NSManagedObjectContextDidSaveNotification called multiple times - ios

Question

Hi I have a FriendsViewController where I display my friends records fetched from coreData. I have another View Controller AddFriendViewController Which is presented by FriendsViewController to add a new friend the and it saves the the Context in it. I am listening to this notification of changes on the shared MOC in my FriendsViewController.
[[NSNotificationCenter defaultCenter]
addObserverForName:NSManagedObjectContextDidSaveNotification
object:appdelegate.context queue:nil
usingBlock:^(NSNotification * _Nonnull note) {
NSLog(#"Re-Fetch Whole Friends Array from core data and Sort it with UILocalizedIndexedCollation and reloadData into table");
}];
In AddFriendsViewController just create a friend object and i
Friend *friend= [NSEntityDescription
insertNewObjectForEntityForName:#"Friend"
inManagedObjectContext:appdelegate.context];
friend.name=nameTextfield.text;
[appdelegate.context save:&error];
[self.navigationController popViewControllerAnimated:YES];
Now when I perform save on the context from a AddFriendViewController the above block in FriendsViewController is triggered couple of times instead of one time which cause more processing because re-fetching whole data from core data.I cannot use Fetched Results Controller because I am using UILocalizedIndexedCollation to sort my array into section. So my question is why it is being called twice or sometimes even thrice? Or is there any alternative for this ?

Answer 1

Only you know when you want the notification observer to be active.
However, two common paradigms are:
If you want to be notified anytime during the life of the view controller, then you register the observer in viewDidLoad and remove the observer in dealloc.
If you want to be notified anytime the view is active, you register the observer in viewWillAppear and remove in viewWillDisappear.
EDIT
I used this statement to remove all entries [[NSNotificationCenter
defaultCenter]removeObserver:self]; And it was still showing same
behaviour. Then I used addObserver: selector: name: object: method
which worked. But Why the other one was not removed ? – Asadullah Ali
That's because you added a block-based observer, which returns an observer object. You remove the object it returned to you. You really should read the documentation of each method that you use.
If you use the block-observer method, the add/remove will look like this.
id observer = [[NSNotificationCenter defaultCenter]
addObserverForName:SomeNotification
object:objectBeingObserved
queue:nil
usingBlock:^(NSNotification *note) {
// Do something
}];
[[NSNotificationCenter defaultCenter] removeObserver:observer];
If you use the selector-observer method, you need to remove the observer that you provide to the add call.
[[NSNotificationCenter defaultCenter]
addObserver:someObject
selector:#selector(notificationHandler:)
name:SomeNotification
object:objectBeingObserved];
[[NSNotificationCenter defaultCenter]
removeObserver:someObject
name:SomeNotification
object:objectBeingObserved];

Answer 2

First, I would strongly suggest using a NSFetchedResultsController instead of building your own observer.
Second, sounds like you are adding the observer several times. You should add it in the -viewDidLoad and remove it in -dealloc.
Again, a NSFetchedResultsController is a better solution. You will have better performance and avoid refetching like you are doing now.

Answer 3

You must get rid of the observer (as others already have stated here). The easiest way would be to use a "one-time observer" which removes itself when activated. In your example code this would be something like:
id __block observer;
observer = [[NSNotificationCenter defaultCenter]
addObserverForName:NSManagedObjectContextDidSaveNotification
object:[PubStore sharedStore].managedObjectContext queue:nil
usingBlock:^(NSNotification * _Nonnull notification) {
[[NSNotificationCenter defaultCenter] removeObserver:observer];
NSLog(#"Re-Fetch Whole Friends Array from core data and Sort it with UILocalizedIndexedCollation and reloadData into table");
}];
Note that you must store the observer in a __block variable to be able to use it inside the block executed when the observer is triggered.