This will create a timer to call the doSomething method on self in 5.0 seconds.

[NSTimer scheduledTimerWithTimeInterval:5.0
         target:self 
         selector:@selector(doSomething) 
         userInfo:nil 
         repeats:NO];

Setting the repeats parameter to false/NO indicates that we want the timer to fire only once. If we set this to true/YES, it would fire every five seconds until manually invalidated.

[timer invalidate];
timer = nil;

This will stop the timer from firing. Must be called from the thread the timer was created in, see Apple's notes (opens new window):

You must send this message from the thread on which the timer was installed. If you send this message from another thread, the input source associated with the timer may not be removed from its run loop, which could prevent the thread from exiting properly.

Setting nil will help you next to check whether it's running or not.

if(timer) {
    [timer invalidate];
    timer = nil;
}

//Now set a timer again.

[timer fire];

Calling the fire method causes an NSTimer to perform the task it would have usually performed on a schedule.

In a non-repeating timer, this will automatically invalidate the timer. That is, calling fire before the time interval is up will result in only one invocation.

In a repeating timer, this will simply invoke the action without interrupting the usual schedule.

When creating a timer, you can set the userInfo parameter to include information that you want to pass to the function you call with the timer.

By taking a timer as a parameter in said function, you can access the userInfo property.

NSDictionary *dictionary = @{
                             @"Message" : @"Hello, world!"
                            }; //this dictionary contains a message
[NSTimer scheduledTimerWithTimeInterval:5.0
     target:self 
     selector:@selector(doSomething) 
     userInfo:dictionary
     repeats:NO]; //the timer contains the dictionary and later calls the function

...

- (void) doSomething:(NSTimer*)timer{
    //the function retrieves the message from the timer
    NSLog("%@", timer.userInfo["Message"]);
}