NSNotificationCenter removeObserver not working

Question

-(void)viewDidAppear:(BOOL)animated {
            NSOperationQueue *mainQueue = [NSOperationQueue mainQueue];
                [[NSNotificationCenter defaultCenter] addObserverForName:UIApplicationUserDidTakeScreenshotNotification object:nil queue:mainQueue usingBlock:^(NSNotification *note) {
                    NSLog(@"SShot");
            }];
        }

- (void)viewWillDisappear:(BOOL)animated{
        [[NSNotificationCenter defaultCenter] removeObserver:self name:UIApplicationUserDidTakeScreenshotNotification object:nil];
    NSLog(@"VWD");
        }

 -(void)viewDidDisappear:(BOOL)animated {
        [[NSNotificationCenter defaultCenter] removeObserver:self name:UIApplicationUserDidTakeScreenshotNotification object:nil];
        NSLog(@"VDD");
    }

I am getting SShot logged in console even after I have removed the observer.

Is there any other way to remove UIApplicationUserDidTakeScreenshotNotification observer.

score 21 · Answer 1 · edited Nov 28 '19 at 16:44

Here is how to do it in Swift 4...

    private var observer: Any!

    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
        observer = NotificationCenter.default.addObserver(forName: NSNotification.Name("SomeNotification"), object: nil, queue: nil) { notification in
            //do something
        }
    }

    override func viewDidDisappear(_ animated: Bool) {
        super.viewDidDisappear(animated)
        NotificationCenter.default.removeObserver(observer)
    }

Leonardo · Accepted Answer · 2015-10-28T11:13:35.573

From Apple Doc:

To unregister observations, you pass the object returned by this method to removeObserver:. You must invoke removeObserver: or removeObserver:name:object: before any object specified by addObserverForName:object:queue:usingBlock: is deallocated.

NSNotificationCenter *center = [NSNotificationCenter defaultCenter];
[center removeObserver:self.localeChangeObserver];

You're trying to remove the worng observer, self is not the observer here, the observer is the object returned by the add method

score 3 · Answer 3 · edited Feb 11 '19 at 20:39

Harris code is correct except a small detail that for Swift 4 it is now

private var observer: Any! and not private var observer: NSObjectProtocol! So code should be:

private var observer: Any!

override func viewWillAppear(_ animated: Bool) {
    super.viewWillAppear(animated)
    observer = NotificationCenter.default.addObserver(forName: NSNotification.Name("SomeNotification"), object: nil, queue: nil) { notification in
        //do something
    }
}

override func viewDidDisappear(_ animated: Bool) {
    super.viewDidDisappear(animated)
    NotificationCenter.default.removeObserver(observer)
}

Piyush Sharma · Answer 4 · 2015-10-28T11:03:01.333

Try using this code

To Add Observer

- (void)viewWillAppear:(BOOL)animated {
    [super viewWillAppear:animated];
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(userDidTakeScreenshot) name:UIApplicationUserDidTakeScreenshotNotification object:nil];
}

- (void)userDidTakeScreenshot {
    // Screenshot taken, act accordingly.
}

And to Remove a Particular Observer

- (void)viewWillDisappear:(BOOL)animated {
    [super viewWillDisappear:animated];
    [[NSNotificationCenter defaultCenter] removeObserver:self name:UIApplicationUserDidTakeScreenshotNotification object:nil];
}

to Remove All Observer

- (void)viewWillDisappear:(BOOL)animated {
        [super viewWillDisappear:animated];
        [[NSNotificationCenter defaultCenter] removeObserver:self];
    }

Let me know if it worked for you !!!!

NSNotificationCenter removeObserver not working

4 Answers4