navigationController keeps running after pop - iphone

Question

Please help me understand the mystery of navigationController. I have a HomeViewController which gets invoked from the didFinishLaunchingWithOptions. From HomeViewController user press a button and my code is
-(IBAction)showMap:(id)sender
{
MapViewController *mapViewController = Nil;
mapViewController = [[MapViewController alloc] initWithNibName:#"MapView-iPad" bundle:nil];
AppDelegate *appDelegate = (AppDelegate *)[[UIApplication sharedApplication] delegate];
[appDelegate.navigationController pushViewController:mapViewController animated:YES];
}
When a user wants to go back from MapViewController, I use the code
-(IBAction)goBackToHome:(id)sender
{
AppDelegate *appDelegate = (AppDelegate *)[[UIApplication sharedApplication] delegate];
[appDelegate.navigationController popViewControllerAnimated:YES];
}
I was under impression that once I am out of MapViewController, all my resources get freed associated with MapViewController. To verify that I put this code inside initWithNibName in MapViewController.
- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil
{
self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
if (self) {
// Custom initialization
[NSTimer scheduledTimerWithTimeInterval:5.0 target:self selector:#selector(printMessage) userInfo:nil repeats:YES];
}
return self;
}
-(void) printMessage
{
NSLog(#"I am inside Map View Controller");
}
To my surprise, even after popping out the MapViewController, the printMessage still continues. Please help me understand what is going on and why MapViewController is still running. Is there any way I can verify the MapViewController is freed up ?

Answer 1

Take a look at the documentation for NSTimer. In particular, the scheduledTimerWithTimeInterval:target:selector:userInfo:repeats: method. The documentation notes that
target
The object to which to send the message specified by aSelector when the timer fires. The target object is retained by the timer and released when the timer is invalidated.
This means that your timer is causing the MapViewController to be retained. Without that timer holding onto a reference to the controller, it should indeed be deallocated when you pop it from the navigation controller.

Answer 2

MapViewController is still being retained: The timer retains the ViewController, and the timer is retained by the run loop it is scheduled on. To get your MapViewController to be released you have to remove the timer from the run loop by using [yourTimer invalidate]. One way to do this is to keep a weak reference to your timer in your MapViewController:
#property (weak) NSTimer *timer;
When you create the timer, assign it to the property:
self.timer = [NSTimer scheduledTimerWithTimeInterval:5.0 target:self selector:#selector(printMessage) userInfo:nil repeats:YES];
Now you can invalidate the timer when you no longer need it, e.g. in viewWillDisappear:
- (void)viewWillDisappear {
[self.timer invalidate];
[super viewWillDisappear];
}