NSNotification is being called multiple times from UITabBarController

Question

I have a UITabBarController, which has 4 tabs. Each one of those tabs is a separate UIViewController. I have objects on each one of those 4 VC\'s t

Answer 1

You must have implemented the NSNotificationCenter's -addObserver:selector:name:object: method in the -viewDidLoad of every viewController

Example:

- (void)viewDidLoad
{
    //...
    [NSNotificationCenter defaultCenter] addObserver:self
                                            selector:@selector(doSomething:) 
                                                name:@"TestNotification"
                                              object:nil];
}

Instead of having this in -viewDidLoad, move it within -viewWillAppear and implement removeObserver:name:object: in -viewWillDisappear.

This way, only the viewController that is currently on will respond to the notification.

Example:

- (void)viewWillAppear:(BOOL)animated
{
    //...
    [NSNotificationCenter defaultCenter] addObserver:self
                                            selector:@selector(doSomething:) 
                                                name:@"TestNotification"
                                              object:nil];
}

- (void)viewWillDisappear:(BOOL)animated
{
    //...
    [NSNotificationCenter defaultCenter] removeObserver:self
                                                   name:@"TestNotification"
                                                 object:nil];
}

---

- (void)doSomething:(NSNotification *)userInfo
{
    //...
    //if you push a viewController then the following is all you need
    [self.navigationController pushViewController:vcSomething
                                         animated:YES];

    //however.... if you're instead presenting a viewController modally then
    //you should implement "-removeObserver:name:object: in this method as well

    //[NSNotificationCenter defaultCenter] removeObserver:self
    //                                               name:@"TestNotification"
    //                                             object:nil];
    //[self presentViewController:vcSomething
    //                   animated:YES
    //                 completion:nil];

    //OR... in the completion parameter as:

    //[self presentViewController:vcSomething
    //                   animated:YES
    //                 completion:^{
    //                     [NSNotificationCenter defaultCenter] removeObserver:self
    //                                                                    name:@"TestNotification"
    //                                                                  object:nil];
    //                 }];
}

---

EDIT:

You (@Jonathan) commented:

I really appreciate your answer and it has helped me out a lot! I actually ran into one more scenario where this issue occur's and I'm not sure how to figure it out. Right now I have a VC that presents another VC modally. Each one has observers for the same NSNotification. Everything performs perfectly well when I'm in the modally presented VC, but once I dismiss that VC and return to the underlying one I have the same issue where the notification is being called multiple times. Do you have an idea for a solution in this case?

Now... regarding this...

FIRSTLY... NOTE:

1. Multiple -addObserver:selector:name:object: will register the specified notification multiple times (means... same notification being registered for N times will call invoke the target selector N times)
2. Presenting a ViewController (call it Child) from, say, Parent viewController will NOT invoke the -viewWillDisappear: of the Parent
   where as...
3. Dismissing the Child viewController will still invoke -viewWillAppear: of the Parent

This creates an imbalance in the logic and if not handled (as per the commented lines in the code example of the doSomething method above), it results in the Parent registering for the notification multiple times (as it's -viewWillAppear: method is called more often than not -viewWillDisappear:)

also see: similar question

Answer 2

it should be called only once so that it never gets called again

static dispatch_once_t onceToken;

dispatch_once(&onceToken, ^{
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(methodName:) name:@"name" object:nil];
});