removeObserver with NSNotification... what am I doing wrong?

Question

Basically, I have a view1 which at some point, calls view2 (via presentModalViewController:animated:). When a certain button in view2 is pressed, view2 is calls a notification method in view1 and immediately afterward is dismissed. The notification method pops up an alert.

The notification method works fine and is called appropriately. The problem is, every time view1 is created (only one view1 should exist at a time), I presumably get another NSNotification being created because if I go from view0 (the menu) to view1, then back and forth a few times, I get a series of the same alert message, one after another, from the notification method as many times as I opened a view1.

Here is my code, please tell me what I'm doing wrong:

View1.m

-(void) viewDidLoad {
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(showAlert:) name:@"alert" object:nil];
}

-(void) showAlert:(NSNotification*)notification {
// (I've also tried to swap the removeObserver method from dealloc
// to here, but it still fails to remove the observer.)
// < UIAlertView code to pop up a message here. >
}

-(void) dealloc {
[[NSNotificationCenter defaultCenter] removeObserver:self];
[super dealloc];
}

View2.m

-(IBAction) buttonWasTapped {
[[NSNotificationCenter defaultCenter] postNotificationName:@"alert" object:nil];
[self dismissModalViewControllerAnimated:YES];
}

-(void) dealloc {
[[NSNotificationCenter defaultCenter] removeObserver:self];
[super dealloc];
}

Answer 1

Calling -dealloc doesn't automatically happen after the view controller is dismissed — there can still be some "life" left in the view controller's lifetime. In that timeframe, that view controller is still subscribed for that notification.

If you remove the observer in -viewWillDisappear: or -viewDidDisappear:, this will have a more immediate effect:

- (void) viewWillDisappear:(BOOL)animated {
    [super viewWillDisappear:animated];
    [[NSNotificationCenter defaultCenter] removeObserver:self name:@"alert" object:nil];
}