Parse: PFObject favorite methods

Favorite.m

@implementation

/*...*/

- (void)toggleFavorite:(BOOL)isFavorite
{
    PFUser *user = [PFUser currentUser];
    if (!user) {
        return;
    }
    
    NSUserDefaults *userDefaults = [NSUserDefaults standardUserDefaults];
    
    NSArray *favorites = [userDefaults objectForKey:kDefaultsFavoriteKey];
    BOOL contains = [self isFavorite];

    if (!(contains ^ isFavorite)) {
        return; // Status quo is fine; the UI shouldn't have allowed this case in the first place.
    }

    NSNotification *notification;
    if (isFavorite) {
        if (favorites) {
            favorites = [favorites arrayByAddingObject:self.objectId];
        } else {
            favorites = @[ self.objectId ];
        }
        PFRelation *favoritesRelation = [user relationForKey:@"favorite{Object}s"];
        [favoritesRelation addObject:self];
        [user saveEventually];
        notification = [NSNotification notificationWithName:Favorite{Object}AddedNotification object:self];
    } else {
        favorites = [favorites filteredArrayUsingPredicate:[NSPredicate predicateWithBlock:^BOOL(NSString *objectId, NSDictionary *bindings) {
            return ![objectId isEqualToString:self.objectId];
        }]];
        PFRelation *favoritesRelation = [user relationForKey:@"favorite{Object}s"];
        [favoritesRelation removeObject:self];
        [user saveEventually];
        notification = [NSNotification notificationWithName:Favorite{Object}RemovedNotification object:self];
    }
    [userDefaults setObject:favorites forKey:kDefaultsFavoriteKey];
    [userDefaults synchronize];
    [[NSNotificationCenter defaultCenter] postNotification:notification];
}

- (BOOL)isFavorite
{
    NSSet *favorites = [NSSet setWithArray:[[NSUserDefaults standardUserDefaults] objectForKey:kDefaultsFavoriteKey]];
    return [favorites containsObject:self.objectId];
}

/*...*/
@end