: self];
[inv setSelector:@selector(onTick:)];

NSTimer *t = [NSTimer timerWithTimeInterval: 1.0
                      invocation:inv
                      repeats:YES];

and after that, you start the timer manually whenever you need like this:

NSRunLoop *runner = [NSRunLoop currentRunLoop];
[runner addTimer: t forMode: NSDefaultRunLoopMode];

And as a note, onTick: method looks like this:

-(void)onTick:(NSTimer *)timer {
   //do smth
}