Core data: concurency conflict between save and fetch

Question

These two function calls seem to be conflicting:

    MagicalRecord.save({ (localContext) in
        let items = NewsItem.staleNewsItems(in: localContext)
        if ((items?.count)! > 0){
            items?.forEach({ (item) in
                if let object = item as? NSManagedObject {
                    object.mr_deleteEntity(in: localContext)
                }
            })
        }
    })

and

- (void) buildAndFetchFRCsInContext:(NSManagedObjectContext*)context {
self.newsItemsFRC = [self buildFetchResultsControllerForClass:[NewsItem class] sortedBy:@"id" withPredicate:nil inContext:context];

[context performBlock:^{
    __unused NSDate* start = [NSDate date];
    NSError* error;

    [self.newsItemsFRC performFetch:&error]; // this line crashes
    [self calculateAndBroadcastCounts];
}];
}

Is this save call thread safe? If so what could cause these two functions to cause each-other to crash?

Answer 1

The issue is I'm modifying the news items outside of the context they were created in. So to fix the issue I had to move the code to the main thread. I switched from using magical records save to just performBlockAndWait which is guaranteed to run on the calling thread:

private static func cleanUpNewsItems() -> Void {
    let context = NSManagedObjectContext.mr_()
    context.performAndWait { 
        var itemsToDelete = [NSManagedObject]()
        if let items = NewsItem.staleNewsItems(in: context) {
            items.forEach({ (item) in
                itemsToDelete.append(item as! NSManagedObject)
            })
        }
        for item in itemsToDelete {
            context.delete(item)
        }
        do {
            try context.save()
        } catch let error as NSError {
            print("Error While Deleting Note: \(error.userInfo)")
        }
    }
}