Question

Not much code here, but this problem is seriously annoying me!

.h:

@interface processController : UIViewController {
         NSTimer *timer;
}
@property (nonatomic, retain) NSTimer *timer;

.m:

- (void)viewDidLoad {
    [super viewDidLoad];
    timer = [NSTimer scheduledTimerWithTimeInterval:0.5 target:self selector:@selector (main) userInfo:nil repeats:YES];
}
- (void)main {
    NSLog(@"testing");
}
- (void)viewDidUnload {
    [self.timer invalidate];
    self.timer = nil;
}

The weird thing is, when I fire this code:

ViewController *main = [[ViewController alloc] initWithNibName:nil bundle:nil];
    [self presentModalViewController:main animated:YES];

and switch views, the timer keeps firing! What am I doing wrong?

Answer 1

viewDidUnload is not called when the view is removed from the screen but only when a memory warning is received by the application. It’s not called when the UIViewController is deallocated either. From Apple’s documentation:

When a low-memory condition occurs and the current view controller’s
views are not needed, the system may opt to remove those views from
memory. This method is called after the view controller’s view has
been released and is your chance to perform any final cleanup. If your
view controller stores separate references to the view or its
subviews, you should use this method to release those references. You
can also use this method to remove references to any objects that you
created to support the view but that are no longer needed now that the
view is gone. You should not use this method to release user data or
any other information that cannot be easily recreated.

You most likely meant to use viewWillDisappear

- (void)viewWillDisappear:(BOOL)animated
{
    [super viewWillDisappear:animated];
    [self.timer invalidate];
    self.timer = nil;
}   

You probably want to start your timer in the viewWillAppear: as viewDidLoad is called as soon as the controller’s view property is accessed. Not nercessarely when the view is shown.

- (void)viewWillAppear:(BOOL)animated
{
    [super viewWillAppear:animated];
    timer = [NSTimer scheduledTimerWithTimeInterval:0.5 target:self selector:@selector(main) userInfo:nil repeats:YES];
}

I’d recommend reading the discussions for viewDidLoad and viewDidUnload as well as viewWillAppear and viewWillDisappear from the UIViewController documentation.