Ios – NSFetchedResultsController and NSOrderedSet relationships

The last two are identical; "atomic" is the default behavior (note that it is not actually a keyword; it is specified only by the absence of nonatomic -- atomic was added as a keyword in recent versions of llvm/clang).

Assuming that you are @synthesizing the method implementations, atomic vs. non-atomic changes the generated code. If you are writing your own setter/getters, atomic/nonatomic/retain/assign/copy are merely advisory. (Note: @synthesize is now the default behavior in recent versions of LLVM. There is also no need to declare instance variables; they will be synthesized automatically, too, and will have an _ prepended to their name to prevent accidental direct access).

With "atomic", the synthesized setter/getter will ensure that a whole value is always returned from the getter or set by the setter, regardless of setter activity on any other thread. That is, if thread A is in the middle of the getter while thread B calls the setter, an actual viable value -- an autoreleased object, most likely -- will be returned to the caller in A.

In nonatomic, no such guarantees are made. Thus, nonatomic is considerably faster than "atomic".

What "atomic" does not do is make any guarantees about thread safety. If thread A is calling the getter simultaneously with thread B and C calling the setter with different values, thread A may get any one of the three values returned -- the one prior to any setters being called or either of the values passed into the setters in B and C. Likewise, the object may end up with the value from B or C, no way to tell.

Ensuring data integrity -- one of the primary challenges of multi-threaded programming -- is achieved by other means.

Adding to this:

atomicity of a single property also cannot guarantee thread safety when multiple dependent properties are in play.

Consider:

 @property(atomic, copy) NSString *firstName;
 @property(atomic, copy) NSString *lastName;
 @property(readonly, atomic, copy) NSString *fullName;

In this case, thread A could be renaming the object by calling setFirstName: and then calling setLastName:. In the meantime, thread B may call fullName in between thread A's two calls and will receive the new first name coupled with the old last name.

To address this, you need a transactional model. I.e. some other kind of synchronization and/or exclusion that allows one to exclude access to fullName while the dependent properties are being updated.

1 - Yes. It is possible. When you create your mapping file, make sure you change the relationship's type from an NSSet to an NSArray, like so:

    @class Tags

    @interface Entity : NSManagedObject
    ...
    @property (nonatomic, retain) NSOrderedSet *tags;
    ...
    @end

    @interface Entity (CoreDataGeneratedAccessors)
    ...
    - (void)addEntityTags:(NSOrderedSet *)values;
    - (void)removeEntityTags:(NSOrderedSet *)values;
    ...
    @end

Change to:

    @class Tags

    @interface Entity : NSManagedObject
    ...
    @property (nonatomic, retain) NSArray *tags;
    ...
    @end

    @interface Entity (CoreDataGeneratedAccessors)
    ...
    - (void)addEntityTags:(NSArray *)values;
    - (void)removeEntityTags:(NSArray *)values;
    ...
    @end

2 - You can specify a sort descriptor when creating your fetch request, like so:

    NSFetchRequest *fetchRequest [NSFetchRequest new];
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"Entity" inManagedObjectContext:myManagedObjectContext];

    [fetchRequest.setEntity: entity];
    [fetchRequest setSortDescriptors:[NSArray arrayWithObject:[NSSortDescriptor sortDescriptorWithKey:@"tags" ascending:<YES|NO>]];
    [fetchRequest setPredicate:[NSPredicate predicateWithFormat:@"ANY tags == %@", yyy]];

    NSFetchResultsController *myFetchResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:myManagedObjectContext setcionNameKeyPath:nil cacheName:nil];
    myFetchResultsController.delegate = self;

    [fetchRequest release]; // Forget that if using ARC.

    NSError *error = nil;

    if (![myFetchResultsController performFetch:&error]) {
        NSLog(@"Failed to fetch data: %@", error);
    }

3 - If you use the property, you shouldn't need to specify a predicate for your tags. If you want to further sort your tags, you can simply use a sort descriptor on that array, like this:

    NSSortDescriptor *tagDescriptor = [NSSortDescriptor sortDescriptorWithKey:@"<tags field to sort on>" ascending:<YES|NO>];
    NSArray *sortedTags = [tagsProperty sortedArrayUsingDescriptors:[NSArray arrayWithObject:descriptors]];

You can simply read from that property for your tags.

Hope this helps!