Issue while calling NSNotificationCenter

Question

I want to call a method from another class via NSNotificationCenter.Everything is working fine.

The problem is my method called up two times.

ViewController.m

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

// Do any additional setup after loading the view, typically from a nib.
}

- (void)removeAllSubViews:(NSNotification *)notification
{
NSLog(@"%@",notification.object);

NSLog(@"Print");


}

ViewController2.m

- (void)viewDidLoad {
[super viewDidLoad];
 [[NSNotificationCenter defaultCenter] postNotificationName:@"getTheRequest" object:@"mySite"];
// Do any additional setup after loading the view.
}

When I run, I get this in console:

Console output

Why my method is called up two times ?

Edit

When I use this code in ViewController2.m it works fine. But Why?? **

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

http://stackoverflow.com/questions/7751191/how-to-stop-the-observer-in-nsnotification-to-called-twice — IKKA, Jun 17 '15 at 13:26
Set a breakpoint at the postNotification and take a look at the call stack. It look like the viewDidLoad is getting called twice, or maybe you have multiple instances of ViewController2. — picciano, Jun 17 '15 at 13:26
Same above code and create new project into my xcode `Version 6.3.2 (6D2105)` and `IOS 8.3`. — Nimit Parekh, Jun 17 '15 at 13:30
When i remove the observer it works fine.. but why the method called up two times ? — Sid, Jun 17 '15 at 13:31

score 0 · Answer 1 · answered Jun 17 '15 at 13:33

0

You have to remove observer after use of it inside the method.

- (void)removeAllSubViews:(NSNotification *)notification
{
 [[NSNotificationCenter defaultCenter] removeObserver("getTheRequest")]

NSLog(@"%@",notification.object);

NSLog(@"Print");

}

answered Jun 17 '15 at 13:33

Kirit Modi

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It doesn't works for me.. when i use your code i still get two times NSLogs. – Sid Jun 17 '15 at 13:35
When i remove the observer in ViewCOntroller.m didLoad it works fine – Sid Jun 17 '15 at 13:36
You typically don't want to remove the observer as soon as the related selector is called, because that would prevent it from responding multiple times in the life of a view controller, which in many cases is desired. The most common pattern is to remove the observer in dealloc. – Mike Jun 17 '15 at 13:45

score 0 · Answer 2 · answered Jun 17 '15 at 13:36

I've seen this before when I had a retain cycle in my view controller, so every time a instance of this view controller was created, it was added as an NSNotificationCenter observer, but because of the retain cycle when the view controller was dismissed, it was never actually deallocated/released from memory, so technically it was still an observer.

You might want to try to add the following to your view controller:

- (void)dealloc {
    NSLog(@"Dealloc called.");

    [[NSNotificationCenter defaultCenter] removeObserver:self];
}

This should remove your view controller as an observer when it is dismissed, if there is no retain cycle, and if there is the NSLog will never be called - which is indicative of a larger memory-related issue, where what you're seeing is just a side-effect.

score 0 · Answer 3 · answered Jun 17 '15 at 13:37

You are probably seeing multiple calls to that method, because you register the observer multiple times (e.g. you have navigated to that view controller before), but did not remove it again at the appropriate places. Anyways, viewDidLoad is very likely not the ideal place to register an observer. A common place to do this is the designated initializer, and removing it again in dealloc.

As a side note (and without seeing enough code for a very informed opinion), your use case ("remove all subviews") does not sound like notifications are a good approach. Have you considered using delegation?

0yeoj · Answer 4 · 2015-06-17T13:50:35.713

It is possible that there is a double NSNotificationCenter registration.

I think you have declared another:

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(removeAllSubViews:) name:@"getTheRequest" object:nil];

somewhere.. trying finding the other one..

And just a tip, when you register for NSNoticationCenter try removing first the observer like:

// removes the observer
[[NSNotificationCenter defaultCenter] removeObserver:YourObserver name:YourName object:YourObject];

followed by:

// register
[[NSNotificationCenter defaultCenter] addObserver:YourObserver selector:YourSelector name:YourName object:YourObject];

Just to remove the existing one, if any..

Issue while calling NSNotificationCenter

4 Answers4