Notifications being removed

Question

I have a model object and a window controller. I would like them to be able to communicate via notifications. I create both during the App Delegate's -applicationDidFinishLaunching: method. I add the observers after the window controller's window is loaded, like this:

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification {
    WordSetWindowController* windowController = [[WordSetWindowController alloc] initWithWindowNibName:@"WordSetWindowController"];

    model = [[WordSetModel alloc] init];

    NSWindow*   window = windowController.window;

    [[NSNotificationCenter defaultCenter] addObserver:windowController
                                             selector:@selector(handleNotification:)
                                                 name:kNotification_GeneratingPairs
                                               object:model];

    [[NSNotificationCenter defaultCenter] addObserver:windowController
                                             selector:@selector(handleNotification:)
                                                 name:kNotification_ProcessingPairs
                                               object:model];

    [[NSNotificationCenter defaultCenter] addObserver:windowController
                                             selector:@selector(handleNotification:)
                                                 name:kNotification_UpdatePairs
                                               object:model];

    [[NSNotificationCenter defaultCenter] addObserver:windowController
                                             selector:@selector(handleNotification:)
                                                 name:kNotification_Complete
                                               object:model];

    [model initiateSearch];
}

The -iniateSearch method kicks off some threads to do some processor-intensive calculations in the background. I'd like those threads to send notifications so I can update the UI while processing is occurring. It looks like this:

- (void)initiateSearch;
{
    [[NSNotificationCenter defaultCenter] postNotificationName:kNotification_GeneratingPairs
                                                        object:self];

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        // ... do the first part of the calculations ...

        // Notify the UI to update
        self->state = SearchState_ProcessingPairs;
        dispatch_async(dispatch_get_main_queue(), ^{
            [[NSNotificationCenter defaultCenter] postNotificationName:kNotification_ProcessingPairs
                                                                object:self];
        });

        // ... Do some more calculations ...

        // Notify the UI that we're done
        self->state = SearchState_Idle;
        dispatch_sync(dispatch_get_main_queue(), ^{
            [[NSNotificationCenter defaultCenter] postNotificationName:kNotification_Complete
                                                                object:self];
        });

    });
}

The first notification works properly, but none of the notifications that happen in the dispatch_async() call ever cause the notification handler to be called. I've tried calling -postNotificationName:: both on the background thread and the UI thread (both by using dispatch_async(dispatch_get_main_queue(),...) and by calling -performSelectorOnMainThread:::) and neither had any effect.

Curious, I added a call via an NSTimer that waits 5 seconds after the big dispatch_async() call at the end of -initiateSearch: and found that even though that all occurs on the main UI thread, it also does not fire the notification handler. If I simply call postNotification::: immediately after the dispatch_async() call returns, it works properly, though.

From this, I'm concluding that the observers are somehow getting removed from the notification center, despite the fact that my code never calls -removeObserver:. Why does this happen, and how can I either keep it from happening or where can I move my calls to -addObserver so that they aren't affected by this?

Answer 1

Is suspect your window controller is getting deallocated.

You assign it to WordSetWindowController* windowController, but that reference disappears at the end of -applicationDidFinishLaunching:, and likely your window controller with it.

Since the window itself is retained by AppKit while open, you end up with an on-screen window without a controller. (Neither NSWindow nor NSNotificationCenter maintain strong references to its controller/observers.)

The initial notification works because those are posted before -applicationDidFinishLaunching: ends and/or the autorelease pool for that event is drained.

Create a strong reference to your window controller in your application delegate, store the window controller's reference there, and I suspect everything will work as advertised.

Something very similar happened to me with a window controller that was also a table delegate; the initial setup and delegate messages would work perfectly, but then later selection events mysteriously disappeared.