I have the following code aiming to catch the event of a NSUserDefaults value changing for a particular key.
[[NSUserDefaults standardUserDefaults] addObserver:self
forKeyPath:SOME_NSSTRING_VARIABLE
options:NSKeyValueObservingOptionNew
context:NULL];
- (void)observeValueForKeyPath:(NSString *) keyPath ofObject:(id) object change:(NSDictionary *) change context:(void *) context
{NSLog (@"Changed for key %@", keyPath); }
But the observeValueForKeyPath is never getting called. I even tried replacing the SOME_NSSTRING_VARIABLE too with a string as mentioned in Observing value changes to an NSUserDefaults key but it has not helped.
Update: I am changing the NSUserDefaults from a tabview. The above code to monitor changes is in a different tab of the same tabviewcontroller. In the tab where I monitor for the changes (the tab where above code exists), if I add a :
- (void)viewWillAppear:(BOOL)animated
{
[super viewWillAppear:animated];
// NSLog(@"viewillappear");
NSUserDefaults *FUDefaults = [NSUserDefaults standardUserDefaults];
NSLog(@"The username obtained is: %@", [FUDefaults objectForKey:SOME_NSSTRING_VARIABLE]);
}
the updated NSUserDefaults value is obtained correctly, but the observeValueForKeyPath was never called.
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
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")
}
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")
}
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