iOS NSUserDefaults Watching the change of values for a single key

Question

I have the following code aiming to catch the event of a NSUserDefaults value changing for a particular key.

 [[NSUserDefaults standardUserDefaults] addObse         


        

Answer 1

Swift Version:

func setUserDefaultsListener(){
    NSUserDefaults.standardUserDefaults().addObserver(self, forKeyPath: "keyPath", options: NSKeyValueObservingOptions.New, context: nil)
}

override func observeValueForKeyPath(keyPath: String?, ofObject object: AnyObject?, change: [String : AnyObject]?, context: UnsafeMutablePointer<Void>) {
    if keyPath == "keyPath" {
        //Do something
    }
}

deinit {
    NSUserDefaults.standardUserDefaults().removeObserver(self, forKeyPath: "keyPath")
}

Answer 2

Swift 5 Version:

extension UserDefaults {
    @objc dynamic var keyPath: Int {
        return integer(forKey: "keyPath")
    }
}

HINT: make sure the var has exactly the same as the keyPath

and where you use the observer do:

var observer: NSKeyValueObservation?

override func viewDidLoad() {
    super.viewDidLoad()
    observer = UserDefaults.standard.observe(\.keyPath, options: [.initial, .new], changeHandler: { (defaults, change) in
        //Do something
    })
}

deinit {
    observer?.invalidate()
}

Answer 3

Edit: viewDidUnload is now deprecated, use dealloc instead to removeObserver

This should work perfectly, I have just tested here.

- (void)viewDidLoad
{
    [super viewDidLoad];

    [[NSUserDefaults standardUserDefaults] addObserver:self
                                            forKeyPath:@"SomeKey"
                                               options:NSKeyValueObservingOptionNew
                                               context:NULL];
    // Testing...
    NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults]; 
    [defaults setObject:@"test" forKey:@"SomeKey"];
    [defaults synchronize];
}

- (void)viewDidUnload
{
    [super viewDidUnload];

    [[NSUserDefaults standardUserDefaults] removeObserver:self forKeyPath:@"SomeKey"];
}

- (void)observeValueForKeyPath:(NSString *) keyPath ofObject:(id) object change:(NSDictionary *) change context:(void *) context
{
    if([keyPath isEqual:@"SomeKey"])
    {
       NSLog(@"SomeKey change: %@", change);
    }
}

Things you could test.

• Put a break point in viewDidUnload and make sure the view isn't disappearing on you (since you are changing SomeKey from another viewController) If this is the case then maybe move your register/de-register code into init/dealloc depending on how your VC works.

• Use explicit KeyPath like @"SomeKey" not a substitution like SOME_NSSTRING_VARIABLE

Answer 4

Swift 3 Version:

override func viewDidLoad() {
    super.viewDidLoad()
    UserDefaults.standard.addObserver(self, forKeyPath: "keyPath", options: .new, context: nil)
}

override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
    if keyPath == "keyPath" {
        //Do something
    }
}

deinit {
    UserDefaults.standard.removeObserver(self, forKeyPath: "keyPath")
}