NSUserDefaults and KVO issues

Question

I'm using NSUserDefaults in my app and I would like to be notified when a particular value is changed. For that, I added the following lines in viewDidLoad:

NSUserDefaults *settings = [NSUserDefaults standardUserDefaults];
[settings synchronize];
[settings addObserver:self forKeyPath:@"pref_server" options:NSKeyValueObservingOptionNew context:NULL];

And the method to be notified:

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{

    NSLog(@"Change");

    NSUserDefaults *settings = [NSUserDefaults standardUserDefaults];
    if (object == settings && [keyPath isEqualToString:@"pref_server"])
    {
        NSLog(@"Server did change");
    }

}

Unfortunately, the latter is never called...@"pref_server" is the item identifier I have set in Root.plist, in Settings.bundle. What am I doing wrong?

Answer 1

Interesting observation:

[NSUserDefaults standardUserDefaults] seems to be KVO compliant now as I am able to observe and bind to it's values. I'm running 10.7.2, using Xcode 4.2, SDK 10.7, LLVM compiler 3.0 .

I can't seem to find this new behavior documented anywhere in the release notes.

Answer 2

I suggest making use of the appropriate notification: NSUserDefaultsDidChangeNotification.

Search for AppPrefs in the Apple Documentation within Xcode and it'll show an example app which does exactly what you want to do. Just compile and run! It makes use of the NSUserDefaultsDidChangeNotification.

This is the code being used to register an observer:

[[NSNotificationCenter defaultCenter] addObserver:self
                                         selector:@selector(defaultsChanged:)
                                             name:NSUserDefaultsDidChangeNotification
                                           object:nil];

Answer 3

NSUserDefaults is not KVO compliant, but NSUserDefaultsController is. So you'd use:

NSUserDefaultsController *defaultsc = [NSUserDefaultsController sharedUserDefaultsController];
[defaultsc addObserver:self forKeyPath:@"values.pref_server" 
               options:NSKeyValueObservingOptionNew 
               context:NULL];