Adding and removing observer issue

Question

I have a view controller "A" which is registered as an observer like this,

-(void)viewWillAppear:(BOOL)animated
{
    [[NSNotificationCenter defaultCenter] removeObserver:self name:@"localActionTaken" object:nil];

    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(receiveLocalActionNotification:) name:@"localActionTaken" object:nil];
}

I am also removing observer in dealloc method.

From view controller "A" user goes to view controller "B" where "localActionTaken" notification is posted.

Everything works fine till this

Now I have another view controller "C" from which user is pushed to view controller "A" and from "A" to "B". But the issue is in this "C"->"A"->"B", if notification is triggered in "B" then observerver "A" is notified twice!

Please let me know if I am missing here anything.

Note : I am using ECSlidingview and "A" and "C" are top view controllers.

----------------- Update -----------------

I had to move removeObserver to viewWillDisappear based on check for specific view controllers in stack.

How count called you -(void)viewWillAppear:(BOOL)animated ? maybe @"localActionTaken" notification sent twice. — Sergey Petruk, Feb 16 '15 at 11:34
Is it possible that "A" is being registered as an observer twice even if I call "removeObserver" before "addObserver"? — Praful Kadam, Feb 16 '15 at 11:43
try it: - (void)viewWillDisappear:(BOOL)animated { [super viewWillDisappear:animated]; [[NSNotificationCenter defaultCenter] removeObserver:self name:@"localActionTaken" object:nil]; } — Sergey Petruk, Feb 16 '15 at 12:12
@Spetruk : after removing it in viewWillDisappear of "A" how could "A" listen when "B" post notification — Praful Kadam, Feb 16 '15 at 12:48

score 1 · Answer 1 · answered Feb 16 '15 at 12:09

Does the number of notifications observed accumulate (grow beyond twice) when navigating between A and B. The viewWillAppear will keep getting called when moving from A <-> B but the dealloc will not be called yet. So perhaps there is something wrong the removeObserver in viewWillAppear that is leading to multiple observers being added on A and this would cause multiple notifications to be observed. The code sample you have included seems to work for adding and removing and in dealloc when popping back to C but I haven't tried with "ECSlidingview"

score 0 · Answer 2 · edited Feb 16 '15 at 13:32

0

Add the observer in viewDidLoad

- (void)viewDidLoad {
       [super viewDidLoad];
       [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(receiveLocalActionNotification:) name:@"localActionTaken" object:nil];
    }

edited Feb 16 '15 at 13:32

nycynik

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answered Feb 16 '15 at 12:20

Vineesh TP

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score 0 · Answer 3 · answered Feb 16 '15 at 12:28

0

An important thing you forgot it is that you need to be symmetric in your class.

If you add the notification in the init method, you will remove the notification in the dealloc method.

If you add the notification in the viewWillAppear method, you will remove the notification in the viewWillDisappearmethod (as @Spetruk said in a comment).

So, you can do :

-(void)viewWillAppear:(BOOL)animated
{
    [super viewWillAppear:animated];

    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(receiveLocalActionNotification:) name:@"localActionTaken" object:nil];
}

- (void)viewWillDisappear:(BOOL)animated 
{ 
   [super viewWillDisappear:animated]; 

   [[NSNotificationCenter defaultCenter] removeObserver:self name:@"localActionTaken" object:nil]; 
}

answered Feb 16 '15 at 12:28

Jordan Montel

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If I remove observer in viewWillDisappear of "A", then I couldn't post notification in "B" as I go from "A"->"B" – Praful Kadam Feb 16 '15 at 12:46
If you want to pass data or make an action from B to A, you should use a protocol and delegate instead of notifications. – Jordan Montel Feb 16 '15 at 12:57
I would have preferred delegation but I have multiple senders. – Praful Kadam Feb 16 '15 at 13:03

Adding and removing observer issue

3 Answers3