NSUserDefaultsDidChangeNotification: What's the name Of the Key, that Changed?

Question

This code will call the method "defaultsChanged", when some value in UserDefaults changed

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

This Code will give me the VALUE that changed

- (void)defaultsChanged:(NSNotification *)notification {
    // Get the user defaults
    NSUserDefaults *defaults = (NSUserDefaults *)[notification object];

    // Do something with it
    NSLog(@"%@", [defaults objectForKey:@"nameOfThingIAmInterestedIn"]);
}

but how can I get the NAME of the key, that changed??

Answer 1

As others stated, there is no way to get the info about the changed key from the NSUserDefaultsDidChange Notification. But there is no need to duplicate any content and check for yourself, because there is Key Value Observing (KVO) which also works with the NSUserDefaults, if you need to specifically be notified of a certain property :

First, register for KVO instead of using the NotificationCenter:

NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];
[defaults addObserver:self
           forKeyPath:@"nameOfThingIAmInterestedIn"
              options:NSKeyValueObservingOptionNew
              context:NULL];

don't forget to remove the observation (eg in viewDidUnload or dealloc)

NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];
[defaults removeObserver:self forKeyPath:@"nameOfThingIAmInterestedIn"];

and finally implement this method to receive KVO notifications

-(void)observeValueForKeyPath:(NSString *)keyPath 
                 ofObject:(id)object
                   change:(NSDictionary *)change
                  context:(void *)context 
{
    NSLog(@"KVO: %@ changed property %@ to value %@", object, keyPath, change);
}

Answer 2

通知的userInfo字典中没有提供数据，所以看起来你运气不好，除非你想在其他地方保存存储在NSUserDefaults中的数据的另一个副本并在两个字典上执行diff。

Answer 3

Use custom notifications to determine what exactly happened, eg:

NSDictionary *options = [NSDictionary dictionaryWithObjectsAndKeys:self.event, @"eventObject", nil];
[[NSNotificationCenter defaultCenter] postNotificationName:@"newEventCreated" object:nil userInfo:options];

If it is not an option with userDefaults, then just read all user defaults everytime you get your NSUserDefaultsDidChangeNotification notification and compair it with previous ones.

Answer 4

just add [[NSNotificationCenter defaultCenter] removeObserver:self name:NSUserDefaultsDidChangeNotification object:nil];

to your appDidBecomeActive method and then add

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

to your applicationDidEnterBackground

then use KVO observer as shown above when in the foreground

Answer 5

You can use dictionaryRepresentation to obtain the entire copy of NSUserDefaults as NSDictionary . Then it's a matter of comparing previous values and new values.

NSUserDefaults is not KVO compliant, the fact that it may fire some KVO notifications should not be relied upon and is a subject to change apparently.

For example:

- (void)setupUserDefaults {
    self.userDefaults = [NSUserDefaults standardUserDefaults];
    [self.userDefaults registerDefaults:@{ /* ... */ }];

    self.userDefaultsDictionaryRepresentation = [self.userDefaults dictionaryRepresentation];

    [notificationCenter addObserver:self 
                           selector:@selector(userDefaultsDidChange:) 
                               name:NSUserDefaultsDidChangeNotification 
                             object:self.userDefaults];
}

- (void)userDefaultsDidChange:(NSNotification *)note {
    NSDictionary *oldValues = self.userDefaultsDictionaryRepresentation;
    NSDictionary *newValues = [self.userDefaults dictionaryRepresentation];

    NSArray *allKeys = @[ /* list keys that you use */ ];

    for(NSString *key in allKeys) {
        id oldValue = oldValues[key];
        id newValue = newValues[key];

        if(![oldValue isEqual:newValue]) {
            [self notifyObserversForKeyChange:key oldValue:oldValue newValue:newValue];
        }
    }

    self.userDefaultsDictionaryRepresentation = newValues;
}