NSNotification being added multiple times

Question

I have added an observer in viewDidLoad and the issues is if I pop back and come to the same class again, it adds observer multiple times.

Below is the code as how observer is being added:

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

The notification is being posted from a different view controller after some processes. Below is the code for the same:

ViewController B:

-(void)CardAccepted  
 {
   [[NSNotificationCenter defaultCenter] postNotificationName:@"PaymentRecieved" object:self userInfo:nil];
 } 

The main issue is removeObserver is not working. If I pop back 5 times and then push to same screen 5 times then the observer is added and fired all 5 times.

I have tried everything that was available on Stack Overflow but nothing seems to be working. Below are few of the things that I tried:

Trial 1:

 BOOL isPaymentObserverAdded = [DefaultsValues getBooleanValueFromUserDefaults_ForKey:@"isPaymentObserverAdded"];
 if (!isPaymentObserverAdded) {
 [[NSNotificationCenter defaultCenter] removeObserver:self name:@"PaymentRecieved" object:nil];
    [[NSNotificationCenter defaultCenter] addObserver:self
                                        selector:@selector(PaymentRecieved:)
                                            name:@"PaymentRecieved"
                                          object:nil];
  } 

 }

 [DefaultsValues setBooleanValueToUserDefaults:YES ForKey:@"isPaymentObserverAdded"];  

Trial 2:

static dispatch_once_t lock;
dispatch_once(&lock, ^{
   [[NSNotificationCenter defaultCenter] addObserver:self
                                        selector:@selector(PaymentRecieved:)
                                            name:@"PaymentRecieved"
                                          object:nil];
});  

I know there are so many similar questions on SO for the exact same issue but none of them is solving my problem. Apart from this, I also have couple of other notifications set on this screen.

Can somone please help me to understand what I am doing wrong?

Answer 1

Instead of removing the observer in your viewDidLoad, you should add it to a dealloc method.

- (void)dealloc {
    [[NSNotificationCenter defaultCenter] removeObserver:self];
}

This will remove all notification observers the object has subscribed to, and is good practice when using observers with pretty much all custom objects.

Answer 2

Try to removeObserver when the user pops from the screen. Like in your back action method.

Like This :

-(void)backAction{

[NSNotificaitonCenter defaultCenter] removeObserver:self name:@"Your Notification name" object:nil]];

}

Answer 3

您可以删除后退按钮上的观察者。确保它可以正常工作。我一周前通过执行此操作解决了此问题