Warning for iOS/iPhone users about duplicate NSNotification observations

Question

This isn't a question so much as a warning to others to save them some time.

NSNotificationCenter on iOS 3/iPhone OS 3 (I'm assuming also Mac OS X and iOS 4) has the following behavior:

If you register yourself multiple times for the exact specific notification, NSNotificationCenter will NOT recognize the redundancy and instead will fire off as many notifications to you as you've registered an observation for.

This is almost never the behavior you want to see and is almost always accidental.

Example:

I want my view controller to receive notifications from a singleton network object when new data comes in:

- (void) viewDidLoad 
{
    [super viewDidLoad];

    [[NSNotificationCenter defaultCenter] addObserver:self
            selector:@selector(newDataArrived:) 
                name:NewDataArrivedNotification
              object:[NetworkListener sharedNetworkListener]];
}

but earlier I'd already put the same thing in viewWillAppear:

- (void) viewWillAppear
{
    [super viewWillAppear];

    [[NSNotificationCenter defaultCenter] addObserver:self
            selector:@selector(newDataArrived:)
                name:NewDataArrivedNotification
              object:[NetworkListener sharedNetworkListener]];
}

Note that it's exactly the same notification, resolving to the same observer, sender and notification name.

In this case, if I don't remove one of those addObserver calls, I'll get duplicate notifications to my view controller.

In a multi-threaded environment, this is a world of hurt. Trust me.

Just putting this out there in case there are others who run into something like this.

Answer 1

NSNotificationCenter on iOS 3/iPhone OS 3 (I'm assuming also Mac OS X and iOS 4) has the following behavior:

If you register yourself multiple times for the exact specific notification, NSNotificationCenter will NOT recognize the redundancy and instead will fire off as many notifications to you as you've registered an observation for.

This is almost never the behavior you want to see and is almost always accidental.

Example:

I want my view controller to receive notifications from a singleton network object when new data comes in:

- (void) viewDidLoad 
{
    [super viewDidLoad];

    [[NSNotificationCenter defaultCenter] addObserver:self
            selector:@selector(newDataArrived:) 
                name:NewDataArrivedNotification
              object:[NetworkListener sharedNetworkListener]];
}

but earlier I'd already put the same thing in viewWillAppear:

- (void) viewWillAppear
{
    [super viewWillAppear];

    [[NSNotificationCenter defaultCenter] addObserver:self
            selector:@selector(newDataArrived:)
                name:NewDataArrivedNotification
              object:[NetworkListener sharedNetworkListener]];
}

Note that it's exactly the same notification, resolving to the same observer, sender and notification name.

In this case, if I don't remove one of those addObserver calls, I'll get duplicate notifications to my view controller.

In a multi-threaded environment, this is a world of hurt. Trust me.

Just putting this out there in case there are others who run into something like this.

Answer 2

You should and always clean up your observers.
The easiest way to do it is : [[NSNotificationCenter defaultCenter] removeObserver:self]
viewDidLoad is not a good place to add observers, because this functions may get called multiple times, this happens when viewDidUnload is triggered. A good place to put your addObservers in viewWillAppear, and removeObservers in viewWillDisappear.

Answer 3

As you said yourself, NSNotificationCenter makes no check for duplicates, which may be annoying for some, but makes sense when concidering the full system behind it.

The same logic applies to adding targets to certain objects, but there is often a key recognition on those.

Thank you for the insight, and for a good, SEO-friendly warning :)