MPMusicPlayerController not posting notifications?

Question

I"m trying to use the MPMusicPlayerController to play music, and I also want to receive the notification MPMusicPlayerControllerPlaybackStateDidChange. I set up my player and notification registration pretty much just like the sample (which works, BTW - it receives notifications correctly):

- (id) initWithPlaylist:(MPMediaPlaylist*)list {
    if (self = [super init]) {
        player = [MPMusicPlayerController applicationMusicPlayer];
        [player retain];

        NSLog(@"setting up player");
        [plaayer setQueueWithItemCollection:list];
        [player setShuffleMode:MPMusicShuffleModeOff];
        [player setRepeatMode:MPMusicRepeatModeNone];

        NSLog(@"registering MPMusicPlayerController Notifications");
        [[NSNotificationCenter defaultCenter] addObserver:self 
                                                 selector:@selector(handle_itemChanged:) 
                                                     name:MPMusicPlayerControllerNowPlayingItemDidChangeNotification 
                                                   object:nil];
        [[NSNotificationCenter defaultCenter] addObserver:self
                                                 selector:@selector(handle_stateChanged:)
                                                     name:MPMusicPlayerControllerPlaybackStateDidChangeNotification
                                                   object:nil];

        NSLog(@"turning on player notifications");
        [player beginGeneratingPlaybackNotifications];
    }
}

I get a whole lot of bupkis. The methods handle_itemChanged: and handle_stateChanged: are just empty except for an NSLog statement to show they've been hit, and it never appears that they get hit. The NSLog statements in initWithPlaylist: are printed to the log as expected. The above is just a business object in my app. It's not a view or view controller.

Any ideas? The bizarre thing is that the AddMusic sample works just fine for me, and I can't tell that I"m doing anything differently with regard to the MPMusicPlayerController and its notifications.

Update: I've added this line in my app delegate to see the full flood of notifications:

[[NSNotificationCenter defaultCenter] addObserverForName:nil object:nil queue:nil usingBlock:^(NSNotification *n) { NSLog(@"notification: %@", n); }];

I see all kinds of notifications being printed to the console, but none from the media player controller.

Answer 1

You should add another line above these lines:

[[NSNotificationCenter defaultCenter] addObserver:self
                                         selector:@selector(handle_itemChanged:)
                                             name:MPMusicPlayerControllerNowPlayingItemDidChangeNotification
                                           object:nil];

that is:

[myPlayer beginGeneratingPlaybackNotifications];

it worked for me.

Answer 2

Finally, I figured out the answer: the player has to be sent messages on the main thread. It makes some sense in retrospect, but it was completely non-obvious until the moment it dawned on me. I modified the bug I opened to be a bug on the documentation because I never did spot where they mentioned that the player must be operated on the main thread.