Yesterday we talk about how to make use of the NSTimer for scheduling a task.
iPhone – NSTimer Example
 

Sometimes, you may want to stop the scheduled task. Then you can use the invalidate method. If you invalidate a unfired NSTimer, that’s fine. But if you invalidate a fired NSTimer, the application will crashed. So how to by pass the problem?

A simple solution is setting the fired NSTimer to nil and check if it is nil before invalidating them. so in the iPhone – NSTimer Example. I set the aTimer to nil once it is fired. the following new MyViewController.m has a stopTimer method which will invalidate the aTimer if this method is called.

MyViewController.m

...
@synthesis aTimer;
...

/* Schedule a task whenever the view is load */
- (void)viewDidLoad {
	[super viewDidLoad];
	// Schedule the runScheduledTask in 5 seconds
	aTimer = [NSTimer scheduledTimerWithTimeInterval:5.0 target:self selector:@selector(runScheduledTask) userInfo:nil repeats:NO];
}

/* runScheduledTask */
- (void)runScheduledTask {
	// Do whatever u want
	...
	// Set the timer to nil as it has been fired
	aTimer = nil;
}

/* stopTimer */
- (void)stopTimer {
	// Check if aTimer is nil
	if (aTimer) {
		// Only invalidate aTimer if it is not yet fired
		[aTimer invalidate];
	}
}

/* Don't forget to release the objects */
- (void)dealloc {
	[aTimer release];
	[super dealloc];
}

Done =)

Reference: How to use an IF statement to check if a NSTIMER is running?