How to add notification observers in NSManagedObject subclass to avoid call to unrecognized selector - objective-c

Question

I've been experiencing a problem with saving managed objects (in a background thread) resulting in calls to unrecognised selectors which seems to be related to the way I'm handling the observation of NSManagedObjectContextObjectsDidChangeNotification. Intermittently it will fail with -[NSFetchRequest myManagedObjectContextDidChange:]: unrecognized selector sent to instance. It's not always a NSFetchRequest, sometimes it's a NSKeyValueObservance or unspecified which makes me believe that the observer is still around after a managed object has been released.
I'm adding and removing the observer to NSManagedObjectContextObjectsDidChangeNotification as seen below. Is there anything wrong with that?
#interface Foo ()
#property (assign, nonatomic, getter = isObserving) BOOL observing;
#end
#implementation Foo
#synthesize observing = _observing;
- (void)awakeFromInsert {
[super awakeFromInsert];
self.addedDate = [NSDate date];
self.modificationDate = [self.addedDate copy];
[self commonAwake];
}
- (void)awakeFromFetch {
[super awakeFromFetch];
[self commonAwake];
}
- (void)awakeFromSnapshotEvents:(NSSnapshotEventType)flags {
[super awakeFromSnapshotEvents:flags];
[self commonAwake];
}
- (void)commonAwake
{
if (self.isObserving) return;
[[NSNotificationCenter defaultCenter] addObserver:self
selector:#selector(myManagedObjectContextDidChange:)
name:NSManagedObjectContextObjectsDidChangeNotification
object:self.managedObjectContext];
self.observing = YES;
}
- (void)willTurnIntoFault
{
[[NSNotificationCenter defaultCenter] removeObserver:self name:NSManagedObjectContextObjectsDidChangeNotification object:self.managedObjectContext];
self.observing = NO;
[super willTurnIntoFault];
}
- (void)myManagedObjectContextDidChange:(NSNotification*)notification {
NSDictionary *userInfo = [notification userInfo];
NSMutableSet *changedObjects = [NSMutableSet new];
NSSet *objects = nil;
objects = [userInfo objectForKey:NSInsertedObjectsKey];
[changedObjects unionSet:objects];
objects = [userInfo objectForKey:NSUpdatedObjectsKey];
[changedObjects unionSet:objects];
objects = [userInfo objectForKey:NSDeletedObjectsKey];
[changedObjects unionSet:objects];
NSPredicate *predicate = [NSPredicate predicateWithFormat:#"SELF IN %#", self.bars];
NSSet *updatedObjects = [changedObjects filteredSetUsingPredicate:predicate];
if (updatedObjects.count > 0) {
NSDate *now = [NSDate date];
if (self.modificationDate == nil || [now timeIntervalSinceDate:self.modificationDate] > 1.0) {
self.modificationDate = now;
}
}
}
#end

Answer 1

Check to see when didTurnIntoFault is called. If it is called, try:
[[NSNotificationCenter defaultCenter] removeObserver:self]
Note that the contextDidChange notification might be called on a different thread then the didTurnIntoFault. And so you might have a race condition. Make sure the adding and removing of the observe is done on the same thread (which should be the one on which the managedOject and hence the managed object itself was created).