NSUserDefaultsDidChangeNotification: What's th

问题:

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??

回答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 (e.g. 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);
}

回答2:

There is no data provided in the notification's userInfo dictionary, so it looks like you're out of luck unless you want to keep another copy of the data stored in NSUserDefaults elsewhere and perform a diff on the two dictionaries.

回答3:

Use custom notifications to determine what exactly happened, e.g.:

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.

回答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

回答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;
}