ios - Why is my NSNotification its observer called multiple times?

Question

Within an App I make use of several viewcontrollers. On one viewcontroller an observer is initialized as follows:

[[NSNotificationCenter defaultCenter] removeObserver:self name:@"MyNotification" object:nil];
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(myMethod:) name:@"MyNotification" object:nil];

Even when removing the NSNotification before initializing the number of executions of myMethod: is being summed up by the amount of repeated views on the respective viewcontroller.

Why does this happen and how can I avoid myMethod: being called more then once.

Note: I made sure by using breakpoints that I did not made mistakes on calling postNotification multiple times.

Edit: This is how my postNotification looks like

NSArray * objects = [NSArray arrayWithObjects:[NSNumber numberWithInt:number],someText, nil];
NSArray * keys = [NSArray arrayWithObjects:@"Number",@"Text", nil];
NSDictionary * userInfo = [NSDictionary dictionaryWithObjects:objects forKeys:keys];
[[NSNotificationCenter defaultCenter] postNotificationName:@"myNotification" object:self userInfo:userInfo];

edit: even after moving my subscribing to viewwillappear: I get the same result. myMethod: is called multiple times. (number of times i reload the viewcontroller).

-(void)viewWillAppear:(BOOL)animated {

    [super viewWillAppear:animated];
    [[NSNotificationCenter defaultCenter] removeObserver:self name:@"MyNotification" object:nil];
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(myMethod:) name:@"MyNotification" object:nil];
}

edit: something seems wrong with my lifecycle. ViewDidUnload and dealloc are not getting called, however viewdiddisappear is getting called.

The way I push my Viewcontroller to the stack is as follows where parent is a tableview subclass (on clicking the row this viewcontroller is initiated:

detailScreen * screen = [[detailScreen alloc] initWithContentID:ID andFullContentArray:fullContentIndex andParent:parent];
[self.navigationController pushViewController:screen animated:YES];

Solution:

Moving removal of nsnotification to viewdiddisappear did the trick. Thanks for guidance!

Answer 1

Based on this description, a likely cause is that your viewcontrollers are over-retained and not released when you think they are. This is quite common even with ARC if things are over-retained. So, you think that you have only one instance of a given viewcontroller active, whereas you actually have several live instances, and they all listen to the notifications.

If I was in this situation, I would put a breakpoint in the viewcontroller’s dealloc method and make sure it is deallocated correctly, if that’s the intended design of your app.