didBecomeActive posts a notification, my view controller never hears it

Question

App delegate:

- (void)applicationDidBecomeActive:(UIApplication *)application {
    [[NSNotificationCenter defaultCenter] postNotificationName:APP_REFRESH_NOTIFICATION object:nil];
}

In my view controller:

- (void)viewDidLoad {
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(doStuff) postNotificationName:APP_REFRESH_NOTIFICATION object:self];
}

- (void)doStuff never gets called. Why?

Answer 1

I assume that you've typed your question incorrectly and you'd meant to write addObserver:selector:name:object:, instead of addObserver:selector: postNotificationName:object: (such method doesn't exist).

In the documentation of - (void)addObserver:(id)notificationObserver selector:(SEL)notificationSelector name:(NSString *)notificationName object:(id)notificationSender we can read:

notificationSender

The object whose notifications the observer wants to receive; that is, only notifications sent by this sender are delivered to the observer. If you pass nil, the notification center doesn’t use a notification’s sender to decide whether to deliver it to the observer.

So in your case, as you're passing object:nil in postNotificationName:object:, you also have to set object:nil in addObserver:selector:name:object:.

According to the documentation you also should replace the method doStuff with:

- (void)doStuff:(NSNotification *)notification

and use @selector(doStuff:) in addObserver:selector:name:object:.

Answer 2

You're passing self as the object parameter to addObserver:selector:name:object:, but doStuff doesn't accept any parameters, so the method call fails (silently). Your viewDidLoad should look like this:

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

Answer 3

You're app delegate is posting a notification when the app becomes active, but your view controller isn't subscribing to that until its view gets loaded. If your app delegate is creating your view controller and loading it (which is probable) then your controller doesn't even exist at the time the notification is posted, which is why it isn't receiving it. If you use a storyboard, and that controller is the entry point in the storyboard, AND you use the info.plist for your app to set that storyboard as the main interface, then it will have already instantiated the controller and loaded its view by the time -applicationDidBecomeActive: is called, solving your problem.